The Influence of Vibrational Excitation on the Photoisomerization of trans-Stilbene in Solution

Physical Chemistry PhD Thesis from the University of Wisconsin Madison, 2010.

 

Abstract:

We prepare electronically excited trans-stilbene molecules in deuterated chloroform using both
one-photon excitation and excitation through an intermediate vibrational state to explore the influence of vibrational energy on excited-state isomerization in solution. After infrared excitation of either two quanta of C-H stretch vibration |2νCH> or the C-H stretch-bend combination |νCH+νbend> in the ground electronic state, an ultraviolet photon intercepts the vibrationally excited molecules during the course of vibrational energy flow and promotes them to the electronically excited state. The energy of the infrared and ultraviolet photons together is the same as that added in the one-photon excitation. Transient broadband continuum absorption monitors the lifetime of electronically excited molecules. The lifetime of excited state trans-stilbene after one-photon electronic excitation is indistinguishable from the excited-state lifetimes for the cases of excitation through |2νCH> and |νCH+νbend>. Vibrational relaxation in the electronically excited state prepared by the two-photon excitation scheme is most likely faster than the barrier crossing, making the isomerization insensitive to the method of initial state preparation.

We show preliminary attempts to monitor vibrational relaxation with femtosecond stimulated Raman spectroscopy. In these studies, we prepare a C-H stretch overtone |2νCH> in the neat solvent acetonitrile or cyclohexane and probe the transient dynamics with a narrowband Raman pump and a broadband Raman probe. Previous work measured the static Raman spectrum of cyclohexane and we build upon this study by using Raman to probe a non-equilibrium distribution of vibrational states and developing near-resonance Raman probing to increase the signal strength.