Electron-impact dissociation of HCl: Translational energy and angular distributions of excited hydrogen atoms

Abstract

Electron-impact dissociation of HCl for the formation of excited hydrogen atoms (n=4) has been investigated by measuring Doppler profiles of the Balmer lines and their angular dependence at a high optical resolution using a Fabry–Perot interferometer. The translational energy distribution (TED) and the angular difference Doppler profile were obtained. There are five major dissociation processes for the formation of H* (n=4). The threshold energy and the TED peak of the five components are (1) 19 and 2.5; (2) 25 and 7.2; (3) 29 and 1.7; (4) 36 and 5.1; and (5) ≳40 and 8–12 eV, respectively. Formation of components 1 and 4 is anisotropic and parallel with respect to the electron beam. Component 1 should be produced by predissociation through the Rydberg states with the Σ symmetry converging to either the 4Σ or 2Σ− state and then those converging to the A 2Σ+ state. The asymmetry parameter (β) of component 1 was determined to be about 0.62, and the intermediate excited state for the formation of component 1 has a lifetime equal to the rotational period. Component 2 would be produced through the Rydberg states converging to the 2Π state. Component 3 would be produced through high-lying doubly excited Rydberg states converging to either the (4)2Π or (4)2Σ+ state. Component 4 should be produced through doubly excited repulsive states with the Σ symmetry.