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 ultr afast 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 ti me 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 naphthalen e), 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 in strumental-limited rise (<5 ps) is observed. The rise time of the cation Ra man intensity does not correlate with the dielectric relaxation time but de pends on the polarity of the solvent. This result indicates that the picose cond relaxation process is not controlled by the dielectric solvent relaxat ion alone. The positional changes and the band narrowings of the cation Ram an bands occur on a 10-20 ps time scale. These are associated with intermol ecular vibrational relaxation of the cation radical toward a thermal equili brium with solvents. The time scale of the intermolecular vibrational relax ation is the same as that of the rise component of the cation Raman intensi ty. 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.