State-changing collisions of low-Rydberg Na atoms with ground state He atoms: A low-energy study involving spherical and nonspherical initial target states

Low-energy (E less than or equal to 1 eV) state-changing collisions of exci ted atom, particularly the low-Rydberg (n < 10, 1 = n - 1) Na atoms, with t he ground state He atoms are studied theoretically by applying the semiclas sical impact-parameter method, in which a coupled molecular states expansio n augmented with the electron translation factor is used to represent the e lectronic wave function. Cross sections are presented for Na (6s/8s/9s; 5p --> rrl) processes. The de-excitation processes still dominate the quenchin g and the s-to-f (and I > 3) propensity rule holds. The explanation lies in the ordering of the relevant adiabatic energy curves and the nature of the coupling. The Stueckelberg interference oscillations are found in the inel astic cross sections. The role of the nonspherical target state before the collision is discussed. The total thermal energy quenching rates are calcul ated and compared with the experiment results. (C) 2000 John Wiley & Sons, Inc.