The goal of the CHIANTI database is to reproduce the emission spectrum of astrophysical sources at wavelengths greater than about 1 Å. This initial version was developed with the expectation that it would be essentially complete for reproducing the emission line spectra at wavelengths above 50 Å. This was done in order to meet the immediate needs for analyses of astrophysical spectra obtained by experiments such as EUVE, Hubble, and SOHO. Consequently, ions in the helium isoelectronic sequence have not been included and He II is the only ion in the hydrogen isoelectronic sequence to be included in the current version. We expect to include these ions in the near future. Many transitions at wavelengths shorter than 50 Å\ are included in the database but it is not complete at the shorter wavelengths.

For He II the 25 fine structure levels of the 1s,
2*l*, 3*l*,
4*l*, and 5*l* configurations have been
included. Observed
energies are from Kelly (1987).
Radiative constants (*gf* and *A* values)
were taken from Wiese et al. (1966)
for dipole transitions. For the 1s
magnetic dipole
and two photon
electric dipole transitions, the *A* values of
Parpia & Johnson (1972)
are used.

Aggarwal et al. (1991b) have
performed R-matrix calculations of
collisions strengths for transitions among the
*n*=1-5 levels of He II
(i.e. 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, 5s,
5p, 5d, 5f, and 5g).
Aggarwal et al. (1992) noted that
the 1991 calculations were performed
at energies up to 7 Ryd and that the collision
strengths in the
intermediate energy region above those energies
where the threshold
resonances occur are overestimates of the actual
collision strength.
Consequently, they combined the close-coupling
collision strengths of
Unnikrishnan et al. (1991) in the
4.4 to 14.71 Ryd energy range with
the collision strengths of Aggarwal et al.
(1991b) in the threshold
region to arrive at better estimates of the
collision strengths to the
2s, 2p, 3s, 3p, and 3d levels. We have used
these values. For the *n*=4
and *n*=5 levels, we have used the original
calculations of Aggarwal
et al. (1991b). To distribute the
collision strengths between 2 LS
states among the fine structure levels, we have
scaled the collision
strengths following the rules for the
distribution of *gf* values under
LS coupling.

The atomic structure of the lithium-like ions is relatively simple. Since there are no intersystem transitions, the calculation of collision rates and radiative data is relatively straightforward. The consequent lack of metastable levels means that density sensitive line ratios are not available but the interpretation of lithium sequence line intensities in terms of emission measures is simplified. The transitions are quite strong and provide good diagnostics over a range of temperatures, from C IV in the transition regions, through Mg X and Si XII in the quiet corona and to Fe XXIV in flares. The electron excitation data available for the lithium-like ions seems to be very accurate (McWhirter 1994).

The lithium-like ions provide an opportunity for a precise comparison between laboratory measurements of electron excitation rate coefficients () and theoretical work. The recent laboratory measurements for C IV, N V and O VI agree well with theory, to within the typical uncertainties (20%) quoted. However, there remains a long-standing discrepancy for Be II.

Also, they provide an opportunity for determining the electron temperature in the solar atmosphere using the intensity ratios of the relative to transitions (McWhirter 1976). In particular, the O VI ratio 1032 Å/173 Å is a potentially useful diagnostic for SOHO (Bely-Dubau 1994).

For the lithium isoelectronic sequence ions C IV
and N V, the
configurations 2s^{2}2s, 2s^{2}2p, 2s^{2}3s,
2s^{2}3p, and 2s^{2}3d
were included. Values for the observed energy
levels were taken from
Kelly (1987). Oscillator strengths
and spontaneous transition
probabilities (*A* values) were taken from
Wiese et al. (1966) and
Martin
et al. (1993). Gau & Henry
(1977) provided analytical fits to their
calculations of the necessary collision strengths
for several ions in
the lithium isoelectronic sequence and these have
been used here.
Their functional form () provides the
correct high energy behavior () but
it has been necessary
to modify their parameter *c*_{2} to ensure an
exact match to the
Coulomb-Bethe high energy limit.

Laboratory measurements of the absolute electron excitation rate coefficients for the transition in C IV have been carried out by Savin et al. (1995). These measurements agree well with the 9 state CC calculations of Burke (1992).

**Figure 1:** Collision strengths (Zhang et al.
1990) for Mg X 1s^{2}2s
^{2}S_{1/2} - 1s^{2}3p ^{2}P_{3/2}. Upper
left, the original
collision strengths, upper right, the scaled
collision strengths with
the high energy limit from the Bethe
approximation, lower left, the
values of , the collision strengths
convolved with a
Maxwellian over the energy range from threshold
to infinity, and, lower
right, the scaled values of following
the Burgess & Tully
(1992) scaling for an allowed transition

Observed energies for the 24 fine structure
levels of 2*l*, 3*
l*, 4*l*, 5*l* configurations for O VI,
Ne VIII and Ar XVI are
obtained from Kelly (1987) and for
Mg X, Al XI, Si XII, S XIV, Ca
XVIII, Fe XXIV, and Ni XXVI are obtained from the
NIST database.

Zhang et al. (1990) have calculated relativistic distorted wave collision strengths for the the lithium isoelectronic sequence from O VI through Ni XXVI. In addition, they provide oscillator strengths from which the radiative data has been calculated. Some additional oscillator strengths come from Martin et al. (1993).

As an example of the Burgess and Tully collision
strength scaling, the
data for the Mg X 1s^{2}2s ^{2}S_{1/2} to
1s^{2}3p ^{2}P_{3/2}
dipole transition is shown in Fig. 1 (click here).
The spline fits to the scaled
's show an average deviation of about
0.2%.

The beryllium-like ions contain metastable levels
(2s2p ^{3}P_{0,1,2}
in the first excited configuration. Since the
early work with Skylab,
these ions have provided useful electron density
diagnostics for the
solar transition region (cf. Gabriel &
Jordan 1972; Dupree
et al. 1976; Dere & Mason
1981). In particular the diagnostic
potential of the C III lines has been extensively
explored (Doschek
1997).

A comparison of the laboratory and theoretical transition probabilities for the resonance and intercombination lines in the Be-like sequence has recently been published by Ralchenko & Vainshtein (1996). Although on the whole the agreement is good, there still remain some discrepancies to resolve.

The available electron excitation rates for the
beryllium-like ions was
reviewed by Berrington (1994). For
the low ion stages, it is important
to include the channel coupling and resonance
effects of the *n* = 3
states on the *n*=2 to *n*=2 transitions.
R-matrix calculations are
therefore recommended. For the highly ionized
systems the CC and DW
results are found to be in good agreement.

The adopted atomic model for C III includes 6
configurations: 2s^{2},
2s2p, 2p^{2}, 2s3s, 2s3p and 2s3d, corresponding
to 20 fine structure
levels. Experimental energies from the NIST
database are available for
all levels, and they have been used to calculate
transition
wavelengths. Radiative transition probabilities
for all possible
transitions between the 20 levels have been taken
from
Bhatia &
Kastner (1992). No radiative transition
probability for the level is
2s2p ^{3}P_{0} is available in the literature.
The decay would involve
a forbidden *J*=0 to *J*=0 transition to the
ground level. Collisional
data for transitions between 2s^{2}, 2s2p and
2p^{2} levels have been
taken from the R-Matrix calculations of
Berrington et al. (1985).
Maxwellian averaged collision strengths are
calculated for 9 values of
electron temperature in the range 10^{4}-10^{6} K.
Adopted collisional
data for transitions from *n*=2 to *n*=3 and
between *n*=3 levels are
reported in Berrington et al.
(1989): they provide R-Matrix effective
collision strengths for 10 values of electron
temperature. Both
R-Matrix calculations adopt the LS coupling
scheme for all levels. In
order to transform to intermediate coupling their
thermal
averaged collision strengths have been scaled
using the statistical
weights of the levels.

The adopted atomic model for N IV includes 6
different configurations:
2s^{2}, 2s2p, 2p^{2}, 2s3s, 2s3p and 2s3d,
corresponding to 20
fine-structure energy levels. Experimental energy
levels come from the
NIST database and have been used to calculate all
transition
wavelengths. Theoretical energy levels come from
Ramsbottom
et al. (1994). *A* values and oscillator
strengths come from the
unpublished calculations of Bhatia and are
available for all possible
transitions between the 20 energy levels. Again,
no radiative
transition probability is available for 2s2p
^{3}P_{0} level.
Maxwellian averaged collision strengths are taken
from the R-Matrix
calculations of Ramsbottom et al. (1994).
values are
provided for 31 values of electron temperature
for all transitions
between the 20 fine structure levels in the range
10^{3}-10^{6} K.

For some transitions from Ramsbottom et al. the
behavior of the
effective collision strengths as a function of
electron temperature is
quite complex and the 5-point spline
interpolation technique is not
able to reproduce with the required accuracy
their dependence on .
For this reason the values of the effective
collision strengths for
temperatures lower than 10^{3.6} K have been
omitted from the CHIANTI
database, so that the interpolation technique
could reproduce
adequately the behavior of the curve. This
omission of points should
not have any consequence for studies of
transition region and coronal
plasmas where the temperature is much higher than
10^{3.6} K and the
temperature of maximum abundance for N IV is
K.

