Time-resolved Raman studies of photoionization of aromatic compounds in polar solvents: Picosecond relaxation dynamics of aromatic cation radicals

Picosecond time-resolved Raman spectroscopy has been used to study the ultrafast relaxation dynamics of aromatic cation radicals following two-photon ionization. In acetonitrile, integrated Raman intensities due to the cation radicals rise in tens of picoseconds, and reach their maxima at a delay time of 40-60 ps from the pump pulse. Such a slow-rise component is observed in all the cation radicals treated (biphenyl, trans-stilbene and naphthalene), suggesting that the picosecond relaxation process increasing the cation Raman intensities occurs after the photoionization of aromatic molecules. In weak polar solvents such as ethyl acetate, on the other hand, only an instrumental-limited rise (<5 ps) is observed. The rise time of the cation Raman intensity does not correlate with the dielectric relaxation time but depends on the polarity of the solvent. This result indicates that the picosecond relaxation process is not controlled by the dielectric solvent relaxation alone. The positional changes and the band narrowings of the cation Raman bands occur on a 10-20 ps time scale. These are associated with intermolecular vibrational relaxation of the cation radical toward a thermal equilibrium with solvents. The time scale of the intermolecular vibrational relaxation is the same as that of the rise component of the cation Raman intensity. From these observations, it is suggested that the thermal excitation of the solvent shell disturbs the solvation structure of the cation radical, which causes the observed picosecond change in the cation Raman intensity.