High-resolution ion translational energy spectroscopy of electronic excitation of homonuclear diatomic molecules in ion molecule collisions

Abstract

Excitation processes in H₂, N₂ and O₂ molecules induced by collisions with H⁺ and H₂⁺ projectile ions have been studied by means of energy loss spectrometry. The relatively simple collision systems were chosen to demonstrate the performance of a new, high-resolution, multi-sector ion translational energy spectrometer. The instrument provides monoenergetic projectile ion beams whose full-width at half maximum is in the range 14-20 meV at a collision energy of 3 keV. The exceptional energy resolution was expected to aid in the elucidation of the principle factors governing collisional processes. The singlet manifold of excited electronic states of neutral target molecules is accessed when excitation is induced in collisions with H⁺ projectiles; both singlet and triplet states are accessed in collisions using H₂⁺ projectiles. Nitrogen exhibits the richest spectrum which can be fully interpreted. Both hydrogen and oxygen targets exhibit strong dissociative excitation features. Excitation beyond the target molecule's ionization limit is observed, but not to its ionic states or neutral molecule Rydberg series and remains to be interpreted. Conclusions on the validity of quantum selection rules were made. Spin conservation was found to be strongly adhered to, whilst orbital angular momentum and total angular momentum were not. The specific selection rules for homonuclear diatomics g ↔ g and u ↔ u were found to be allowed, in contradiction to optical spectroscopy, and there was strong experimental evidence that the optically forbidden ∑ ⁺-∑^- symmetry transition was not observed, except in one case at small energy defect. For H₂⁺ projectiles, excitation to triplet states was strongly favoured over excitation to singlet states by a factor of four.