The internal conversions of trans- and cis-1,3,5-hexatriene in cyclohexane solution studied with sub-50 fs UV pulses

Subpicosecond internal conversion dynamics of cis- and trans-1,3,5-hexatriene in cyclohexane were studied using sub 50 fs ultraviolet pulses. This time resolution allows direct observation of the 1B (S-2) and 2A (S-1) lifetimes. The 1B lifetime is 55 +/- 20 fs for trans-hexatriene, and less than or equal to 50 fs for cis-hexatriene. The subsequent 2A lifetime of trans-hexatriene is found to be 190 +/- 30.fs, and is controlled by the rate of intramolecular vibrational energy redistribution. The 2A lifetime in cis-hexatriene is 250 +/- 30 fs, and is affected by modification of the excited state potential energy surface from coupling with the solvent. (C) 2000 Elsevier Science B.V. All rights reserved.