ADIABATIC AND NONADIABATIC EFFECTS IN VIBRATION-ROTATIONAL SPECTRA OF DIATOMIC-MOLECULES

For the vibration-rotational motion of a diatomic molecule, various forms of an effective Hamiltonian which includes the corrections of the Born-Oppenheimer approximation in the form of radial functions are reviewed. A procedure to fit vibration-rotational and pure rotational transitions is proposed which is insensitive to the specific representation of the Born-Oppenheimer corrections but is capable of extracting the mass-independent Born-Oppenheimer potential from the experimental data. This method is applied to LiH successfully. The truncation of the power expansion of the Born-Oppenheimer potential introduces ambiguity depending on the mathematical form of the expansion coordinate. We reconsider the question of separability of the adiabatic and nonadiabatic corrections which can be derived from the atomic mass dependence of the molecular energy levels. We demonstrate that by means of a measured molecular rotational g-factor g(J), the adiabatic and nonadiabatic terms of the rotational energy become separable in lowest order. The possibility to estimate the lowest order for the correction function of the vibrational kinetic energy operator is briefly discussed. The inclusion of this contribution is important to allow a comparison of the various existing models for analysis of vibration-rotational spectra of great precision. In the case of LiH it is shown that the higher order mass corrections are dominated by the contributions of the rotating and vibrating electric dipole moment. (C) 1994 Academic Press, Inc.