Collision-induced electronic excitation of CH + ions by impact on rare gas atoms and some diatomic molecules

Abstract

Efficient excitation of the A 1Π and B 1Δ electronic states of CH + ions was observed in collisions of CH +(X 1+) with noble gas atoms or diatomics (H 2, D 2, N 2, O 2, NO, CO) at kinetic energies up to 1 keV lab. This process and the competing charge transfer forming CH(A 2Δ) were investigated by spectral analysis of the subsequent optical emission. In the energy range available, the excitation functions increase steadily with the collision energy. For systems where charge transfer excitation is negligible, the cross-section depends only moderately on the target gas species. Population of the CH +(B) state, which requires simultaneous excitation of two electrons in the collision, proceeds with unexpectedly high efficiency. The energy dependence here is somewhat different for rare gases and for diatomics. The total cross-sections for collisional formation of the electronically excited A and B states of CH + are of the order of several Å 2. From the ground singlet state only radiating singlet states of CH + are collisionally populated, even when target gases are used for which the spin-conservation rule would permit population of the CH +(b 3−) state; the collisional transfer to this state does, however, occur from the CH +(a 3Π) metastable state present in the projectile ion beam. The population of rotational states of CH +(A) corresponds to temperatures of 4000 K at 100 eV lab and 3000 K at 1000 eV lab, independent of the target gas.