Study of excited electronic states of the $^{39}$KCs molecule correlated with the K($4^2$S)+Cs($5^2$D) asymptote: experiment and theory

TL;DR

This study combines experimental polarisation labelling spectroscopy and advanced electronic structure calculations to analyze and interpret the complex excited electronic states of the $^{39}$KCs molecule near the K($4^2$S)+Cs($5^2$D) asymptote, revealing strong spin-orbit coupling effects.

Contribution

It provides a detailed experimental and theoretical analysis of highly excited $^{39}$KCs states, demonstrating the effectiveness of combined spectroscopy and electronic structure calculations.

Findings

Strong spin-orbit coupling causes perturbations in the spectra.

Excellent agreement between experimental data and theoretical calculations.

Validated the use of polarisation labelling spectroscopy for dense electronic states.

Abstract

Using the polarisation labelling spectroscopy, we performed the detailed analysis of the level structure of excited electronic states of the $^{39}$KCs molecule in the excitation energy interval between 17500~cm$^{-1}$ and 18600~cm$^{-1}$ above the $v=0$ level of the $X^1\Sigma^+$ ground state. We prove that the observed states are strongly coupled by spin-orbit interaction above 18200~cm$^{-1}$, as manifested by numerous perturbations in the recorded spectra. The spectra are interpreted with the guidance of accurate electronic structure calculations on KCs, including potential energy curves, transition electric dipole moments, and representation of the spin-orbit interaction with a quasi-diabatic effective Hamiltonian approach. The agreement between theory and experiment is found remarkable, clearly discriminating among the available theoretical data. This study confirms the accuracy of the polarisation labelling spectroscopy to analyse highly-excited electronic molecular states which present a dense level structure.