HIGH-RESOLUTION ION TRANSLATIONAL ENERGY SPECTROSCOPY OF ELECTRONIC EXCITATION OF HOMONUCLEAR DIATOMIC-MOLECULES IN ION-MOLECULE COLLISIONS

Excitation processes in H-2, N-2 and O-2 molecules induced by collisio ns with H+ and H-2(+) projectile ions have been studied by means of en ergy loss spectrometry. The relatively simple collision systems were c hosen 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 exceptiona l energy resolution was expected to aid in the elucidation of the prin ciple factors governing collisional processes. The singlet manifold of excited electronic states of neutral target molecules is accessed whe n excitation is induced in collisions with H+ projectiles; both single t and triplet states are accessed in collisions using H-2(+) projectil es. Nitrogen exhibits the richest spectrum which can be fully interpre ted. Both hydrogen and oxygen targets exhibit strong dissociative exci tation features. Excitation beyond the target molecule's ionization li mit is observed, but not to its ionic states or neutral molecule Rydbe rg series and remains to be interpreted. Conclusions on the validity o f quantum selection rules were made. Spin conservation was found to be strongly adhered to, whilst orbital angular momentum and total angula r momentum were not. The specific selection rules for homonuclear diat omics g <----> g and u <----> u were found to be allowed, in contradic tion to optical spectroscopy, and there was strong experimental eviden ce that the optically forbidden Sigma(+)-Sigma(-) symmetry transition was not observed, except in one case at small energy defect. For H-2() projectiles, excitation to triplet states was strongly favoured over excitation to singlet states by a factor of four. (C) 1997 Elsevier S cience B.V.