For O V, the configurations 2s^{2}, 2s2p, 2p^{2},
2s3s, 2s3p and 2s3d
are included. Energy levels from the NIST
database are available for
all of the 20 fine structure levels. Zhang
& Sampson (1992) provide
electric dipole oscillator strengths and *A*
values for all allowed

transitions. Additional radiative transition
probabilities are taken
from Muhlethaler & Nussbaumer
(1976) for forbidden and
intercombination transitions. For transitions
from the *n*=3 levels, the
*A* values of Hibbert (1980) have
been used.

Zhang & Sampson (1992) also
provide relativistic DW collision
strengths among the *n*=2 levels. Collision
strengths to the *n*=3 levels
are provided by Kato et al. (1990)
who use the R-matrix calculations of
Berrington & Kingston (1990). The
values of Kato et al. have been
recently updated (Kato 1996).

For these members of the beryllium-like sequence,
the
configurations 2s^{2}, 2s2p, 2p^{2}, 2s3s, 2s3p,
2s3d, 2p3s, 2p3p and
2p3d are included. Energy levels from the NIST
database are available
for many but not all of the 46 fine structure
levels. The number of
unidentified levels increases with *Z*.
Zhang & Sampson (1992) provide
electric dipole oscillator strengths and *A*
values for all allowed
transitions. Additional radiative transition
probabilities are taken
from Muhlethaler & Nussbaumer
(1976) for forbidden and
intercombination transitions. Zhang &
Sampson (1992) also provide
relativistic DW collision strengths among the
*n*=2 levels. The *n*=2 to
*n*=3 transition probabilities and
Coulomb-Born-exchange collision
strengths are taken from Sampson et al.
(1984).

We note that R-matrix electron impact excitation
rates for Ne VII
transitions up to the *n*=3 configurations have
recently been published
by Ramsbottom et al. (1994, 1995).
These will be assessed in the next
release of CHIANTI.

The configurations 2s^{2}, 2s2p and 2p^{2}, are
included. Energy
levels from the NIST database are available for
all of the 10 fine
structure levels. Zhang & Sampson
(1992) provide electric dipole
oscillator strengths and *A* values for all
allowed

transitions.
Additional radiative transition probabilities are
taken from
Muhlethaler & Nussbaumer (1976)
for forbidden and intercombination
transitions which have been interpolated for Al X
and Ar XV. Zhang &
Sampson (1992) also provide relativistic DW
collision strengths among
the *n*=2 levels. For S XIII and higher,
transitions from the *n*=3 levels
occur at wavelengths shorter than 50 Å. The
*n*=3 levels for these
ions have not been included in the present
version of the CHIANTI
database since the additional levels incur a
penalty in computational
speed when solving for level populations.
Nevertheless, they will be
included in future releases of CHIANTI.

Berrington (1994) found very good agreement between the DW results for Ca XVII (Bhatia & Mason 1983) and the R-matrix calculations by Dufton et al. (1983c), even for quite low temperatures. This gives us confidence in the adoption of relativistic DW calculations for the highly ionized Be-like ions.

The adopted atomic model for Fe XXIII includes 9
configurations,
2s^{2}, 2s2p, 2p^{2}, 2s3*l* and 2s4*l*
corresponding to 30
fine structure energy levels. Experimental
energies come from the NIST
database and have been used to calculate the
transition wavelengths.
When no experimental energy level was available,
the theoretical value has
been used. Theoretical energies for
configurations 2s^{2}, 2s2p and
2p^{2} come from Zhang & Sampson
(1992). Theoretical values for the
remaining configurations, oscillator strengths
and *A* values come from
the unpublished calculations of Bhatia.
Radiative transition
probabilities are available for all transitions
between the levels of
the adopted atomic model.

Adopted collision strengths for transitions
between levels belonging to
2s^{2}, 2s2p and 2p^{2} configurations come from
Zhang & Sampson
(1992) (see previous subsection).
Collision strengths to the 3*l*
and 4*l* come from the unpublished distorted
wave calculations of
Bhatia (1996). Collision strengths
are provided for 3 values of the incident
electron energy at 115, 345 and 695 Rydberg for
all transitions between
the 30 levels.

The 's from Zhang & Sampson have been
compared with those
determined from the earlier work by Bhatia
& Mason (1981, 1986). On the
whole the agreement is found to be excellent at
around 10^{7} K.
However, for a few (non-dipole) transitions, the
high temperature
behaviour differs. This could be due to lack of
convergence for the
high partial waves in Bhatia and Mason's
calculations.
Berrington
(1994) found good agreement between the
Fe XXIII distorted wave
calculations (Bhatia & Mason 1986)
and the sophisticated, fully
relativistic (Dirac) R-matrix calculations
(Norrington & Grant 1987;
Keenan et al. 1993). The
calculations of Bhatia and Mason do not contain
the
2p3*l* and
2p4*l* configurations. This places a severe
limitation on the accuracy
of the collision strengths for some of the *n*=2
to *n*=3, 4 transitions. This
problem is discussed by Bhatia & Mason
(1981).

The boron isoelectronic sequence has proven a
rich source for
diagnostic ratios in the solar atmosphere (cf.
Vernazza & Mason
1978). For the low ion stages,
transitions from the metastable
levels in the excited configuration ,
which fall at around
1400 Å, have been used as a primary diagnostic
for measuring the
electron pressure in the transition region (cf.
Dere et al. 1982). For
the coronal ions (Mg VIII, Si X, A XIV, Ca XVI),
the relative change in
population in the ground levels -
is reflected in the
intensities of the UV transitions from the
excited configuration
. The intensity ratios of the UV
spectral lines have been used
to determine electron densities in solar flares
(cf. Dere
et al. 1979) and other solar features
(see references in
Dwivedi
1994 and Keenan 1996).
X-ray and XUV lines from the ion Fe XXII arise
from the transitions between the excited
configurations ,
and and the ground
configuration (Mason & Storey
1980). These have been recorded in
spectra of solar flares and more
recently in astrophysical sources (Dupree
et al. 1993;
Monsignori
et al. 1994b). These spectral lines can
be used for electron
density determination if the electron density is
high (> 10^{12}
).

Electron excitation data for the B-like ions was
reviewed by
Sampson
et al. (1994). Extensive datasets are
available for this sequence
and include those of

Sampson et
al. (1986) who used their own
non-standard
CB exchange method, with relativistic
corrections. These are generally
in very good agreement with the IC DW
calculations using the UCL code.
These data have now been superseded by more
recent R-matrix and fully
relativistic DW calculations.

The observed energy levels for the 2s^{2}2p,
2s2p^{2}, 2p^{3},
2s^{2}3s, and 2s^{2}3p configurations are from
Kelly (1987) and are
complete. Oscillator strengths and *A* values
are taken from Dankwort
& Trefftz (1978), Nussbaumer &
Storey (1981),
Lennon et al. (1985),
and Wiese & Fuhr (1995).
Radiative transition probabilities for C II
have been measured in the laboratory by
Fang et al. (1993). R-matrix
values of the collision strengths among these
levels have been provided
by Blum & Pradhan (1992). For the
allowed transitions, the collision
strengths generally tend to extrapolate to the
proper high energy limit
derived from the oscillator strength.

Observed energies for the 20 fine structure
levels of the 2s^{2}2p,
2s2p^{2}, 2p^{3}, 2s^{2}3s, 2s^{2}3p and
2s^{2}3d configurations are
from Moore (1993) and Kelly
(1987). Oscillator strengths and *A*
values
have been provided for all of the same levels by
Stafford et al. (1993)
except for the ground level fine structure
transition where the *A* value
is from Nussbaumer & Storey
(1979). Stafford et al.
(1994) report
R-matrix calculations of Maxwellian-averaged
collision strengths among
all these configurations.

**Figure 2:** Collision strengths (Zhang et al.
1994) for Si X 2s^{2}2p
^{2}P_{1/2} - 2s2p^{2} ^{2}D_{3/2}. Left,
the original Maxwellian
averaged collision strengths , right,
the scaled collision
strengths with the high energy limit from the
Bethe approximation. The
scaling follows that of Burgess & Tully (1992)

The 125 fine structure levels of the 2s^{2}2p,
2s2p^{2}, 2p^{3} and
2*l*2*l*'3*l*'' configurations have been
included for
these ions. The energy levels are from the NIST
database

(Martin
et al. 1995) except for the 2s2p^{2}
^{4}P levels which are from
Edlen
(1981). Widing (1996) has
pointed out that the Edlen values seem to
identify several lines of Ar XVI arising from the
2s2p^{2} ^{4}P
levels. For the *n*=2 configurations, the
observed energies are known.
For the 2*l*2*l*'3*l*'' configurations, most
of the
levels are known for O IV but as *Z* increases,
the number of levels for
which there are observed energies become fewer
and for Fe XXII only a
minority of these levels have been identified.
Oscillator strengths
and *A* values for the *n*=2 levels were obtained
from the same sources as
for C II above with additional data from
Bhatia et al. (1986). For the
*n*=3 levels, the oscillator strengths from the
unpublished calculations
of Zhang & Sampson (1995) were
used.

