Barely fluorescent molecules. II. Twin-discharge jet laser-induced fluorescence spectroscopy of HSnBr and DSnBr

HSnBr and DSnBr have been detected for the first time by a combination of laser-induced fluorescence (LIF), fluorescence hole-burning, and wavelength resolved emission spectroscopies. The transient molecules were produced in a twin-discharge jet using separate precursor streams of SnH₄/SnD₄ and HBr/DBr, both diluted in high pressure argon. The ùA″-X̃¹A' spectrum of HSnBr only consists of the 0₀ ⁰ and 2₀ ¹ cold bands that show clearly resolved subband structure with fluorescence lifetimes varying from 526 to 162 ns. The DSnBr LIF spectrum exhibits four bands (0₀ ⁰, 2₀ ¹, 2₀ ², and 1₀ ¹) whose fluorescence lifetimes decrease from 525 ns (0⁰) to 175 ns (1¹). Single vibronic level emission spectra have provided extensive information on the ground state vibrations, including all the anharmonicities and the harmonic frequencies. Fluorescence hole-burning experiments have shown that a few higher HSnBr nonfluorescent levels are very short-lived but still detectable. The ab initio studies of Tarroni and Clouthier [J. Chem. Phys. 156, 064304 (2022)] show that these molecules dissociate into SnBr + H on the excited state potential surface and this is the cause of the short fluorescence lifetimes and breaking off of the LIF spectra. HSnBr is a barely fluorescent molecule in the sense that only vibrational levels less than or equal to 317 cm^-1 in the excited state emit detectable photons down to the ground state.