Born-Oppenheimer breakdown effects in the determination of diatomic internuclear potentials: Application of a least-squares fitting procedure to the HCl molecule

Abstract

This paper describes a least-squares fitting procedure for the reduction of measured line positions in 1Σ diatomic spectra to effective internuclear potentials. The procedure, which is based on first-order perturbation theory/Hellman-Feynman theorem, can be applied to data within a single electronic state, or to such data in combination with those of an electronic transition. Following recent theoretical work on Born-Oppenheimer breakdown, a modified, or effective, vibration-rotation Hamiltonian is employed to take account of non-adiabatic effects. An unlimited amount of data can be fitted simultaneously, with appropriate weighting. Results are described from an application of the procedure to the extensive data available for H 35Cl ( X 1Σ +), including those from a recent study of the B → X system. Combined with similar results from vibration-rotation data for H 37Cl and D 35Cl, the effective potentials for the ground states of the three isotopes lead to a determination of the Born-Oppenheimer potential for HCl ( X 1Σ +). The synthetic vibration-rotation spectrum of D 37Cl, obtained from the eigenvalues of the calculated effective Hamiltonian for this isotope, is found to be in excellent agreement with the precise experimental data from Fourier transform spectroscopy.