Picosecond Time-Resolved Transient Grating Method for Heat Detection: Excited-State Dynamics of FeCl3 and o-Hydroxybenzophenone in Aqueous Solution

Transient grating signals after photoexcitation to electronically excited states of FeCl 3 and o -hydroxybenzophenone ( o -HBP) in aqueous solutions were investigated at various temperatures with short-pulsed laser systems. The observed signals can be expressed by a superposition of the density grating (Dens.G) signal and the temperature grating (Temp.G) signal. Above 4 °C, an acoustic oscillation is predominant in the signal, while the acoustic oscillation disappears and a pure Temp.G signal is observed clearly at 4 °C. The rise profile of the pure Temp.G signal was analyzed in terms of the deactivation from the photoexcited state, and it is found that a very fast thermalization process takes place among the solvent molecules. The lifetime of the charge-transfer state of [Fe(OH)(H 2 O) 5 ] 2+ is determined to be 55 ps from the rise profile of the Temp.G signal. Using a subpicosecond laser system and the population grating signal, two time constants (400 fs and 7.5 ps) are obtained for the dynamics of o -HBP. The dynamics of the photoexcited states of FeCl 3 and o -HBP are also discussed. Since the temporal response of the Temp.G component is 10−100 times shorter than that of the Dens.G signal, this signal can be used for studying ultrafast heat-releasing processes.