Jet-cooled laser-induced fluorescence spectra of the B̃ ←
X̃ electronic transition of 1-methylvinoxy and of a
cis/trans mixture of 2-methylvinoxy are presented. We observe some
50 vibronic bands in the spectrum of
1-methylvinoxy. The complexity is due to a change in the preferred
methyl rotor orientation on electronic
excitation. The B̃-state barrier to internal rotation is
approximately 750 cm-1. The
B̃-state fluorescence
lifetimes decrease from about 130 ns near the origin to 26 ns for
internal energy of 2700 cm-1. We
observe
some 22 vibronic bands in the spectrum of the cis/trans mixture of
2-methylvinoxy. The B̃-state fluorescence
lifetimes decrease gradually from about 190 ns near the origin to 140
ns for internal energy of 1170 cm-1
and
then very sharply at higher energy. The new spectra serve as
fingerprints for identification of specific
methylvinoxy isomers as products of chemical reactions of
O(3P) with alkenes, as recently observed by
Bersohn
and co-workers.
The fluorescence excitation and emission spectra of trans-β-methyl styrene have been measured in a supersonic jet. A complete vibrational assignment of the S0 and S1 states’ frequencies is reported, assisted by ab initio quantum chemical calculations and by comparison with the IR spectrum. The fluorescence lifetime, τf, of the isolated molecule changes monotonously from 24.5 to 15 ns as the excitation energy increases from the origin band to an excess of 3000 cm−1. The fluorescence quantum yield from the zero-point energy level of S1 is about 38%, similar to the liquid solution value; The major radiationless process being intersystem crossing to a triplet level. The increasing congestion of the emission spectra as the excitation energy is increased is interpreted as due to intramolecular vibrational energy redistribution. The data are consistent with the fact that in the isolated molecule intramolecular vibrational energy redistribution is faster than intersystem crossing. Beyond an excess energy of about 3200 cm−1, a more pronounced decrease in τf is observed, indicating that the barrier to trans–cis isomerization on the S1 surface, in the isolated molecule is higher than 3200 cm−1.
The vibronic structure of the fluorescence excitation and emission spectra of trans-β-methyl styrene have been measured and analyzed, using an ab initio calculation. Good agreement between the experimental and calculated data, which indicate that the molecule is planar in both the ground and the S1 states, is obtained.
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