| Abstract
| - Picosecond time-resolved Raman spectroscopy has been used to study the ultrafast relaxation dynamics ofaromatic cation radicals following two-photon ionization. In acetonitrile, integrated Raman intensities due tothe cation radicals rise in tens of picoseconds, and reach their maxima at a delay time of 40−60 ps from thepump pulse. Such a slow-rise component is observed in all the cation radicals treated (biphenyl, trans-stilbeneand naphthalene), suggesting that the picosecond relaxation process increasing the cation Raman intensitiesoccurs after the photoionization of aromatic molecules. In weak polar solvents such as ethyl acetate, on theother hand, only an instrumental-limited rise (<5 ps) is observed. The rise time of the cation Raman intensitydoes not correlate with the dielectric relaxation time but depends on the polarity of the solvent. This resultindicates 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 equilibriumwith solvents. The time scale of the intermolecular vibrational relaxation is the same as that of the risecomponent of the cation Raman intensity. From these observations, it is suggested that the thermal excitationof the solvent shell disturbs the solvation structure of the cation radical, which causes the observed picosecondchange in the cation Raman intensity.
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