Zhang et al. (1994) have provided
R-matrix calculations of Maxwellian
averaged collision strengths between the 15 fine
structure levels in
the 2s^{2}2p, 2s2p^{2} and 2p^{3} configurations.
The behavior of the
collision strengths at the lowest temperatures
(100 - 500 K) can often
vary so rapidly that they can not be well
represented with a 5 point
spline. Often these values have been ignored so
that the accuracy at
the lowest temperature is about only a factor of
2. Collision
strengths to the *n*=3 levels (110 fine structure
levels) have been
calculated by Zhang & Sampson
(1995) using the Coulomb-Born method.

One complication for the boron sequence is a
level crossing by the
2s2p^{2} ^{2}S and ^{2}P
levels somewhere
between calcium (*Z*=20) and iron (*Z*=26).
Zhang (1995) has confirmed
that their published collision strengths
involving these two levels
must be exchanged to reflect this.
Dankwort & Trefftz (1978) appear
to have simply exchanged labels so that of the
two, the ^{2}P levels always has the higher energy.

Figure 2 (click here) shows an example of the use of
the Zhang et al. R-matrix
collision strengths for the case of Si X. It has
been necessary to
truncate the calculated collision strengths to
temperatures above 3
10^{4} K. Since Si X is formed near 10^{6} K in
collisional
equilibrium, there should be no loss of accuracy
for collisionally
dominated plasmas. The high temperature limit is
determined from the
oscillator strength. In general, it would be
useful to have the
oscillator strength calculated for the same
atomic model used for the
collisional calculations to compare the
values to. The average deviation of
the spline fit to the scaled collision strengths
is about 0.5%.

Ait-Tahar et al. (1996) have
recently carried out a Dirac R-matrix
calculation for transitions between levels in the
*n*=2 configurations
of Fe XXII. For the high energies, they find 10%
agreement with RDW (Zhang & Sampson
1994) and R-matrix (BP + TCC)
(Zhang & Pradhan 1994, 1995). For
low energies they find some
discrepancies in the resonance structures which
need further
investigation. They have published sample plots
of 's and
their 's are not yet available.

The atomic model included the same levels as the
O IV - Fe XXII ions
above. The values for the observed energy levels
were taken from NIST
(Martin et al. 1995) and
Edlen (1981). For the *n*=2
levels, the
oscillator strengths and *A* values were
generally from
Dankwort &
Trefftz (1978) except for the 2s2p^{2}
^{2}S and
^{2}P levels which were from
Sampson et al. (1986). The
ground term fine structure *A* value was obtained
by extrapolating the
values of Bhatia et al. (1986).
Oscillator strenghts and *A* values
involving the *n*=3 levels are from Zhang
& Sampson (1995). For the
*n*=2 levels of Ni XXIV, the collision strengths
were taken from the
Coulomb-Born-Exchange calculations of
Sampson et al. (1986). For the
*n*=3 levels, the collision strengths are those
of Zhang & Sampson
(1995).

Lines from ions in the carbon isoelectronic sequence have been studied in spectra from the transition region, corona and flares. The ground configuration comprises , and levels. The relative populations of these levels can be sensitive functions of density. References are given in Mason & Monsignori Fossi (1994), Dwivedi (1994) and Keenan (1996). Monsignori Fossi & Landini (1994c) assessed the electron excitation data for the C-like ions. Burgess et al. 1991 have critiqued the R-matrix calculations of Aggarwal (1985b) for Mg VII. They find his treatment for the allowed transitions inadequately accounts for the long range potential. Errors on the order of 15% could arise and they point out that there may be similar problems with allowed transitions of other ions in this isoelectronic sequence calculated in a similar manner by Aggarwal.

In addition to the density diagnostic ratios for the coronal lines, the solar flare lines from Fe XXI are of particular interest. The UV line at 1354.1 Å has been studied from Skylab and SMM observations and more recently stellar spectra have been obtained with the Goddard High Resolution Spectrograph on the Hubble Space Telescope (Maran et al. 1994). Highly ionized iron lines have been recorded for several stars with EUVE, and electron density values have been deduced for the stellar atmospheres from the Fe XXI line ratios (Dupree et al. 1993).

The configurations 2s^{2}2p^{2}, 2s2p^{3},
2s^{2}2p3s, 2s^{2}2p3p,
corresponding to 23 fine-structure levels have
been included. The
experimental energy levels of the NIST database
were adopted. The
oscillator strengths and *A* values of Bell
et al. (1995) have been
adopted. Since Bell's work did not provide
radiative transition
probabilities for several transitions, a new
calculation has been
performed using the SSTRUCT package of the
University College of
London. The new calculation included 17
configurations: 2s^{2}2p^{2},
2s2p^{3}, 2p^{4}, 2s^{2}2p3s, 2s^{2}2p3p,
2s^{2}2p3d, 2s2p^{2}3s,
2s2p^{2}3p, 2s2p^{2}3d, 2s^{2}2p4s, 2s^{2}2p4p,
2s^{2}2p4d, 2s^{2}2p4f,
2s2p^{2}4s, 2s2p^{2}4p, 2s2p^{2}4d and
2s2p^{2}4f. The resulting
radiative transition probabilities have been
corrected for the
differences between experimental and theoretical
values of the
transition's wavelengths. The corrected values
have been inserted
where Bell's values were unavailable.

The effective collision strengths for
all transitions are
taken from the R-matrix calculations of
Stafford et al. (1994).
Effective collision strengths are calculated in
the electron
temperature range 5 10^{3} to 1.25 10^{5} K.
Lennon & Burke (1994)
have also recently published collision data for
the C-sequence,
including N II. Some of their values differ from
Stafford et al. by
more than 50%. The problem is that the position
of a large resonance
near threshold dramatically effects the low
temperature
values. The differences between the two sets of
calculations must be
taken as a measure of the uncertainty in the
calculations. The exact
position of such resonances is extremely
difficult to determine.

The adopted atomic model for O III includes 46
fine structure levels
of the 2s^{2}2p^{2}, 2s2p^{3}, 2p^{4},
2s^{2}2p3s, 2s^{2}2p3p, and
2s^{2}2p3d configurations. Experimental energies
for all levels come
from the NIST database and have been used to
calculate transition
wavelengths. Oscillator strengths and *A* values
for all possible
transitions between the 46 energy levels are
taken from Bhatia &
Kastner (1993a). For transitions
between the levels of the ground
configuration (first 5 levels) the R-Matrix
Maxwellian averaged
collision strengths of Lennon & Burke
(1994) were adopted. They report
values in the electron temperature range
10^{3}-10^{5} K. Effective
collision strengths for optically allowed
transitions between the
levels belonging to 2s^{2}2p^{2} and 2s2p^{3},
2p^{4} configurations
have been calculated by Aggarwal
(1985a) for temperatures between 5
10^{3} and 5 10^{5} K. Collision strengths for
the remaining
forbidden and intercombination transitions among
these levels are
provided by Aggarwal (1983) for
temperatures between 2.5 10^{3} and 6
10^{5} K. Both calculations have been performed
using the R-Matrix
method. A comparison between the Maxwellian
averaged collision
strengths from Lennon & Burke
(1994) and
Aggarwal (1983) for transitions
between the ground configuration levels shows no
significant

differences. Collision
strengths for transitions to
the *n*=3 levels are taken from the distorted
wave calculations of
Bhatia
& Kastner (1993a). Collision strengths
are reported for 5 values of
incident electron energy at 4, 6, 8, 10 and 12
Rydberg.

The adopted atomic model for Ne V includes 6
configurations,
2s^{2}2p^{2}, 2s2p^{3}, 2s^{2}2p3s, 2p^{4},
2s^{2}2p3p, and 2s^{2}2p3d,
corresponding to 46 fine structure energy levels.
Experimental energy
levels are taken from the NIST database for all
the levels, with the
exceptions of levels 2p^{4} ^{1}D_{2} and
^{1}S_{0}, which come from
Edlen (1985). *A* values and
oscillator strengths come from
Bhatia &
Doschek (1993b), and are provided for
all the possible transitions
between the 46 energy levels.

Maxwellian averaged collision strengths for
transitions between the
ground levels (configuration 2s^{2}2p^{2} and
2s2p^{3} ^{5}S_{2}) are
taken from Lennon & Burke (1994).
Collision strengths for all other
the transitions come from the distorted wave
calculations of Bhatia &
Doschek (1993b). Collision strengths are
provided for 3 values of
incident electron energies at 10, 15 and 20
Rydberg. Collision
strengths have also been calculated using the
R-Matrix by Aggarwal
(1985b, 1986) for allowed transitions
and by Aggarwal (1984) for
forbidden and
intercombination transitions.

