PHOTOCHEMICAL RING-OPENING REACTIONS ARE COMPLETE IN PICOSECONDS - A TIME-RESOLVED UV RESONANCE RAMAN-STUDY OF 1,3-CYCLOHEXADIENE

Photoproduct formation, vibrational, and conformational relaxation kin etics in the photochemical ring-openings of 1,3-cyclohexadiene (CHD) a nd alpha-phellandrene (alpha-PHE) are determined by picosecond, time-r esolved UV Stokes and anti-Stokes resonance Raman spectroscopy. The fr equency-doubled output from an amplified, synchronously pumped dye las er system was used to perform transient, two-color resonance Raman Sto kes experiments on the photoconversion of CHD to cis-hexatriene(c-HT) and of alpha-PHE to 3,7-dimethyl-1,3,5-octatriene(OT). The appearance time of the Stokes scattering from ground-state c-HT is 6 +/- 1 ps, wh ile that of OT is 11 +/- 2 ps. In both reactions, the photoproduct ant i-Stokes ethylenic intensity appears with a time constant of 8 +/- 2 p s and decays in 9 +/- 2 ps. This similarity demonstrates that the phot oproduct appearance and intermolecular vibrational relaxation kinetics are not dramatically affected by the presence of alkyl substituents. Analysis of the photoproduct spectral evolution in the Stokes and anti -Stokes data as well as the observation of Raman lines characteristic of the all-cis conformer in the anti-Stokes data demonstrates that all -cis-HT first appears on the ground-state surface and then undergoes c onformational relaxation to produce mono-s-cis-HT with a time constant of 7 ps. The photoproduct anti-Stokes ethylenic and single-bond stret ch intensities further demonstrate that the initial photoproduct tempe rature at 4 ps is 1500 +/- 500 K and that the cooling time is 15 ps. T his is the first complete analysis of the photoproduct formation, vibr ational, and conformational relaxation dynamics characteristic of phot ochemical pericyclic ring-opening reactions.