The spectrum, of rubidium hydride, RbH - II. Potential curves, wave functions, and intensity distribution

In the preceding paper an account has been given of the analysis of the spectrum of the rubidium hydride molecule, RbH. It is the purpose of this paper to derive the potential curves from the observed constants and to study the interesting vibrational intensity distribution which is a feature of this spectrum. In addition to a very open Franck-Condon parabola, similar to that obtained for the other alkali-metal hydrides, there is for RbH a well-marked second parabola and a weak third parabola. Such subordinate parabolae have been observed previously, e.g. Herzberg (1932) noted the effect for phosphorus, P2, and Almy and Kinzer (1935) in their analysis of the spectrum of arsenic, As2, drew attention to indications of several parabolae as loci of the strong bands. The classical Franck-Condon treatment of the vibrational intensity distribution accounts only for the main parabola; to explain the others it is necessary to apply the methods of wave mechanics. Hitherto, these subordinate parabolae have only been observed for molecules with extensive close-structured band systems; in the spectrum of RbH is a beautiful example combined with an open struc­- ture, the rotational analysis of which has been completed, thus enabling the potential energy functions to be derived accurately. This is therefore an especially favourable case in which to study the intensity distribution in detail.