CIS-STILBENE PHOTOCHEMISTRY - DIRECT OBSERVATION OF PRODUCT FORMATION AND RELAXATION THROUGH 2-COLOR UV PUMP-PROBE RAMAN-SPECTROSCOPY

The formation of hot trans-stilbene by photoisomerization of cis-stilbene, and its subsequent vibrational cooling, have been monitored by picosecond two-color UV pump-probe anti-Stokes Raman spectroscopy. Pairs of pump (295 nm) and probe (278 nm) pulses at a 500 Hz repetition rate interrogate flowing samples of cis-stilbene in cyclohexane or methanol, and the anti-Stokes Raman scattering due mainly to the trans-stilbene product is detected. Hot ground-state trans-stilbene appears promptly within our 10 ps experimental time resolution, and the anti-Stokes intensity decays with a time constant of 39 +/- 10 ps in cyclohexane and 17 +/- 5 ps in methanol. The decay of the anti-Stokes intensity is accompanied by a shift of the C=C stretching vibrations by about 20 cm-1 to higher frequencies. The question of whether or not the initial vibrational distribution is statistical is addressed, but the present data do not allow firm conclusions to be drawn.