Collisional Deactivation of Highly Vibrationally Excited SO₂: A Time-Resolved FTIR Emission Spectroscopy Study

Time-resolved Fourier transform IR emission spectroscopy, capable of 10^-8 s and 0.1 cm^-1 spectral resolution, has been used to study the collisional deactivation of highly vibrationally excited SO₂ by bath-gas molecules Ar, N₂, O₂, CO₂ and SF₆. The vibrationally excited SO₂ were initially prepared with 32,500 cm^-1 energy in the X˜¹A₁ state by the pulsed 308 nm laser excitation followed by internal conversion. The entire collisional deactivation process of the excited SO₂ was monitored by time-resolved IR emission spectra through the IR active transitions. The average energy, <E>, of excited SO₂ was extracted from the IR emission bands using known vibrational constants and selection rules. <E>is further used to derive the average energy loss per collision, <E>, by each of the bath-gas molecules. The results show that <E> increases from mono- and di-atomic quenchers to more complex polyatomic molecules, as V-V energy transfer contributes to V-T/R. For all bath molecules, <E> increases with <E> and displays a marked increase at <E> ≈ 20,000 cm-1. The observed threshold behavior most likely arises from intramolecular vibronic coupling within SO₂ and implies the importance of long range interaction in intermolecular energy transfer.