At low temperatures the value obtained for
depends quite
critically on the energy dependence of the cross section near threshold.
Since the target is a neutral
atom,
for sufficiently small *E*_{j}, but, for most of the transitions, the data
produced by the CCC calculations are not sufficiently detailed at low
energies to delineate this. The numbers given here for
were obtained by assuming that
falls linearly to zero in
the interval between *E*_{j} = 0 and the lowest value of *E*_{j} at which
the cross section was calculated. For our tabulated
range, we have
,
where the error due to this threshold effect does
not exceed a few percent. Further details will be given elsewhere.

The CCC approximation takes full account of continuum states, whereas the
R-matrix method used by both BK and SB does not. Allowing for the continua
has two opposite effects: (a) part of the flux which would go to
discrete states is now diverted towards continuum states, so reducing
collision strengths for transitions between discrete states; (b)
intermediate continuum states may provide alternative routes for flux to
reach discrete states,
thereby increasing collision strengths. By
comparing effective collision strengths we reach the following
conclusions: for transitions ,
,
the ratio
is close to one for ,
and is significantly less than one if
(i.e. *n*=4).
This we
interpret to mean that (a) is the dominant effect for these latter
transitions. However, for some of the
transitions the ratio is
appreciably greater than one, indicating that (b) dominates. This
suggests that the continuum states are capable of either decreasing or
increasing collision strengths particularly if both the initial and
final levels are close to the ionisation threshold.

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