Microscopic Mechanism of Solute−Solvent Energy Dissipation Probed by Picosecond Time-Resolved Raman Spectroscopy

TLDR

Based on the observed correlation between cooling rates and solvent thermal diffusivity, the authors propose a simple numerical model for solute–solvent energy dissipation in solution. They analyze the cooling kinetics with this macroscopic numerical model, treating the process as a two‑step energy transfer from the excited solute to the nearest solvent molecules and then to the bulk. The results demonstrate that photoexcited trans‑stilbene transfers excess energy to adjacent solvent molecules within a few picoseconds, and the overall dissipation rate is governed by the solvent’s thermal diffusivity, as confirmed by picosecond Raman thermometry.

Abstract

The excess energy dissipation process of photoexcited S1 trans-stilbene in solution has been studied with picosecond time-resolved Raman spectroscopy. The peak position of the 1570-cm-1 band (CC stretch) is shown to be useful as an indicator of picosecond temperature changes; a picosecond time-resolved Raman spectrometer can be regarded as a "picosecond Raman thermometer". The cooling rates of S1 trans-stilbene thus observed in 10 different solvents show a strong correlation with the thermal diffusivities of the bulk solvents. Based on this observation, a simple numerical model is proposed for the solute−solvent energy dissipation process in solution. The observed cooling kinetics are analyzed with this macroscopic model. It is concluded that the excess energy is first shared among the solute and the nearest solvent molecules in a few picoseconds or faster. The further heat conduction to outer-sphere solvent molecules determines the whole dissipation rate, which explains the observed correlation between the vibrational cooling rate and the thermal diffusivity of the solvent.