A Theoretical Study on Crossings Among Electronically Excited States and Laser Cooling of Group VIA (S, Se, and Te) Hydrides.

Various electronically excited states and the feasibility of direct laser cooling of SH, SeH, and TeH are investigated using the highly accurate ab initio and dynamical methods. For the detailed calculations of the seven low-lying Λ-S states of SH, we utilized the internally contracted multireference configuration interaction approach, considering the spin-orbit coupling (SOC) effects. Our calculated spectroscopic constants are in very good agreement with the available experimental results. It is found that, from SH to TeH, the crossing points among the A2Σ+ and three electronically excited states gradually shift downward toward the ground vibrational level of the A2Σ+ state. This is consistent with our previous findings in other molecular systems and makes the laser cooling of TeH unfeasible. Our calculations indicate that the three crossing points, respectively, between the A2Σ+ and a4Σ-, A2Σ+ and B2Σ-, and A2Σ+ and b4Π states of SH, all lie above the v' = 1 vibrational level of the A2Σ+ state, as a result of which the crossings involving electronic states of higher energy would not hinder its laser cooling. Based upon our study on various excited states, we have constructed a viable laser-cooling scheme for SH, utilizing three laser beams and leveraging the A2Σ+ → X2Π transition. This transition possesses a very large vibrational branching ratio R00 (0.9558), an abundant number of scattered photons (9.30 × 103), and a short radiative lifetime (787 ns). Our work underscores the important role of excited-state crossings in molecular laser cooling and demonstrates that SH emerges as a very good candidate for ultracold molecules.