Production of excited states in the collision of He+ with HCl and HBr: selective production of the v = 0 level in the A 2Σ of the molecule-ion

The emission of light over the 2000–6000-Å wavelength region resulting from the collision of the He+ ions on HBr and HCl has been investigated at ion energies from 0.3 to 4.0 keV. Emission is observed from the A 2Σ → X 2Πi transition of HBr+ and HCl+, from atomic hydrogen, and from Br+ and Cl+ ions. Relative cross sections were determined for the formation of individual H-atom quantum levels and vibrational levels of the molecular ion. Vibrational populations were proportional to the Franck-Condon factors for a transition between the neutral ground state and the A 2Σ state. An unusual dependence upon target gas pressure was observed for the intensity of the (0, 0) band of both HCl+ and HBr+. The intensity increases quadratically with target gas pressure in the 10−4-torr region. Similar studies of other ions impinging on HCl and HBr do not show this effect. The most plausible mechanism satisfying all of the experimental observations is that two molecules of target gas are involved in the excitation process. In the first step He+ reacts with HCl or HBr to form a reactive intermediate that subsequently reacts with a second molecule to produce the A 2Σ state of the molecule-ion in the v=0 level only.