The configurations 2s^{2}2p^{2}, 2s2p^{3},
2s^{2}2p3s, 2p^{4},
2s^{2}2p3p, 2s^{2}2p3d, corresponding to 46
fine-structure levels, were
included in the atomic models for these 2 ions.
Values for the
observed energy levels were taken from the NIST
database. Transition
wavelengths were calculated using observed energy
levels, but where
these values were not available theoretical
energy levels were used.
Theoretical energy levels, oscillator strengths,
collision strengths
and spontaneous transition probabilities for
these ions are taken from
the calculations of Bhatia & Doschek
(1993c, 1995b) for Si IX and Mg
VII respectively. Collision strengths were
calculated using the
distorted wave approximation for three values of
the incident electron
energy in the range 12-36 Rydberg (Mg VII) and
20-60 Rydberg (Si IX).
For these ions, the incident energies are fairly
high above threshold
and closely spaced in energy. This presents a
problem in extrapolating
the collision strengths over the complete energy
range from threshold
to infinite energy.

R-matrix calculations of allowed transitions in Mg VII and Si IX have been performed by Aggarwal (1985b, 1986) but may be somewhat inaccurate as discussed by Burgess et al. (1991).

The adopted atomic model for S XI includes the
configurations
2s^{2}2p^{2}, 2s2p^{3}, 2p^{4}, 2s^{2}2p3s,
2s^{2}2p3p and 2s^{2}2p3d,
corresponding to 46 fine structure energy levels.
Experimental
energies were taken from the NIST database while
the theoretical
energies for all the levels come from
Bhatia & Kastner (1987).
Wavelengths have been calculated with the
experimental energy levels
but where these values were not available the
theoretical energies have
been used. Radiative transition probabilities
and oscillator strengths
are reported by Bhatia & Kastner
(1987) for all possible transitions
between the levels of the adopted atomic model.

Collision strengths for transitions between all
the 46 levels are
available from Bhatia & Kastner
(1987). In this paper collision
strengths
are calculated for only one value of the incident
electron energy at 25
Rydberg. The calculation of the
Maxwellian-averaged collision
strengths is then very arbitrary. For this
reason collision strengths
for all transitions among the ground levels
(2s^{2}2p^{2}
configuration) and between the ground
configuration and the 2s2p^{3}
configuration have been taken from Mason &
Bhatia (1978). In this
paper collision strengths are reported for 3
values of incident
electron energy, and the calculation of the
Maxwellian-averaged
collision strengths is better defined. For the
remaining transitions,
the effective collision strengths have been
calculated using the single
energy collision strengths. For these transitions
we have artificially
introduced the values
at threshold and for
so that they are similar to energy
dependence of the
same transition for other members of the
isoelectronic sequence. A
comparison between the effective collision
strengths of Mason &
Bhatia (1978) and those of Bhatia
& Kastner (1987) show differences of
up to 30%.

The adopted atomic model for Ar XIII includes the
configurations
2s^{2}2p^{2} and 2s2p^{3}, corresponding to 15
fine structure energy
levels. Experimental energies come from the NIST
database and have
been used to calculate transition wavelengths.
Theoretical energy
levels, *A* values, oscillator strengths and
collision strengths for all
the possible transitions have been taken from
Dere et al. (1979).
Collision strengths are calculated using the
distorted wave
approximation, including the configurations
2s^{2}2p^{2}, 2s2p^{3}
and 2p^{4}, for 3 values of the incident electron
energy at 15, 30
and 45 Rydberg.

The adopted atomic model for Ca XV includes the
configurations
2s^{2}2p^{2} and 2s2p^{3}, corresponding to 15
fine structure energy
levels. Experimental energies come from the NIST
database and have
been used to calculate transition wavelengths.
Oscillator strengths
and *A* values are from the calculations of
Froese-Fischer & Saha (1985).

For the Ca XV 2s^{2}2p^{2} and 2s2p^{3}
configurations, we use the
R-matrix collision strengths of Aggarwal et
al. (1991a). We have also
considered distorted wave calculations for this
ion (Bhatia & Doschek
1993a; Dere et al. 1979).
For the transitions within the *n*=2
configurations, it was not possible to obtain
satisfactory fits to the
Bhatia & Doschek (1993a)
collision strengths because the incident
energies are too closely bunched and too high
above threshold. In
these cases, extrapolation of the collision
strength values to
threshold and the high energy limit (nondipole)
is very uncertain.
Dere et al. (1979) published
collision strengths for Ca XV at lower
energies. Although most of their values matched
those of Bhatia &
Doschek at 45 Ryd, a few transitions differed.
The probable reason is
that Dere et al. did not carry out the DW
calculations to partial wave
values as high as Bhatia and Doschek. The Dere et
al. values at 45 Ryd
may not have fully converged. Consequently, it
was not possible to
construct a consistent set of distorted wave
collision strengths that
spanned a sufficient range in energy. In
addition, the resonance
contribution is significant for the forbidden
transitions within the
ground configuration. Indeed, for some of these
transitions, the
's at 3 10^{6} derived from
Bhatia & Doschek (1993a)
differ by more than a factor 2 from Aggarwal's
values.

For Fe XXI, 36 fine-structure levels in the
configurations
2s^{2}2p^{2}, 2s2p^{3}, 2s^{2}2p3s, 2s^{2}2p3d,
2p^{4} were included.
The observed energies are mostly from the NIST
database and
supplemented by a few from Mason et al.
(1979). Oscillator strengths
and *A* values have been computed using the
SSTRUCT package. The
adopted atomic model includes 17 configurations:
2s^{2}2p^{2},
2s2p^{3}, 2p^{4}, 2s^{2}2p3s, 2s^{2}2p3p,
2s^{2}2p3d, 2s2p^{2}3s,
2s2p^{2}3p, 2s2p^{2}3d, 2s^{2}2p4s, 2s^{2}2p4p,
2s^{2}2p4d, 2s^{2}2p4f,
2s2p^{2}4s, 2s2p^{2}4p, 2s2p^{2}4d and
2s2p^{2}4f. The calculated
radiative transition probabilities have been
corrected for the
differences between experimental and theoretical
values of each
transition's wavelength.

Collision strengths from Aggarwal
(1991) have been adopted for
transitions involving the 2s^{2}2p^{2}, 2s2p^{3}
2p^{4}
configurations. They have been calculated using
the close-coupling
method for 20 values of electron energy in the
range 20-300 Rydberg.
Collision strengths for transitions from the
ground configuration to
2s^{2}2p3s and 2s^{2}2p3d configuration come from
Mason et al. (1979).
They are calculated with the distorted wave
method for 3 values of
electron energy in the range 20-100 Rydberg.
The energies specified for
the incident electron cover a sufficiently large
range, at least for
the *n*=2 excitations, that the Maxwellian
integral is well determined.
For several transitions, a comparison with the
R-matrix calculations of
Aggarwal (1991) was made and
generally the agreement was found to be
quite good. It is noted that electron collision
rates to the *n*=4
levels have recently been published by
Phillips et al. (1996). These
will be included in future releases of CHIANTI.

The nitrogen isoelectronic sequence provides some interesting diagnostics for coronal ions, cf. Bhatia & Mason (1980a) and recent work by Dwivedi, Mohan and colleagues referenced in Dwivedi (1994). Kato (1994) has reviewed the existing data for the nitrogen isoelectronic sequence. She found that for several important ions of this sequence very few data are available (Ne IV to Mn XIX), while O II and Fe XX have been studied quite extensively. Most of the existing calculations have been carried out using the distorted wave approximation.

The adopted atomic model for O II includes the
configurations
2s^{2}2p^{3}, 2s2p^{4} and 2p^{5}, corresponding
to 15 fine-structure
energy levels. The experimental energies are
taken from the NIST
database. These energies were used to calculate
the transition
wavelengths. Since no experimental energy values
were available for the
configuration 2p^{5}, wavelengths for transitions
originated from
the 2p^{5} levels were computed using theoretical
energies.

Maxwellian averaged collision strengths are taken
from
McLaughlin &
Bell (1994). Effective collision
strengths are provided for all
transitions between 11 LS levels (corresponding
to configurations
2s^{2}2p^{3}, 2s2p^{4}, and 2s^{2}2p^{2}3s). The
collisional data were
calculated using the R-Matrix method and fitted
with a Chebyshev
polynomial expansion and tabulated. Effective
collision strengths
among the fine structure levels have been
obtained from the reported LS
coupling values by scaling according to the
statistical weights of the
fine-structure levels involved in each
transition. Levels in the
2s^{2}2p^{2}3s configuration have not been
included since radiative
data for the 2s^{2}2p^{2}3s levels were not
available. These levels
will be included in future releases of CHIANTI.

For transitions involving the 2p^{5}
configurations and for transitions
between the levels of each multiplet, collision
strengths have been
taken from the unpublished distorted wave
calculations Bhatia (1996),
provided for 3 values of incident electron energy
at 4, 8 and 12
Rydberg. Since collision strengths for all other
configurations are
provided, a comparison has been carried out
between these data and
those of McLaughlin & Bell (1994).
The two sets of data show
significant differences both in the allowed and
in the forbidden
transitions.

The adopted atomic model for Ne IV includes 3
configurations:
2s^{2}2p^{3}, 2s2p^{4} and 2p^{5}, corresponding
to 15 fine-structure
energy levels. The experimental energies are
taken from the NIST
database and are used to calculate the
wavelengths.

