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Phosphorescence excitation spectroscopy in supersonic jets. The lowest triplet state of pyrazineThe laser induced T 1 (n*)←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 609.68 and 869.02 cm Ϫ1 , respectively. Minima in the potential hypersurface at ϭ61.7°and ␣ϭ42.2°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 Ϫ1 ͒ does not greatly change, S 1 /T 1 ϭ710.8/609.7, whereas the barrier height for the wagging-inversion barrier increases dramatically, 574.4/869.0.
The 000 origin band of the S1←S0, electronic transition that results from n→π* electron promotion has been observed under molecular beam conditions with a pulse amplified ring laser. At low temperatures, ∼0.7 K, the spectrum consisted of 11 lines that originated from Ka″=0 and J″=0 or 1 rotational levels. A rotational analysis revealed that the transition between the a1–a1 torsional levels gives rise to a c-type band, whereas the e–e levels are connected by a hybrid transition that has components along the a, b, and c principal axes. The fluorescence emission from the e levels was greatly reduced at temperatures above 3 K. The interpretation of this photophysical effect requires an intermolecular collision within the molecular beam that quenches the fluorescence from the S1 state.
The T1(n,π*)←S0 laser-induced phosphorescence excitation spectrum of biacetyl has been recorded with a rotating slit nozzle excitation apparatus. The 000 system origin was observed for the first time as a weak band at 19 529 cm−1. Built on this band were the activities of the three large amplitude modes: υ16(au) acetyl torsion, υ15(au) gearing methyl torsion, and υ21(bg), the antigearing methyl torsion. The potential surface for methyl internal rotation extracted from fitting the observed levels yielded a barrier to methyl torsion of 359.6 cm−1.
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