Intensities of KCs $E(4{)}^1{\mathrm{\Sigma }}^{+}\to (a^3{\mathrm{\Sigma }}^{+},X^1{\mathrm{\Sigma }}^{+})$ band system up to dissociation threshold: an interplay between spin-orbit, hyperfine and rovibronic coupling effects

The relative intensity distribution in the rotationally resolved laser-induced fluorescence spectra belonging to the $E(4{)}^1{\mathrm{\Sigma }}^{+}\to (a^3{\mathrm{\Sigma }}_1^{+},X^1{\mathrm{\Sigma }}^{+})$ band systems of the KCs molecule was analyzed. The experimental intensities in doublet $P$,$R$ progressions assigned to spin-allowed $E\to X$ and spin-forbidden $E\to a$ transitions up to their common ground dissociation limit were described in the framework of a coupled-channels (CC) deperturbation model applied for the interacting \Xstate\ and \astate\ 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, \emph{ab initio} estimates for matrix elements $A(R)$ of the hyperfine structure (HFS) , and transition dipole moments $d_{EX}(R)$ and $d_{Ea}(R)$. 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 \Estate\ state with the remote ${}^3\mathrm{\Pi }$ states is responsible for appearance of the $E\to a$ vibrational bands for the intermediate $v_a$-values. In turn, the HFS coupling between \Xstate\ and \astate\ states leads to peculiarities in $E\to (X,a)$ intensities, which are pronounced for high $v_{X\mathrm{/}a}$-values in the vicinity of K(4${}^{2}S$)+Cs(6${}^{2}S$) dissociation threshold. Both adiabatic ro-vibrational and non-adiabatic electronic-rotational interactions explain the abrupt deviation of some observed $P\mathrm{/}R$ intensity ratios from the expected Hönl-London factors.