The radiative data and collision strengths have been taken from unpublished distorted wave calculations of Bhatia (1996). Collision strengths are calculated using the distorted wave method for 3 values of electron energy at 5, 10 and 15 Rydberg.

The atomic model includes two configurations:
2s^{2}2p^{3} and
2s2p^{4}, corresponding to 13 fine-structure
levels. Energy levels and
transition wavelengths have been calculated using
the experimental
energy levels of the NIST database.

Radiative transition probabilities and
collision

strengths for all
transitions within 2s^{2}2p^{3} and between the
2s^{2}2p^{3} and
2s2p^{4} configurations are included and are from
the calculations of
Bhatia & Mason (1980a). The
scattering calculations have been
carried out using the configurations
2s^{2}2p^{3}, 2s2p^{4} and 2p^{5},
with the distorted wave method, and collision
strengths are computed
for 3 values of the electron energy between 10
and 45 Rydberg.

The configurations included in the atomic model
are 2s^{2}2p^{3} and
2s2p^{4}, corresponding to 13 fine-structure
energy levels. The
relevant atomic data for Fe XX are taken from
Bhatia & Mason
(1980b). Both theoretical and
experimental energy levels are provided
by the authors. Transition wavelengths have been
calculated using the
experimental values. Collision strengths have
been calculated using
the distorted wave method for 3 values of
electron energy: 20, 50 and
100 Rydberg.

Spectroscopic diagnostics for the oxygen-like
transition region and
coronal ions were explored by Raju &
Dwivedi (1978).
The solar flare lines from Fe XIX have been
studied in XUV
(Loulergue
et al. 1985) and X-ray

(Bhatia et al. 1989)
spectra.

The oxygen isoelectronic sequence has not been
treated extensively in
the literature. The only close-coupling
calculations are those of
Butler & Zeippen (1994) who have
calculated thermal-averaged
collision strengths for transitions between the
levels within the
ground configuration of ions from F II to Ar XI,
for electron
temperatures ranging from 10^{3} to 10^{5} K.
Distorted wave
calculations have been performed for all the most
abundant ions,
including transitions between the ground
configuration and excited
ones. A comparison between distorted wave and
close-coupling collision
strengths for the lighter ions shows a generally
acceptable agreement.
Only S IX shows some differences between the two
different methods,
where the close coupling data sometimes show
greater thermal-averaged
collision strengths at low temperatures. Since
the maximum abundance
temperature for oxygen-like ions with *Z*
16 is 10^{6} K
we have adopted distorted wave calculations also
for the transitions in
the ground configuration for Si VII and above.

The case of Ne III and Mg V is different. The
electron temperature for
the maximum abundance for these ions falls in the
range
10^{5}-10^{5.5} K and it is important to include
the close-coupling
effective collision strengths since the effects
of resonances are
important.

The main deficiency of the adopted distorted wave collision strengths is that they have been calculated only for three values of the incident electron energy near threshold. Consequently, the behavior of the non-dipole collisions strengths at high energy is not well determined.

For Ne III and Mg V, the 10 fine-structure levels
belonging to the
2s^{2}2p^{4}, 2s2p^{5} and 2p^{6} configurations
have been included.
Observed energy levels are taken from the NIST
database and they are
used to calculate the transition wavelengths.
The radiative data are
from the unpublished calculations of Bhatia
(1996) which also provide
collision strengths for all the transitions
between these 10 levels.
Distorted wave collision strengths for Ne III are
computed for three
values of the incident electron energy in the
range 5-15 Rydberg, and
for Mg V, 4 values of electron energy are
available, in the range 10-30
Rydberg.

Butler & Zeippen (1994) report
close-coupling effective collision
strengths for the ground configuration
transitions. These effective
collision strengths are provided for 11 values of
the electron
temperature ranging from 10^{3} to 10^{5} K.
This temperature range is
insufficient for an accurate extrapolation to
high energy. The
collision strengths provided by Bhatia do allow a
good determination of
the effective collision strengths in the
temperature range 10^{6} -
5 10^{6} K. Thus the two different sets of data
have been merged
and the resulting effective collision strengths
have been inserted in
the CHIANTI database. No significant
inconsistencies between the two
different sets of data were found.

Bhatia's collision strengths have been adopted for all other transitions.

The atomic model includes the 2s^{2}2p^{4},
2s2p^{5} and
2p^{6} configurations, corresponding to 10 fine
structure energy

levels.

Experimental energy levels from the NIST database
have been adopted.
These values have been used to calculate
all the transition
wavelengths. The experimental energy of the
2p^{6} ^{1}S_{0} level of
Ar XI is unknown and all the wavelengths of the
transitions involving
this level have been computed using theoretical
energies. Theoretical
energy levels, radiative transition probabilities
and collision
strengths for Si VII, S IX and Ar XI have been
provided by Bhatia et al.
(1979).

The scattering problem has been solved using the distorted wave approximation. Collision strengths have been calculated for 3 values of electron energy: 10, 15, 20 Rydberg for Si VII and S IX, and 15, 20, 25 Rydberg for Ar XI. Again, this is a fairly restricted range in energy for determining the energy dependence of the collision strengths.

The atomic model of Ca XIII includes 4
configurations: 2p^{4},
2s2p^{5}, 2p^{6} and 2p^{3}3s, corresponding to
20 fine-structure
energy levels. The experimental energies of the
NIST database have
been adopted, but the values for the levels
2p^{3}3s ^{5}S_{2} and
^{3}P_{0,1,2} are unknown. Transition
wavelengths have been
calculated using the experimental energy levels,
but if these values
were not available, the theoretical ones have
been used. The
theoretical energy levels, radiative transition
probabilities and
collision strengths are taken from the
unpublished calculations of
Bhatia (1996).

The collision strengths were computed using the
distorted wave
approximation. They have been calculated for 5
values of the electron
energy in the range between 40 and 60 Rydberg.
The collisional data are
available for all transition between and within
the listed
configurations. These data agree well with
Mason (1975) for the *n*=2
transitions.

Baliyan & Bhatia (1994) published R-matrix collision data for Ca XIII with a more extensive set of target configurations. Unfortunately, their results are in LS coupling and they do not tabulate the 's, so we have not used them in the CHIANTI database. However, they have carried out a very careful comparison of the R-matrix and distorted wave results, giving particular attention to the partial wave convergence. In general, they found good agreement (better than 30%) with the Bhatia distorted wave results, although they suggest that the target wavefunctions could be improved. They also found the resonance structure to be significant for low energies.

The adopted atomic model includes 2s^{2}2p^{4},
2s2p5 and 2p^{6}
configurations, corresponding to 10
fine-structure energy levels.
Theoretical energy levels, radiative transition
probabilities and
collision strengths come from Loulergue et
al. (1985), who also provide
the experimental energy levels. The latter
energies are used to
calculate the transition wavelengths.

The authors show that for calculating the radiative transition probabilities, there is no significant improvement in extending this basis to higher configurations than those listed above. The collision strengths are calculated using the distorted wave method for 3 values of incident electron energy, 22.5, 45 and 90 Rydberg.

For the fluorine sequence ions Ne II through Ca
XII, only the two
lowest configurations having a total of 3 fine
structure levels have
been included: 2s^{2}2p^{5} ^{2}P_{3/2,1/2}
and 2s2p^{6}
^{2}S_{1/2}. The experimental energy levels
are from
Kelly (1987)
for Ne II and from the NIST database
(Martin et al. 1995) for the
others. *A* values between the ground
configuration levels are from
Martin et al. (1995) and from
Blackford & Hibbert (1994) for
the
others. Collision strengths between the 2 ground
configuration levels
are from the calculations of Saraph &
Tully (1994). Sources for
collision strength to the 2s2p^{6} level are: Ne
II (Bhatia 1996,
unpublished), Mg IV (Mohan et al.
1988), Si VI
(Mohan & Le Dourneuf
1990), S VIII
(Mohan et al. 1987) and Ca XII
(Mason 1975).

The 113 fine structure levels of the
2s^{2}2p^{5}, 2s2p^{6},
2s^{2}2p^{4}3*l*, 2s2p^{5}3*l* and
2p^{6}3*l*
configurations are included. Observed energy
levels have been taken
from the NIST database (Martin et al.
1995). The primary source of
oscillator strengths and *A* values were the
calculations of
Sampson
et al. (1991). Additional radiative
rates were taken from
Blackford
& Hibbert (1994) and Cornille et
al. (1992).
Sampson et al. (1991)
have calculated relativistic distorted-wave
collision strengths for
ions with *Z* 22 and have been incorporated
into the CHIANTI
database. These calculations are generally in
good agreement with the
distorted wave calculations of Cornille et
al. (1992) for Fe XVIII.

The adopted atomic model includes 4
configurations: 2s^{2}2p^{6},
2s^{2}2p^{5}3s, 2s^{2}2p^{5}3p and
2s^{2}2p^{5}3d, corresponding to 27
fine-structure levels. The experimental energy
levels are taken from
the NIST database and are used to calculate all
the transition
wavelengths. Radiative transition probabilities
and collision
strengths come from the calculations of
Bhatia et al. (1985). Radiative
data are available for all transition between the
listed
configurations.

