Fourier-transform spectroscopy and relativistic electronic structure calculation on the $c^3{\Sigma }^{+}$ state of KCs

Abstract

The Ti:Saphire laser operated within 13800 - 11800 cm${}^{-1}$ range was used to excite the $c^3{\Sigma }^{+}$ state of KCs molecule directly from the ground $X^1{\Sigma }^{+}$ state. The laser-induced fluorescence (LIF) spectra of the $c^3{\Sigma }^{+}\to a^3{\Sigma }^{+}$ transition were recorded with Fourier-transform spectrometer within 8000 to 10000 cm${}^{-1}$ range. Overall 673 rovibronic term values belonging to both $e/f$-components of the $c^3{\Sigma }^{+}(\Omega =1^{\pm })$ state of ${}^{39}$KCs, covering vibrational levels from $v$ = 0 to about 45, and rotational levels $J\in [11,149]$ were determined with the accuracy of about 0.01 cm${}^{-1}$; among them 7 values for ${}^{41}$KCs. The experimental term values with $v\in [0,22]$ were involved in a direct point-wise potential reconstruction for the $c^3{\Sigma }^{+}(\Omega =1^{\pm })$ state, which takes into account the $\Omega$-doubling effect caused by the spin-rotational interaction with the nearby $c^3{\Sigma }^{+}(\Omega =0^{-})$ state. The analysis and interpretation were facilitated by the fully-relativistic coupled cluster calculation of the potential energy curves for the $B^1\Pi$, $c^3{\Sigma }^{+}$, and $b^3\Pi$ states, as well as of spin-forbidden $c-X$ and spin-allowed $c-a$ transition dipole moments; radiative lifetimes and vibronic branching ratios were calculated. A comparison of relative intensity distributions measured in vibrational $c-a$ LIF progressions with their theoretical counterparts unambiguously confirms the vibrational assignment suggested in [\emph{J. Szczepkovski, et. al.}, JQSRT, \textbf{204}, 133-137 (2018)].

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