Intensities of KCs [formula omitted] band system up to dissociation threshold: an interplay between spin-orbit, hyperfine and rovibronic coupling effects

Abstract

The relative intensity distribution in the rotationally resolved laser-induced fluorescence spectra belonging to the E(4)1Σ+→(a3Σ1+,X1Σ+) band systems of the KCs molecule was analyzed. The experimental intensities in doublet P,R progressions assigned to spin-allowed E→X and spin-forbidden E→a transitions up to their common ground dissociation limit were described in the framework of a coupled-channels (CC) deperturbation model applied for the interacting X1Σ+ and a3Σ+ states. The CC intensity simulation was based solely on fixed electronic structure parameters as functions of the internuclear distance R, namely: accurate empirical potential energy curves for all three states, ab initio estimates for matrix elements A(R) of the hyperfine structure (HFS), and transition dipole moments dEX(R) and dEa(R). A comparison between the measured intensities and their theoretical counterparts demonstrates a strong competition between different intramolecular interactions. A weak spin-orbit coupling of the upper E(4)1Σ+ state with the remote 3Π states is responsible for appearance of the E→a vibrational bands for the intermediate va-values. In turn, the HFS coupling between X1Σ+ and a3Σ+ states leads to peculiarities in E→(X,a) intensities, which are pronounced for high vX/a-values in the vicinity of K(42S)+Cs(62S) dissociation threshold. Both adiabatic ro-vibrational and non-adiabatic electronic-rotational interactions explain the abrupt deviation of some observed P/R intensity ratios from the expected Hönl-London factors.