Observation and modelling of bound-free transitions to the $X^1{\mathrm{\Sigma }}^{+}$ and $a^3{\mathrm{\Sigma }}^{+}$ states of KCs

The oscillation continuum in laser-induced fluorescence spectra of bound-free $c^3{\mathrm{\Sigma }}^{+}\to a^3{\mathrm{\Sigma }}^{+}$ and (4)${}^1{\mathrm{\Sigma }}^{+}\to X^1{\mathrm{\Sigma }}^{+}$ transitions of the KCs molecule were recorded by Fourier-transform spectrometer and modelled under the adiabatic approximation. The required interatomic potentials for ground $a^3{\mathrm{\Sigma }}^{+}$ and $X^1{\mathrm{\Sigma }}^{+}$ states were reconstructed in an analytical Chebishev-polynomial-expansion form in the framework of the regularization direct-potential-fit procedure based on the simultaneous consideration of experimental line positions from [R. Ferber et al, Phys. Rev. A, $\mathbf{\text{80}}$, 062501 (2009)] and the present \emph{ab initio} calculation of short-range repulsive potential data. It was proved that the repulsive part over dissociation limit of the derived $a^3{\mathrm{\Sigma }}^{+}$ potential reproduces the experiment better than the potentials reported in literature. It is also shown that all empirical and semi-empirical potentials available for the $X^1{\mathrm{\Sigma }}^{+}$ state reproduce the bound-free (4)${}^1{\mathrm{\Sigma }}^{+}\to X^1{\mathrm{\Sigma }}^{+}$ spectrum with equal quality in the range of observations.