Fluorescence from the 1 1B u state of t r a n s,t r a n s-1,3,5,7-octatetraene in a free jet

Abstract

The 1 ¹B_u→1 ¹A_g fluorescence spectrum of trans,trans 1,3,5,7‐octatetraene in a supersonic helium jet has been determined. Excitation at the 1 ¹B_u origin (35 549.3 cm⁻¹) gives rise to a well resolved emission spectrum whose vibronic development is dominated by the single and double bond stretch modes of the polyene. There is no gap between the onset of the dispersed emission spectrum and origin of the previously reported fluorescence excitation spectrum. Observed band profiles are well described by the convolution of 12 cm⁻¹ Lorentzians with the spectrometer response function. The fluorescence lifetime for excitation at the 1 ¹B_u origin is shorter than 2 ns; an excited state decay time of 100–300 ps follows from an analysis of the dependence of fluorescence signal on excitation power in the nonlinear regime. The rough mirror symmetry between the 0‐0 excited emission and excitation spectra and the short lifetime indicate that the emission comes exclusively from the 1 ¹B_u state with no significant contribution from the lower lying 2 ¹A_g state. For excitation at the origin the overall emission yield is estimated to be 0.05–0.15.