Abstract

Fluorescence excitation and emission spectra of the symmetry for bidden and 2¹A_g ← 1¹A_g (S₁ ← S₀) and strongly allowed 1¹B_u ← 1¹A_g (S₂ ← S₀) transitions of several unsubstituted and methyl substituted linear polyenes are measured in free jet expansions. Activation barriers of <200 cm⁻¹ and ~2000 cm⁻¹ for nonradiative decay in S₁ states of trienes and tetraenes are determined from measured fluorescence lifetimes. By analogy to previous condensed phase studies, the nonradiative channel is assigned to cis-trans isomerization. Octatetraene fluorescence lifetimes have a quantum-state dependence, which is ascribed to state specific tunneling rates through the isomerization barrier. Short lifetimes are observed mainly for a_u and b_g vibronic symmetry modes, which are more strongly coupled to the isomerization coordinate than are the a_g and b_u symmetry Franck-Condon and Herzberg-Teller active modes. In addition, S₂ ← S₀ spectra of tetraenes show Lorentzian line shapes with energy dependent widths of 10–60 cm⁻¹ due to internal conversion to S₁ states on a <500 fs time scale.

© 1992 Optical Society of America