THE $T_{1}(n\pi^{*})\leftarrow S_{0}$ LASER INDUCED PHOSPHORESCENCE EXCITATION SPECTRUM OF ACETALDEHYDE IN A SUPERSONIC FREE JET: TORSION AND WAGGING POTENTIALS IN THE LOWEST TRIPLET STATE

Author Institution: Department of Chemistry, The University of Akron; Inform\'{a}tica, Universidad de Castilla-La Mancha; Department of Chemistry, University of Wisconsin-Parkside; Department of Chemistry, Brocke UniversityThe laser induced $T_{1}(n\pi^{*})\leftarrow S_{0}$ phosphorescence excitation spectrum of jet-cooled acetaldehyde has been observed for the first time with a rotating slit nozzle excitation system. The vibronic origins were fitted to a set of levels that were obtained from a Hamiltonian that employed flexible torsion-wagging large amplitude coordinates. The potential surface extracted from the fitting procedure yielded barriers to torsion and inversion of 614.30 and $864.45 cm^{-1}$, respectively. Minima in the potential hypersurface at $\theta = 60.30^{\circ}$ and $\alpha = 42.16^{\circ}$ defined the equilibrium positions for the torsion and wagging coordinates. A comparison to the corresponding $S_{1}$-state parameters showed that the torsion barrier (in $cm^{-1}$) does not greatly change, $S_{1}$/$T_{1}$ = 710.8/614.3, whereas the barrier height for the wagging-inversion barrier increases dramatically, 574.4/864.5