The spectroscopy and single vibronic level fluorescence quantum yields of jet cooled trans-stilbene and its van der Waals complexes

This paper reports results on the spectroscopy and dynamics of trans-stilbene seeded in a supersonic free jet. The cooling provided by the expansion results in well-resolved vibronic structure near the origin. Fluorescence excitation spectra reveal two low frequency vibrational modes at 82 and 95 cm−1 above the origin. By 1800 cm−1 above the origin the discrete vibronic structure has given way to a continuous absorption which extends to 6200 cm−1 blue of the origin. The fluorescence excitation spectrum is combined with a direct absorption measurement in the jet using a Xe lamp in order to obtain SVL fluorescence quantum yields for the isolated trans-stilbene molecule. The fluorence quantum yields show no mode dependence, indicating that vibrational relaxation within the trans configuration is much faster than isomerization to the perpendicular configuration. Stilbene van der Waals complexes with helium and argon have been identified in fluorescence excitation spectra at higher nozzle backing pressures. Dispersed fluorescence spectra of these complexes place an upper bound on the stilbene-He binding energy in the excited state of 82 cm−1. Several features of the photodissociation of these complexes are discussed.