Collision strengths were calculated using the distorted wave method for only one value of the incident electron energy, 15 Rydberg. For dipole transitions, a second energy point is available in the Bethe approximation high energy limit so that the integration over a Maxwellian velocity distribution can be performed. For the nondipole transitions, the variation of the collision strength with energy has been estimated from the same transition in Fe XVII, either by setting the collision strength to be constant with energy or to go to zero at high energies.

The adopted atomic model for S VII includes four
configurations:
2s^{2}2p^{6}, 2s^{2}2p^{5}3s, 2s^{2}2p^{5}3p and
2s^{2}2p^{5}3d
corresponding to 27 fine-structure levels. The
adopted experimental
energy levels are taken from the NIST database
and they are used to
calculate all the transition wavelengths. The
theoretical energy
levels, *A* values and oscillator strengths are
taken from
Hibbert
et al. (1993). Radiative data are
calculated for transitions to the
ground configuration, between 2s^{2}2p^{5}3s and
2s^{2}2p^{5}3p,
between 2s^{2}2p^{5}3p and 2s^{2}2p^{5}3d. *A*
values for the forbidden
transitions 2s^{2}2p^{6} ^{1}S_{0} -
2s^{2}2p^{5}3s ^{3}P_{2} and 2s^{2}2p^{5}3s
^{3}P_{1} - ^{3}P_{0} have been extrapolated
from the corresponding
values of the other ions of the neon
isoelectronic sequence.

Thermal-averaged collision strengths are taken
from Mohan
et al. (1990). The collision strengths
are calculated using the close
coupling method for all transitions from the
ground level to
2s^{2}2p^{5}3s, 2s^{2}2p^{5}3p and 2s^{2}2p^{5}3d
configurations. The
data are tabulated for 20 values of electron
temperature ranging from 5
10^{4} to 10^{6} K. These data are calculated
in LS coupling.
Since the ground level consists of a single
level, fine structure
thermal-averaged collision strengths for
transitions involving
multiplets have been distributed according to
their statistical
weights.

The included configurations are 2s^{2}2p^{5}*nl*
and 2s2p^{6}*nl* with
*n*=3, 4 and *l*=0, 1, 2, 3. This atomic model
includes 89 fine structure
levels. The experimental energy levels are taken
from the NIST
database. These values have been used to
calculate transition
wavelengths, but where experimental energy levels
were lacking (Ca XI
and Ni XIX), theoretical values of Zhang et
al. (1987) have been used.

Zhang et al. (1987) also calculated
radiative transition probabilities
for electric dipole transitions between the
ground level and the
excited levels. These rates provide decay rates
for only a few (14) of
the upper levels. Additional oscillator
strengths and *A* values have
been taken from Hibbert et al.
(1993). Nevertheless for the majority
of energy levels belonging to the higher energy
configurations
2s^{2}2p^{5}4*l* and 2s2p^{6}*nl*, radiative data are
not available.
Consequently, it is not possible to construct a
model of the
statistical level populations that include these
levels and so they
have been omitted from the adopted atomic model.
Since these levels
have very high energy and they are not metastable
we do not expect any
significant change in the level populations and
line intensities in the
synthetic spectra of these ions. The adopted
atomic model includes all
levels of configurations 2s^{2}2p^{6},
2s^{2}2p^{5}3s, 2s^{2}2p^{5}3p
and 2s^{2}2p^{5}3d, and the levels 2s2p^{6}3p
^{3}P_{1}, ^{1}P_{1},
2s^{2}2p^{5}4s ^{3}P_{1} and ^{1}P_{1},
2s^{2}2p^{5}4d ^{3}P_{1},
^{3}D_{1}, ^{1}P_{1}, 2s2p^{6}4p ^{3}P_{1} and
^{1}P_{1}. The total
number of energy levels is 36.

Collisional data are taken from Zhang et al. (1987) who employed a Coulomb-Born-exchange method. The collision strengths are calculated for 9 values of the incident electron energy in threshold units in the range 1-15, allowing a straightforward determination of the collision strength values from threshold to the high energy limit. Collisional data are available for all transitions connecting the ground level and the 88 excited levels. Collision strengths for transitions involving the omitted levels have not been inserted in the database, though they are available in CHIANTI format.

The adopted atomic model includes seven
configurations: 2s^{2}2p^{6},
2s^{2}2p^{5}3s, 2s^{2}2p^{5}3p, 2s^{2}2p^{5}3d,
2s2p^{6}3s, 2s2p^{6}3p,
2s2p^{6}3d corresponding to 37 fine structure
energy levels. The
experimental energy levels for the lowest 26
levels are from Bhatia &
Doschek (1992) and the rest are from the
NIST database. It should be
noted that Bhatia & Doschek preserve the term
designations of the
lower *Z* members of the sequence. The
experimental energies have been
used to calculate the transition wavelengths.
Where no experimental
energy values were available, theoretical ones
have been used.
Theoretical energy values, *A* values, oscillator
strengths and collision
strengths are provided by Bhatia & Doschek
(1992) for all
transitions between the 37 levels of the adopted
atomic model.
Distorted wave collision strengths are calculated
at five incident
electron energies in the range 77-254 Rydberg.
These collision
strengths can be compared those from a similar
calculations by
Cornille
et al. (1994).

The 21 fine structure levels of the 3,
4, and 5 configurations have been included. The
observed energy levels are
from the NIST database (Martin et al.
1995). Oscillator strengths and
*A* values are from Sampson et al.
(1990) except for the
transitions where the hydrogenic values of
Wiese et al. (1966) are
used.

Relativistic distorted wave collision strengths
have been provided by
Sampson et al. (1990) over a wide
energy range and are used in the
CHIANTI database.

Cornille et
al. (1997) have calculated distorted
wave results for Fe XVI (*n*=3, 4 and 5). They
discuss the importance of
high partial wave contributions, even for the
electric quadrupole
transitions. Their values agree well with those
of
Sampson et al.
(1990). Tayal (1994)
published R-matrix values for the *n*=3, 4
configurations. He included the resonance
contribution, which is
particularly important at lower temperatures (<
10^{6} K). New R-matrix
calculations are in progress by Eissner et
al. (1996) as part of the
Iron Project. They find that some collision rates
published by Tayal
(1994) are very sensitive to the position
of large resonances near
threshold.

The adopted atomic model for Si III includes the
configurations 3s^{2},
3s3p, 3p^{2}, 3s3d, 3s4s and 3s4p, corresponding
to 20 fine structure
energy levels. The experimental energy levels
are taken from the NIST
database and have been used to calculate the
transition wavelengths.
The theoretical energy levels come from
Baluja & Hibbert (1980).
Oscillator strengths and *A* values come from
Dufton et al. (1983a).
Since only a few transitions are treated in that
paper, we have been
performed new calculations for all the possible
transition in the
adopted atomic model. The calculation has been
carried out with
SSTRUCT, including the configurations 3s^{2},
3s3p, 3p^{2}, 3s3d, 3s4s,
3s4p, 3s4d, 3s4f, 3p3d, 3d^{2}, 3p4s, 3p4p, 3p4d
and 3p4f. The
resulting *A* values and oscillator strengths
have been adopted for all
the transition for which values were not provided
by Dufton
et al. (1983a). A comparison between SSTRUCT
results and Dufton
et al. (1983a) radiative transition probabilities
shows no significant
differences.

Adopted collision data come from Dufton &
Kingston (1989). They
provide coefficients for least-squares polynomial
fits for thermal
averaged collision strengths between the 20
lowest states
(corresponding to the 20 fine-structure levels
adopted by CHIANTI).
Thermal averaged collision strengths have been
calculated with the
R-matrix program for the temperature interval
between 10^{3.8} and
10^{5.2} K, for which the Si III ionization
fraction is significant.
Fine structure transitions between triplets were
reported, while
transitions between singlets and triplets are
presented in LS coupling.
The authors show that the values for fine
structure transitions between
singlets and triplets may be obtained by scaling
the collisional data
by the appropriate degeneracies which we have
done here.

The adopted atomic model for S V, Ar VII, Ca IX
and Ni XVII includes
the configurations 3s^{2}, 3s3p, 3p^{2}, 3s3d and
3s4s, corresponding
to 16 fine-structure levels. The experimental
energy levels are taken
the NIST database. Experimental energy levels
have been used to
calculate the transition wavelengths. Where no
experimental values were
available, the theoretical ones have been used.
Theoretical energy
levels, *A* values, oscillator strengths and
collisional data are taken
from Christensen et al. (1986).
Radiative transition probabilities are
available for electric dipole transitions between
3s^{2} and 3s3p
configurations, and between 3s3p and 3p^{2}, 3s3d
and 3s4s
configurations. Collision strengths are
calculated with the distorted
wave method for three incident electron energies
for all possible
transition between the 16 fine-structure levels
of the adopted atomic
model.

