PICOSECOND DYNAMICS OF N-HEXANE SOLVATED TRANS-STILBENE

In this article, we report studies of the time- and frequency-resolved picosecond dynamics of trans-stilbene solvated with one n-hexane mole cule in a van der Waals complex. Excitations are made to several overt ones of a low-frequency intermolecular vibration and to combination ba nds of these overtones with the symmetric in-plane ethylene bend mode v25 in the S1 state of two distinct conformers, A and C, of these comp lexes. A Franck-Condon analysis of these spectra allows a characteriza tion of the potential for this intermolecular mode in the ground and e xcited electronic state of both conformers. From the time-resolved dyn amics of the A-isomer, the three different types of IVR behavior in th e cluster, as in the bare molecule, are identified and studied. Single exponential fluorescence decays are observed at low excess energies, showing no IVR on the time scale of the lifetime of the molecule. At i ntermediate energies, between 220 and 260 cm-1, quantum-beat modulated decay profiles indicate restricted IVR dynamics. At higher ''cess ene rgies IVR becomes dissipative and biexponential decays are observed. T he decrease in the lowest excess energy at which dissipative IVR behav ior sets in, from 1200 cm-1 in the bare t-stilbene molecule to less th an 300 cm-1 in the complex, is attributed to a large increase in the d ensity of vibrational states of the complex due to the six low-frequen cy intermolecular vibrations of the cluster. A similar study of the C- isomer reveals that the IVR dynamics become even more accelerated. Exc itation of the stilbene mode v25 at 200 cm-1 leads to dissipative ener gy flow from the stilbene molecule into the cluster vibrations within tens of picoseconds, becoming faster for higher excess energies. The r esults present a nice example of the significant impact that low-frequ ency ''solvent-like'' cluster vibrations have on the dynamics of vibra tional energy redistribution and the coherent transfer of vibrational energy between the solute and the cluster modes.