Session G15 - Poster Session I.
POSTER session, Saturday afternoon, April 19
Congressional Hall, Renaissance
In elastic scattering of photons from atoms and ions with atomic configurations involving only fully-filled subshells, rotational invariance allows the scattering amplitude to be expressed as A = \mbox\boldmath \epsilon_i \cdot \mbox\boldmath \epsilon_f \: F(\mbox\boldmath k_i \cdot \mbox\boldmath k_f) + \mbox\boldmath \epsilon_i \cdot \hat k_f \: \mbox\boldmath \epsilon_f^* \cdot \hat k_i \: G(\mbox\boldmath k_i \cdot \mbox\boldmath k_f), where \mbox\boldmath \epsilon_i,\mbox\boldmath k_i (\mbox\boldmath \epsilon_f,\mbox\boldmath k_f) are the polarization and momentum vectors of the initial (final) photon, and F and G are invariant amplitudes. This invariance results from a sum over magnetic quantum numbers at the level of the scattering amplitude for each subshell. More general use of such an averaged amplitude approach (with weighted cross sections according to the number of electrons) may not be appropriate for situations where many or all subshells are only partially-filled, as can be the case for low Z ground state atoms, ions, and excited state configurations. Here there is a non-zero total angular momentum, and, in principle, different polarized scattering cross sections corresponding to the possible projections of the angular momentum. The correct unpolarized cross section is given by a weighted average of the polarized cross sections. The consequences for scattering from such configurations will be discussed, both for the case of an unpolarized target and for a target with a definite orientation. Marked differences from the averaged amplitude approach can occur near the inner-shell photoionization thresholds.