The atomic model for Fe XV includes
configurations 3s^{2}, 3s3p,
3p^{2}, 3s3d, 3p3d, 3s4s, 3s4p, 3s4d, 3p4s, 3p4d,
3p4f.
The radiative data and electron scattering data
are taken from Bhatia et al.
(1997). The collision strengths,
calculated in the distorted wave approximation,
are given for 3
energies, 25, 50 and 75 Rydbergs. Fits were made
from the levels in
the 3s^{2} and 3s3p configurations up to all the
other configurations.
One limitation of these calculations was the
omission of the 3d^{2}
configuration. New calculations for the *n*=3
configurations have just
been completed (Bhatia & Mason 1997). These are
in good agreement
with the earlier distorted wave results by
Christensen et al. (1985).
Dufton et al. (1990) compared their
R-matrix results with Christensen
et al. and also found good agreement (better than
20%).
They did not publish their collision data and it
is not available.
Eissner et al. (1996) are carrying
out new R-matrix calculations for
Fe XV as part of the Iron Project. They find
that, contrary to
Dufton et al. 's conclusions, the resonance
structure is important
for some of the Fe XV transitions and could make
a significant
difference to the excitation rates.

The atomic model for Si II includes five
configurations, 3s^{2}3p,
3s3p^{2}, 3s^{2}3d, 3s^{2}4s and 3s^{2}4p,
corresponding to 15
fine structure levels. The experimental energy
levels are taken from the
NIST database and are used for calculating the
transition wavelengths.
Experimental energies are available for all
levels. Electric dipole
allowed transitions are taken from
Lanzafame (1994). Radiative
transition probabilities for forbidden and
intercombination transitions
are obtained with SSTRUCT. The following
configurations have been
included in the calculation: 3s^{2}3p, 3s3p^{2},
3s^{2}3d, 3s^{2}4s,
3s^{2}4p, 3s^{2}4d, 3s^{2}4f, 3p^{3}, 3s3p3d,
3s3d^{2}, 3p3d^{2},
3d^{3}, 3p^{2}3d, 3s3p4s, 3s3p4p, 3s3p4d and
3s3p4f. These radiative
transition probabilities have been calculated
using the theoretical
energy levels but have been corrected to take
into account the
experimental energies.

Thermal averaged collision strengths are taken
from the R-matrix
calculations of Dufton & Kingston
(1991a). Thermal averaged
collision strengths are tabulated for 6 values of
the electron
temperature ranging from 10^{3.6} to
10^{4.6} K.

The adopted atomic model for S IV includes the 3
configurations
3s^{2}3p, 3s3p^{2} and 3s^{2}3d, corresponding to
12 fine-structure
energy levels. Experimental energies were
available from the NIST
database for all the included levels, and have
been used to calculate
transition wavelengths. Oscillator strengths
and *A* values are mainly
taken from Bhatia et al. (1980),
who provide values for transitions
between all the levels of the adopted atomic
model. Radiative decay
rates for the intercombination lines between
levels 3s^{2}3p ^{2}P and
3s3p^{2} ^{4}P have been taken from

Dufton et al. (1982).

Collisional data come from 3 sources. For
transitions between
^{2}P and 3s3p^{2} ^{4}P, collision
strengths
have been taken from the R-matrix calculations of
Dufton et al. (1982). They present
Maxwellian-averaged collision strengths in
the electron temperature range 10^{4} to
10^{5.6} K. A comparison
between these results and the close-coupling
calculations of
Bhadra &
Henry (1980) and the distorted wave
values of
Bhatia et al. (1980) has
shown that the Dufton values are larger than the
others at low
temperatures, due to scattering resonances. The
main difference between
Dufton's calculations and Bhadra's and Bhatia's
is that the former
explicitly delineates the resonances in the
collision strengths.
Collision strengths for transitions from 3s^{2}3p
^{2}P and
3s3p^{2} ^{4}P to the remaining levels of the
3s3p^{2} configuration
come from Bhadra & Henry (1980). They provide
fine structure
collision strengths for five values of the
incident electron energy in
the range 1.3 to 6 Rydberg, calculated with the
close-coupling method.
For transitions involving the 3s^{2}3d
configuration, the Bhatia
et al. (1980) collision strengths have been
adopted. They are
calculated with the distorted wave method for
three values of incident
electron energy in the range 2 to 6 Rydberg.

The model for Fe XIV includes 12 fine structure levels of the , and configurations. The observed energy levels are from the NIST database. Transition probabilities are taken from Froese Fischer & Liu (1986).

The R-matrix calculations of Dufton &
Kingston (1991b) give upsilons
for all transitions from the ground 3s^{2}3p
configuration to the
3s3p^{2} and 3s^{2}3d configurations and are used
here. No
significant differences were found between this
work, the earlier work
by Mason (1975) and the more recent
work of
Bhatia & Kastner (1993b).
Storey et al. (1996) give upsilons
for the ground
^{2}P_{1/2} - ^{2}P_{3/2} transition
calculated using the R-matrix
codes. Only the upsilons above 10^{4} K were
fitted as the simple 5
point spline could not be applied accurately to
the complete set of
upsilons.

Although the Storey et al. upsilons were around
50% greater than the
Dufton & Kingston upsilons, this had little
effect on the level
balance as the dominant process populating the
^{2}P_{3/2} level is
cascades from upper levels.

The model of the Fe XIII ion includes 27 fine
structure levels belonging
to the 3s^{2}3p^{2}, 3s3p^{3} and 3s^{2}3p3d
configurations. The
observed energy levels are from the NIST
database. A 24 configuration
model of Fe XIII was used in SSTRUCT to generate
transition
probabilities. Most importantly, transition
probabilities were derived
to de-populate the metastable 3s^{2}3p3d
^{3}F_{4} level, which
accounts for some 20% of the level balance at
densities of around
10^{12}-10^{13} cm^{-3}. In the work of
Brickhouse
et al. (1995), this level was omitted as
there existed no data in the
literature to de-populate the level, and explains
the discrepancies
between the CHIANTI results and theirs.

The distorted wave calculations of Fawcett
& Mason (1989) were used in
favor of the more recent R-matrix calculations of
Tayal (1995),
principally because the R-matrix results differed
for some transitions
(e.g., 3-13, 4-14) by a factor of 2, the
reasons for which were not
adequately explained. Also, upsilons were not
published by Tayal for
transitions up to the 3s^{2}3p3d configuration or
for the 3s3p^{3}
^{5}S_{2} level. It is to be noted that
Fawcett & Mason (1989)
provide only collision strengths for transitions
out of the ground
configuration and so the effects of electron
collisional excitation out
of the metastable ^{3}F_{4} level have not been
included.

As with Fe XIII, the model of the Ni XV ion
includes 27 fine structure
levels belonging to the 3s^{2}3p^{2}, 3s3p^{3}
and 3s^{2}3p3d
configurations. Experimental energies were
available for only 17 of
the 27 levels and so theoretical values were used
for the remaining
levels.

Accurate transition probabilities were calculated
using SSTRUCT with a 35
configuration model. Again, we find that magnetic
quadrupole
transitions are significant in helping to
depopulate the metastable
3s3p^{2}3d ^{3}F_{4} level. We also note that
there are no collision
strengths available that might allow this level
to be de-populated by
electron collisional processes to configurations
other than 3s^{2}3p^{2},
which may affect the level population
significantly.

The distorted wave calculations of Mason
(1996b) are used to
provide collision strengths at one energy (10
Ryd) for all of the
3s-3p and 3p-3d transitions. Four
configurations were included in
these calculations--3s^{2}3p^{2}, 3s3p^{3},
3s3p^{2}3d and
3p^{4}--but collision strengths were not
calculated for transitions to
or from 3p^{4}. For dipole

transitions, the single collision strength
and the value of the high energy limit derived
from the oscillator
strength define the energy dependence of the
collision strength. For
other transitions, the collision strength is
assumed to be constant
with energy.

Forbidden lines of S II such as 6717 and
6731 are
observed in planetary nebulae and their intensity
ratio provides a good
indicator of electron density for values near
10^{3} cm^{-3}
(Osterbrock 1989). Fe XII line
ratios are sensitive diagnostics of electron
density for coronal conditions (Dere
1982).

For S II, 28 fine structure levels of the
3s^{2}3p^{3}, 3s3p^{4},
3s^{2}3p^{2}3d, and 3s^{2}3p^{2}4s configurations
have been included.
Energy levels based on observed spectra are from
the NIST database
(Martin et al. 1995). Sources for
oscillator strengths and *A* values
for S II include Mendoza & Zeippen
(1982),
Ho & Henry (1983), Huang
(1984), Fawcett (1986).
There are large differences among these
various sources for the values of the radiative
constants. In
addition, none provide a radiative decay rate for
the
3s^{2}3p^{2}3d ^{4}F_{9/2} level.
Consequently, we have used a
single set of allowed and forbidden radiative
rates for S II calculated
with the SSTRUCT program (Binello
1996). The effective collision rates
have been calculated by Cai & Pradhan
(1993) using the R-matrix
program.

