Scattering angle dependence of electron impact excitation: Intensity variation within a vibrational progression

Abstract

Intensity distributions of electronic transitions in O2 and CO within a vibrational progression resulting from electron impact excitation are studied theoretically and experimentally. The multireference single- and double-excitation configuration interaction (MRD-CI) method is used to elucidate details of selected electronic transitions. In particular, the adiabatic MRD-CI approach can account for the variation of the Franck–Condon envelope with scattering angle that has been reported for the B 1Σ+←X 1Σ+ transition in CO and also was recently observed in the B′ 3Σ−u←X 3Σ−g transition of O2. This behavior contrasts with the relative stability of the intensity distribution observed within the CO A 1Π←X 1Σ+ vibrational progression. In the former cases the excited state undergoes changes with internuclear separation because of the presence of an avoided crossing. Since a transition from the zeroth vibrational level in the ground electronic state to an individual vibrational level in the excited electronic state tends to select a particular internuclear distance (R centroid), each vibrational band may behave as a transition to a separate electronic level. This happens because the excited-state wave function undergoes a compositional change with internuclear separation between the adiabatic partners of the avoided crossing.