MOMENTUM-TRANSFER IN THE PHOTOIONIZATION OF LITHIUM

Selective atomic diffusion in stellar envelopes and atmospheres may re sult in stratification of chemical elements and observed under or over abundances. In order to compute accurately radiative forces, one has t o include photoionization processes, and take into account the fractio n of the absorbed photon momentum that is transfered to the ejected el ectron. We show how to use Quantum Defect Theory to calculate this qua ntum mechanical quantity, and illustrate our approach on the lithium c ase.