Rotationally resolved spectroscopy of the $ÃA21←X̃B21$ transition of $H2S+$ above the barrier to linearity using the mass-analyzed threshold ionization photofragment excitation technique

The $ÃA21←X̃B21$ transitions of $H2S+$ above the barrier to linearity have been investigated with the energy resolution high enough to identify individual rotational transition lines for the first time. The rotational cooling of the cation is achieved either by the direct ionization or mass-analyzed threshold ionization (MATI) technique employed in the vacuum-ultraviolet laser excitation of the jet-cooled $H2S$⁠. Subsequent photoexcitation leads to the $H2S+→H2+S+$ dissociation and the $S+$ product yield taken as a function of the excitation energy gives the photofragment excitation (PHOFEX) spectra. The combined use of MATI and PHOFEX techniques greatly simplifies the spectrum allowing the accurate identification of the rotationally resolved bands which is otherwise a formidable task due to the intrinsic complexity of the $ÃA21←X̃B21$ transition. Highly excited states of $Ã(0,7,0)$⁠, $Ã(0,8,0)$⁠, and $Ã(0,9,0)$ vibronic levels with different $K$ quantum numbers which are located above the barrier to linearity are thoroughly investigated. The bent-to-quasilinear transition of $H2S+$ above the barrier to linearity shows the characteristics of the Renner–Teller effect, showing the large $A$ rotational constant and strong intensity borrowing of the highly vibrationally excited ground levels such as $X̃(0,23,0)$ or $X̃(0,24,0)$ in the dipole-allowed excitation. Spectroscopic parameters of term values, rotational, and spin-orbit coupling constants are precisely determined in this work, providing the most quantitative spectroscopic structure of the $H2S+$ to date. Quantum-state dependent photodissociation dynamics are also discussed from spectral features of PHOFEX.