Inverse distribution of populations of vibrational levels in the A2Sigma state of the HCl/+/ ion excited by electron impact

A method is developed for producing a population inversion of vibrational levels in excited electronic states of molecules or molecular ions by direct electron impact. The dependences of the emission band intensities of hydrogen chloride on the electron beam current and gas pressure are determined. It is found that the ratio of the band intensities to the (1-0) band intensity is independent of the beam current and gas pressure, which indicates that the A2Sigma state is excited by direct electron impact. The populations of the vibrational levels in the A2Sigma state are calculated and these values are found to agree with the theoretical values. This finding indicates the validity of the Franck-Condon principle for such a complex process as the excitation of the HCl(+) ion from the ground state of the HCl molecule by electron impact. It is concluded that the validity of the Franck-Condon principle in the case of excitation by electron impact results in the formation of an inverse distribution of population of the vibrational levels in the upper electronic states. It is suggested that a similar effect may occur in other molecules which have displaced potential curves of the ground and excited electronic states.