RTAB data files are available at:
ftp://www-phys.llnl.gov/pub/rayleigh/RTAB

8. Atomic form factors (data_MF, data_RF, data_NF)

• we wanted all our data to be computed in exactly the same self consistent potential for detailed comparison with our S-matrix predictions;

• we required form factors that could be interpolated with very high accuracy to intermediate momentum transfers;

• in addition to total-atom values, we required systematic access to shell and subshell form factors to implement total-atom S-matrix cross sections.

Although the values are computed in a different atomic model, we have included the non-relativistic form factors (NF) of Hubbell et al. (1975) as a convenience to users of the RTAB database. The NF values are easily the approximation for elastic scattering in the widest use today. It will be natural in any investigation of scattering to relate more sophisticated predictions back to the non-relativistic form factors of Hubbell et al.

In Table 6 we list selected form factors extracted from the RTAB database. Note that the MF and RF form factors are stored as form factor per electron, in contrast to the NF values. As a consequence, the MF and RF values in Table 6 have been multiplied by the appropriate number of electrons. An indication of the accuracy of our numerical evaluation of the form factor can be gleaned from the value of the total-atom RF in Table 6. As f(0) identically equal to N, we see a relative error of less than about 1x10^-8.

We have observed that the MF values approach the same high-energy limit as our SM values within about the overall numerical accuracy of our calculations (see, for example, Kissel et al., 1995). Although not a proof, we suspect that the modified relativistic form factor essentially predicts the correct forward-angle high-energy limit of scattering, and we use MF as our practical evaluation of this limit. Due to this desirable feature of MF predictions, we tend to prefer MF to other form factors. It has also been our general experience, that the order of validity of form-factor-only predictions is MF, followed by NF, with RF yielding the poorest predictions.

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Information date: Sep. 2, 2000 lk