Nonadiabatic investigations of ro-vibrational frequencies within the systems , H2, and prospects for : use of distance-dependent effective masses

Abstract

For small hydrogenic systems the discrepancy between theoretical and experimental studies of ro-vibrational spectra is mostly related to the neglect of adiabatic and non-adiabatic effects on the ro-vibrational eigenvalues. In the case of and its isotopomers, detailed investigations of low lying ro-vibrational excitations have been performed. Using gaussian geminals an absolute accuracy of the Born–Oppenheimer potential energy surface (PES) by ∼0.02 cm−1 was reached. Of similar quality are calculations of relativistic effects and diagonal adiabatic contributions. Non-adiabaticity can be simulated by using atomic masses for vibrational motion and nuclear masses for rotational motion, so that the deviation to experiment can be reduced to a few hundredths of a wavenumber. Recently, a rigorous non-adiabatic theory in terms of a single potential energy surface had been developed and was tested numerically for and H2. Within this new non-adiabatic theory distance-dependent effective nuclear masses have to be used. Our main interest is to use these new ideas for taking into account the effects of non-adiabaticity on the ro-vibrational spectrum of and its isotopomers. Within a perturbative approach we investigate the influence of the operator of nuclear kinetic energy of a triatomic molecule on the electronic wave function. For the asymptotic arrangement of a nearly separated atom–diatomic system we can calculate numerically with high accuracy the distance-dependent effective nuclear masses for and H2. The kinetic energy operator for a triatomic molecule describing ro-vibrational motion has many different terms, where different effective masses have to be taken into account. Within the present work for , the distance-dependent effective mass for the diatomic motion part, using Jacobi coordinates, will be presented.