THERMALIZATION AFTER PHOTOEXCITATION TO THE S-2 STATE OF TRANS-AZOBENZENE IN SOLUTION

A simple method to measure a rate of the translational temperature inc rease after the nonradiative transition with a few ps time resolution is presented. This method uses an acoustic peak shift of the transient grating signal, which can be determined accurately, and is very sensi tive to rates and the relative amount of the thermal energy from the n onradiative transition. Using this method, the thermalization rate aft er the photoexcitation of trans-azobenzene to the S-2(pi pi) State is measured. The acoustic peak shift indicates that the temperature of t he solvents rises within less than similar to 3 ps after the nonradiat ive transition of the S-2 --> S-1 process. The thermalization after th e S-1 --> S-0 transition completes faster than the decay of the S-1 st ate (16 ps in acetonitrile and 18 ps in ethanol). The fast energy tran sfer from the solute to the solvent is discussed. (C) 1998 American In stitute of Physics.