We note that new R-matrix calculations have
recently been carried out for
S II by Ramsbottom et al. (1996),
including 43 fine-structure levels,
up to the *n*=4 configurations. These will be
assessed in the next
release of CHIANTI.

The model of the Fe XII ion includes 41 fine
structure levels belong to
the 3s^{2}3p^{3}, 3s3p^{4} and 3s^{2}3p^{2}3d
configurations. Energy
levels are from the NIST database and Jupen
et al. (1994). Transition
probabilities were taken from a recent SSTRUCT
run performed by Binello
(1996) using a 24 configuration model for
Fe XII.

No advances over the distorted wave calculations
of Flower (1977) have
been made for the important EUV transitions of Fe
XII. Tayal
et al. (1987) presented R-matrix upsilons
for the transitions within
the ground 3s^{2} 3p^{3} configuration, which we
use to supplement the
Flower data. Tayal & Henry
(1988) published R-matrix collision data
between the 3s^{2}3p^{2} and 3s3p^{3}
configurations. Their collision
strengths agree with Flower's values to within
30%. Some strange
effects in the Tayal et al. and Tayal & Henry
data arise through the
use of pseudo-resonances (see Mason
1994). New calculations of
electron excitation data for Fe XII as part of
the Iron Project
(Binello et al. 1996). We await
these results for
incorporation into the CHIANTI database.

We note that the most important requirements for
this ion are
collision strengths to the 3s^{2}3p^{2}3d
configuration and out of the
metastable levels in that configuration, which
can have substantial
populations even at densities of 10^{8} -
10^{10} cm^{-3}.

The model for Fe XI includes 47 fine structure
levels of the
3s^{2}3p^{4}, 3s3p^{5}, 3s^{2}3p^{3}3d and 3p^{6}
configurations. Most
of the energy levels were taken from the NIST
database and a few from
the work of Jupen et al. (1994).
Observed energies for many levels do
not exist. A 13 configuration model of Fe XI was
used to derive
transition probabilities with SSTRUCT. It was
found that adding the
extra configurations significantly altered the
mixing coefficients of
the 3s^{2}3p^{3}3d ^{3}S_{1}, ^{3}P_{1} and
^{1}P_{1} levels, thus
affecting their oscillator strengths.

New distorted wave calculations by Bhatia
& Doschek (1996) have
finally superseded the work of Mason
(1975). Collision strengths at 3
energies were calculated. In view of the changes
in mixing
coefficients just noted and because there were
significant differences
between SSTRUCT and Bhatia & Doschek for other
transitions, we decided
to rescale *all* the 3s^{2}3p^{4} -
3s^{2}3p^{3}3d collision
strengths by the ratio of SSTRUCT oscillator
strengths to the Bhatia &
Doschek oscillator strengths. One consequence of
this was to increase
the strength of the ^{3}P_{2} - ^{1}P_{1}
transition, and so we
identify this as the 188.30 Å line, rather
than the ^{3}P_{2} -
^{3}S_{1} identification of Jupén et al.
(1994).

The model for Fe X includes 54 fine structure
levels in the
3s^{2}3p^{5}, 3s3p^{6}, 3s^{2}3p^{4}3d and
3s3p^{5}3d configurations.
Energy levels are from the NIST database and from
Jupen et al. (1994).
Experimental energies of many of the 3s3p^{5}3d
levels are still
unknown. Oscillator strengths together with all
transition
probabilities were derived using a 12
configuration model in SSTRUCT.
The configurations used were 3s^{2}3p^{5},
3s3p^{6}, 3s^{2}3p^{4}3d,
3s3p^{5}3d, 3s^{2}3p^{3}3d^{2}, 3s^{2}3p^{4}4s,
3s^{2}3p^{4}4p,
3s^{2}3p, 3s^{2}3p,
3s3p^{4}3d^{2} and .
The transition probabilities were found to be
consistent with those of Fawcett
(1991) to within 5%.

Bhatia & Doschek (1995a)
presented new distorted wave calculations for
Fe X that were an improvement over the results of
Mason (1975);
however, for the 3s^{2}3p^{5} ^{2}P - 3s3p^{6}
^{2}S transitions, the
collision strengths were severely under-estimated
and so, for these
transitions only, the accurate SSTRUCT oscillator
strengths were used
to scale upwards the collision strengths. As
part of the Iron
Project, Pelan & Berrington (1995)
calculated upsilons for the ground
^{2}P_{3/2} - ^{2}P_{1/2} transition using
the R-matrix program.
The inclusion of resonance structure was found to
increase the
excitation rate by a *factor of 10* for low
temperatures, having
significant consequences on the level balance of
the ion, and so we use
the Pelan & Berrington upsilons for this
transition. It is to be
noted that R-matrix calculations have been
performed for Fe X by

Mohan
et al. (1994); however question marks
have been raised over this work
by Pelan & Berrington (1995) and
Foster et al. (1996) and so we
retain the distorted wave calculations for the
present.

For Fe IX, the 13 levels of the 3p^{6} and
3p^{5}3d configurations were
included. Energy levels from the NIST database
were used. A 10
configuration model of Fe IX was used in SSTRUCT
to generate transition
probabilities. The distorted wave calculations
of Fawcett & Mason
(1991) were used to fit and scale the
collision strengths among these
levels. Although collisional data was also given
for the 3p^{5}4s
configuration, we note that this would lead to
two extra metastable
levels (^{3}P_{0} and ^{3}P_{2}). However, it is
evident that the
3s3p^{5}3d and 3s3p^{4}3d^{2} configurations are
energetically lower
than 3p^{5}4s, and so the two levels would
actually decay via these
configurations. The apparent metastable nature of
the two levels is
thus purely a consequence of the neglect of the
3s3p^{5}3d and
3s3p^{4}3d^{2} configurations in the level
balance, and so we ignore
the 3p^{5}4s configuration in this work. Our
*gf* value for the ^{1}S
- ^{1}P transition (which gives rise to the
strong line at
171.07 Å) was found to be 25% lower than
that given by Fawcett &
Mason and so their collision strengths, for this
transition only, were
scaled proportionately.

We note that Burgess et al. (1996) have published and fitted non-exchange DW electron excitation data for Ca II using OMEUPS. These will be included in the next release of CHIANTI.

The present model of Fe VIII includes the
3p^{6}3d, 3p^{6}4p, 3p^{6}4f,
3p^{6}5f, 3p^{6}6f, 3p^{6}7f configurations.
Observed energies for the
12 fine structure levels are from the NIST
database. Oscillator
strengths and *A* values are from Czyzak &
Krueger (1966),
Fawcett
(1989) and NIST. Czyzak & Krueger
(1966) have calculated a number of
collision strengths for Fe VIII using the
Coulomb-Born approximation.
Pindzola et al. (1989) have
calculated both close-coupling and
distorted wave cross sections for the 3d-4s and
3d-4p and 4s-4p. Their
close-coupling and distorted wave cross sections
tend to show good
agreement. The values of Pindzola et al.
collision strengths for the
3d-4p are about a factor of 3-4 lower than
the values calculated by
Czyzak & Krueger. The collision rates of
Pindzola for the 3d-4p
transition have been used and the data of Czyzak
& Krueger for the
others. Calculations of excitation rates to the
3p^{6}3d^{2}
configuration are not available and this is a
major weakness of the
model. In fact, the present values of the
collision strengths appear
to be of limited accuracy.

The 9 fine structure levels of the ground term
3p^{6}3d^{2} have
been included. Energy levels are taken from the
NIST database (Martin
et al. 1995) and the oscillator
strengths and *A* values are those of
Nussbaumer & Storey (1982). All
of the lines involve forbidden
transitions and the shortest wavelength is 1490
Å. Collision
strengths among the 9 levels of the ground term
3p^{6}3d^{2} have been
calculated by Norrington & Grant
(1987) and their averages over a
Maxwellian velocity distribution tabulated by
Keenan & Norrington (1987).
Their values are in good agreement with the
distorted wave values of
Nussbaumer & Storey (1982) but
diverge at higher energies. The
values of Keenan & Norrington
(1987) have been included in the CHIANTI
data base. Lines from this ion provide
potentially useful density
diagnostics for electron densities between 10^{5}
and 10^{8}
cm^{-3}.

Fe II is a complex ion and produces a large number of lines that are observed at infrared, visible and ultraviolet wavelengths in a variety of astrophysical sources. It plays a major role in radiative losses from solar and stellar chromospheres (Anderson & Athay 1989).

The model of Fe II uses 142 levels of the quartet
and sextet terms of
the 3d^{6}4s, 3d^{7} and 3d^{6}4p configurations.
New calculations of
dipole transition probabilities have been made by
Nahar (1995). These
calculations used the energy levels of Fe II from
Johansson (1978) as
well as unpublished values by Johansson to arrive
at a fairly complete
description of the energy levels of Fe II. *A*
values for forbidden
transitions are from the NIST compilation
(Martin et al. 1995).
Recently, Zhang & Pradhan (1995)
have calculated collisions strengths
among the 142 fine structure levels levels and
are all included in the
CHIANTI database. At densities characteristic of
stellar
chromospheres, the lowest 23 levels can have
significant populations.

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