Vibrational energy levels and predissociation lifetimes of the AΣ state of SH/SD radicals by photodissociation spectroscopy.

Photo-predissociation of SH and SD radicals in the AΣ state is investigated using the high-n Rydberg atom time-of-flight technique. By measuring the photoproduct translational energy distributions as a function of excitation wavelength, contributions from overlapping AΣ (v') ← XΠ (v″) transitions can be separated, and the H/D + S(P) photofragment yield (PFY) spectra are obtained across various rovibrational levels (SH v' = 0-7 and SD v' = 0-8) of the AΣ ← XΠ bands. The upper AΣ state vibrational levels v' = 5-7 of SH and v' = 3-8 of SD are determined for the first time. The PFY spectra are analyzed with the simulation program PGOPHER [C. M. Western, J. Quant. Spectrosc. Radiat. Transfer 186, 221 (2016)], which gives vibrational origins and linewidths of the rovibrational levels of the AΣ state. The linewidths (≥1.5 cm) of the SH AΣ v' = 3-7 and SD AΣ v' = 2-8 states are characterized for the first time in this work, demonstrating that these levels undergo rapid predissociation with lifetimes on the order of picosecond. The lifetimes of the SD AΣ v' = 0, N' = 1 and 2 levels are determined to be 247 ± 50 ns and 176 ± 60 ns by pump-probe delay measurements, respectively. The experimentally measured lifetimes are in reasonable agreement with the theoretical predictions.