We have studied the ultrafast relaxation dynamics of aromatic cation radicals following photoionization by time-resolved absorption and Raman spectroscopic techniques. In nonpolar solvents, the absorption due to the cation radical decreases in subpicoseconds, which arises from the geminate recombination of the ejected electrons with their parent cations. In polar solvents, on the other hand, the absorption of the cation radical first decreases in the subpicosecond range, and then increases in tens of picoseconds. The subpicosecond decay is ascribed to the geminate recombination of the unsolvated cation radical in the polar solvents. The picosecond rise is observed in all the cation radicals treated (naphthalene, 1,4-diphenylbutadiene, and biphenyl), suggesting that the picosecond relaxation process increasing the absorption intensity occurs after the photoionization of the aromatic molecules in the polar solvents. The observed picosecond relaxation is discussed in terms of the thermal excitation of the neighboring solvent molecules due to intermolecular vibrational relaxation.
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