Atomic data for highly charged B-like ions is of considerable interest in the study and modeling of high temperature astrophysical and laboratory plasmas. The spectra from ions in the Boron isoelectronic sequence are observed from a variety of astrophysical objects such as novae, planetary nebulae, Seyfert galaxies, the interstellar medium and the Sun. The spectral lines provide valuable temperature, density and abundance diagnostics in the wavelength range from the IR to the EUV. Of particular interest is the theoretical atomic data for Fe XXII for spectral lines in the EUV likely to be observed from the Solar and Heliospheric Observatory (SOHO) and the Extreme Ultraviolet Explorer (EUVE). Accurate atomic collision data for Fe XXII is also needed in the laboratory plasma diagnostics, particularly tokamaks.
Among highly charged B-like ions, Fe XXII deserves a more careful examination for the following reasons. (1) The relativistic effects, which give rise to fine-structure level splitting and intermediate-coupling type of mixing, are very important and should not be neglected; (2) resonances and the coupling effects are very pronounced and affect the rate coefficients for some transitions considerably (not only resonances due to the n=2 levels but also the n=3 levels, especially for the transitions to the high-lying n=2 levels); and (3) autoionizing resonances may be subject to radiation damping which has not heretofore been taken into account in the calculation of excitation rates.
Many workers have calculated collision strengths and rate coefficients for Fe XXII, mostly in the distorted-wave approximation (see the data review for B-like ions by Sampson etal. 1994). There are also recent fully relativistic distorted-wave (RDW) results by Zhang & Sampson (1994a,b), which did not include the coupling and resonance effects. In a previous work, we have also calculated collision strengths and rate coefficients for the 105 transitions between the n=2 levels in several Boron-like ions, including Fe XXII, published in Paper III of the present series (Zhang etal. 1994). The calculations were done using the R-matrix close-coupling method, with fine structure and the relativistic effects included rather approximately via an algebraic transformation of the scattering matrices using the relativistic term-coupling-coefficients (TCC) for the target ion (Eissner etal. 1974). However, as pointed out by Zhang & Pradhan (1995a) and further demonstrated in Sect. 3, for highly charged ions such as Fe XXII, the TCC approach is not an adequate approximation to the relativistic effects, and furthermore, the radiation damping effect could be important for some transitions. Also, the target expansion in Paper III did not include n=3 levels for the Fe XXII calculation, so that the resonances arising from coupling with these levels were omitted. Therefore, the results in that work for Fe XXII may not be accurate. In the present work, we employed the relativistic Breit-Pauli (BP) R-matrix method, which treats the intermediate coupling effects ab initio, to calculate electron-impact excitation collision strengths and rate coefficients for Fe XXII, using a 45 level target expansion. Radiation damping of autoionizing resonances using the Bell & Seaton (1985) theory of dielectronic recombination was included, and its effect on the rate coefficients was studied. To our knowledge, this is the first time that all three kinds of effects, 1) the relativistic effects, 2) the resonance and coupling effects and 3) the radiation damping effect, have been included simultaneously in an ab initio calculation of electron-impact excitation rate coefficients for positively charged ions.
The present calculation includes
15 n=2 fine-structure levels
and 30 n=3 levels, as given in detail by Table 1 (click here).
The Coulomb-Bethe approximation was employed to include the high
partial-wave contributions for the optically allowed transitions.
Collision strengths and maxwellian-averaged rate coefficients
have been calculated for the 105 transitions among the 15 fine-structure
levels of n = 2 configurations,
the 450 transitions between these 15 n = 2 levels
and the 30 n = 3 levels,
and the 435 transitions among the 30 n = 3 levels.
As in Paper III, rate coefficients are tabulated for
scaled electron temperatures between
100 and 50 000 
K, where the ion charge z = Z - N with Z and N being
the atomic number of the ion and the number of the electrons per ion.
For Fe XXII, this corresponds to a range of
T(K) from about 44 000 to 
.
A brief description of the computations and the results
are given in the following sections.
The present work is part of an international collaboration known as the IRON Project (Hummer etal. 1993, referred to as Paper I) to obtain accurate electron-impact excitation rates for fine-structure transitions in atomic ions. A full list of the papers in this Atomic Data from the IRON Project series published to-date is given in the references. A complete list of papers including those in press can be found at http://www.am.qub.uk/projects/iron/papers/, where abstracts are also given for each paper.