FLUORESCENCE STUDY OF STYRENE AND STYRENE ADDUCTS IN A SUPERSONIC JET

Author Institution: The Hebrew University of Jerusalem, Jerusalem, Israel 91904.The $S_{0}\leftrightarrow S_{1}$ fluorescence spectra of styrene have been studied experimentally and assigned with the aid of an ab-initio calculation. The vibronic structure of the transition was largely maintained in styrene clusters with rare gases and with ammonia, for in-plane modes. The spectral shifts and the relative intensities were unchanged by comparison with the bare molecule. The out-of-plane $(a^{\prime\prime})$ modes were influenced differently by the cluster formation- there relative intensity was largely reduced, and some of them were difficult to observe over the noise level. This behavior is consistent with a structural model for the cluster that predicts the smaller molecule to be positioned above the molecular plane. The styrene-ammonia in-plane vibronic transitions were all shifted $52 cm^{-1}$ to the blue from the bare styrene lines. Single vibronic level fluorescence (SVLF) recorded from these lines showed great similarity to the corresponding bare molecule SVLF spectra, up to the dissociation energy. Excitation of the styrene-ammonia cluster slightly above the dissociation energy led to an emission spectrum practically identical with that of styrene excited at the 0,0 band, showing a complete dissociation on the excited state potential surface. Excitation of higher bands led to emission from several bare styrene levels simultaneously. Excitation of low lying out-of-plane modes of the styrene-ammonia cluster S1 level showed much weaker lines, consisting of small progressions, suggesting strong coupling between the van der Waals vibrations and the molecular out-of-plane vibrations

Spectroscopy and Dynamics of Jet-Cooled Styrene-Ammonia Clusters

Author Institution: The Hebrew University of Jerusalem, Jerusalem, IsraelThe $S_{0} \leftrightarrow S_{1}$ absorption and emission spectra of styrene have been analyzed with the help of an ab-initio calculation. It was found that out-of-plane vibrations undergo, in general, a red shift upon electronic excitation, while in-plane ones are not appreciably changed. An important exception to this rule is the $\nu_{17}$ mode, which is similar the $\nu_{14}(b_{2u})$ mode of benzene - it shows a considerable frequency increase upon excitation. The analysis led to some new assignments of certain vibronic transitions. In addition, the spectra of styrene-ammonia adducts were recorded. Cluster formation affects the frequencies of some normal modes of $S_{1}$ to a small but easily measured extent. It was found that in general, out-of-plane modes were blue shifted with respect to the bare molecule, while in-plane modes were essentially unchanged. Vibronic bands leading to out-of-plane in the cluster appear to be less intense than in the bare molecule; in contrast, some bands at about $500 cm^{-1}$ are found to have a considerably more intense than the corresponding bare molecule ones. These bands are due to a mode similar to the $\nu_{6}(e_{1m})$ mode of benzene, responsible for the vibronic coupling that makes the $^{1}A_{g} \leftrightarrow ^{1}B_{2u}$ transition weakly allowed. The dissociation energy of the styrene-ammonia cluster was roughly determined, by recording the dispersed emission spectra following excitation of the cluster to different vibronic levels. It was found to be $650 \pm 100$ for $S_{1}$ and $600\pm 100$ for $S_{0}$