BARELY FLUORESCENT MOLECULES I. TWIN DISCHARGE JET SPECTROSCOPY OF HSnCl

Abstract

The divalent tin HSnCl transient molecule has been detected for the first time by LIF spectroscopy. HSnCl and DSnCl were produced in a twin discharge jet using separate precursor streams of SnH$_{4}$ and HCl (DCl), both diluted in high pressure argon. The \~{A}$^{1}$A$''$-\~{X}$^{1}$A$'$ spectrum of HSnCl consists of a single rotationally resolved 0-0 band with a very short fluorescence lifetime ($\sim$25 ns). In contrast, the spectrum of DSnCl exhibits three bands (0$_{0}^{\,0}$, 2$_{0}^{\,1}$ and 2$_{0}^{\,2}$) whose fluorescence lifetimes decrease from 400 ns (0$^{0}$) to less than 10 ns (2$^{2}$). Single vibronic level emission spectra have been recorded, providing information on all three vibrational modes in the ground state. Fluorescence hole burning experiments have shown that a few higher nonfluorescent levels are very short-lived but still detectable. Our detailed ab initio studies indicate that these molecules dissociate into SnCl + H on the excited state potential surface and this is the cause of the short fluorescence lifetimes and breaking off of the fluorescence. It is fortunate that the HSnCl excited state zero-point level is still fluorescent or it would not be detectable by LIF spectroscopy. Our calculations also predict that HSnBr should also fluoresce on excitation of low-lying bending levels in the excited state.