Photochemical timing. Application to intramolecular vibrational redistribution in t-stilbene

Photochemical timing, a spectroscopic method for measuring time-gated fluorescence spectra with picosecond time resolution, is demonstrated in a study of intramolecular vibrational redistribution in the S1 state of supersonically cooled t-stilbene. In the photochemical timing technique, two different color, nanosecond laser pulses are used: the first one prepares the molecule in an S1 vibronic state, while the second one quenches S1 fluorescence by excitation from the S1 state, to a dark Sn state. With this technique, the lifetime of t-stilbene is reduced to <60 ps; this allows observation of fluorescence from the initially excited state in preference to fluorescence from the states populated by vibrational redistribution.