Orientation and alignment of the first excitedpstate in Li+He and Na+He scattering

Orientation and alignment parameters for the first excited p state of Li and Na in collisions with He through direct excitation from the ground state are studied theoretically in the energy region up to Ec.m.=100 keV by using a quasi-one-electron theory. Scattering states are expanded in terms of molecular orbitals, which are calculated by using the pseudopotential method and include electron translation factors. The approach appears to work well for Li+He, giving good agreement for the 2p excitation probability and orientation. For alignment, the situation is less clear because of difficulty in experimental measurement. Two-electron effects and cascades from more highly excited states cause our description of Na+He collisions to be less satisfactory. However, agreement with the experimental 3p excitation probability and orientation parameters where all data are available is fairly good at lower energies (Ec.m.1 keV) and at larger impact parameters (b>1.25 a.u.).