Emission spectra of heteronuclear diatomic rare gas positive ions

An electric discharge in a binary mixture of rare gases produces several discrete emission band groups confined to a narrow wavelength region characteristic of the pair of rare gases and closely related in energy to the ionization energy difference between the two atoms. Such spectra were observed for nine of the ten binary mixtures of He, Ne, Ar, Kr, and Xe. The energy differences between band groups of a given mixture agree closely with the doublet splittings of the lowest states of the atomic ions. The suggestion is offered that the emission spectrum is carried by the heteronuclear ion (XY)+ which undergoes a bound–bound charge–transfer transition X+Y →XY++hν, in which X is the lighter and Y the heavier element in the mixture.