Application of an improved fitting procedure for diatomic internuclear potentials and Born-Oppenheimer breakdown functions: The B1Σ+ → X1Σ+ system of H35Cl and H37Cl

Abstract

A numerical procedure for direct fitting of effective 1Σ Hamiltonians to spectroscopic data of diatomic molecules is described. Approximate Hamiltonians for the X and B states of H37Cl, derived from fitted Hamiltonians for H35Cl, provided predicted line positions in the B → X system of H37Cl that were accurate to about 0.05 cm−1. These calculations permitted the first extensive rotational assignments for 31 bands of this isotopomer (0 ≤ v′ ≤ 6; 7 ≤ v″ ≤ 17) in spectra recorded previously using HCl prepared from naturally occurring chlorine. The entire B → X data set for H35Cl (2010 line positions) and H37Cl (928 line positions), together with 573 vibration-rotation and pure rotation transitions in the X state, has been employed simultaneously in fits to the effective X- and B-state Hamiltonians of the two isotopomers. It is concluded that Born-Oppenheimer breakdown effects in the X1Σ+ state of HCl are dominated at large internuclear separations by coupling to the repulsive 1 1Π state correlating with ground state atoms. Ab initio calculations of the radial variation of matrix elements coupling the X state with excited electronic states would be of much value.