Theoretical study of Ca(4s5p 1P)→Ca(4s5p 3P) transitions in collisions with He: Integral cross sections and alignment effects

Publisher Website
Google Scholar
Abstract
The fully quantum theory is developed for 1P→3P transitions in collisions of electronically excited (nsn’p) alkaline earth atoms with closed shell atoms. Spin‐changing transitions occur by means of the small spin–orbit mixing between the 1P and 3PJ=1 states of the isolated atom, and are facilitated by the crossing between the 1Π and 3Σ potential curves which correlate, respectively, with the 1P and 3P asymptotes. Close‐coupled calculations are carried out for the Ca(4s5p)+He system, based on four different choices of the necessary interaction potentials. Particular attention is devoted to the simulation of the recent experiment of Hale, Hertel, and Leone [Phys. Rev. Lett. 5 3, 2296 (1984)], in which the 1P→3P cross section was determined in a crossed‐beam experiment as a function of the orientation of the initially excited 5p orbital. This polarization dependence depends critically on the long‐range splitting between the 1Π and 1Σ curves. A fully adiabatic description of the collision dynamics is used to interpret the results of the quantum scattering calculations. No clear‐cut theoretical justification is found for the ‘‘orbital following’’ models which have been developed to interpret prior experimental studies of collisions involving excited atoms in P electronic states. Rather, a picture emerges in which the initially selected orientation of the Ca p orbital correlates, at short range, with equal probability to Σ‐like and Π‐like potential curves. Variations in the 1P→3P cross sections are due to long‐range Coriolis coupling between the electronic orbital and nuclear orbital momenta and may reflect quantum interference effects between the Π‐like and Σ‐like adiabatic potentials.