2.1. Electron Impact Excitation

The quantity usually computed for electron impact excitation is denoted as (i, j), which was introduced by Hebb and Menzel (1940) and later termed the ``collision strength'' by Seaton, in analogy with the line strength for radiative transitions. The is dimensionless and symmetric with respect to initial and final states. It is related to the cross section as

(2.2)

in units of the area of the H atom.

The usually tabulated quantity is the maxwellian averaged collision strength, also called the effective collision strength,

(2.3)

The excitation rate coefficient, in cm³ sec^-1, is defined as

(2.4)

related to the de-excitation rate coefficient (E_i < E_j) as

(2.5)

The influence of autoionizing resonances may be seen in Fig. 2.

Figure 2. Collision strength for the forbidden transition 6D9/2 - 6D7/2 in Fe II (Pradhan and Zhang 1993). The (T), at T = 10,000 K, using the close coupling collision strengths shown in Fig. 2 is approximately a factor of three higher than the one calculated using the distorted wave values (diamonds) calculated by Nussbaumer and Storey (1980).

A detailed discussion on the analysis of collision strengths and rate coefficients is given by Burgess and Tully (1992), who describe analytic fitting procedures to (E) and (T) for interpolation and extrapolation in a compact form for the different types of transitions.