TIME-RESOLVED FTIR EMISSION SPECTROSCOPY OF HIGHLY VIBRATIONALLY EXCITED $SO_{2}$

Abstract

Time-Resolved Fourier Transform IR Emission Spectroscopy (TR-FTIRES), with $0.5 \mu$s and $1 cm^{-1}$ resolutions, has been used to characterize the collisional energy transfer of highly vibrationally excited $SO_{2}, SO_{2}$ was excited by 308 nm laser pulses through the $\bar{B}^{1}B_{1} \leftarrow \bar{X}^{1}A_{1}$ trasitions. Collisional induced internal conversion following laser excitation prepares molecules with $32,000 cm^{-1}$ vibrational energy. IR emission spectra from highly vibrationally excited $SO_{2}$ show many broad and red shifted features. The energy distribution of $SO_{2}$ during a collision deactivation process can be monitored through the $\nu_{1}, \nu_{3}$ and $\nu_{1} + \nu_{3}$ emission detected by TR-FTIRES. IR emission can also be detected from bath gas molecules allowing direct observation of the V-V energy transferring collisions. The average energy loss per collision, $\langle \Delta E\rangle$, as a function of average energy, $\langle E\rangle$, of $SO_{2}$ can be extracted from the TR-FTIRE spectra by modeling the intensity of the IR emission. The $\langle \Delta E\rangle-\langle E\rangle$ curve shows a dramatic increase in $\langle \Delta E\rangle$ values above $20,000 cm^{-1}$ energy. Previous studies in our $laboratory^{1}$ have shown that such a dramatic increase is caused by intramolecular vibronic couplings at high vibrational energies. This observation suggests that the origin of the lowest triplet state of $SO_{2}$ is around $20,000 cm^{-1}$.