Femtosecond Resonance-Enhanced Multiphoton Ionization of Perylene in Hexane. Electronic Excitation of the Radical Cation and Evidence of Hydrogen Abstraction from the Solvent

Abstract

We demonstrate that perylene in solution can be highly excited with relatively weak femtosecond radiation via two-photon resonance-enhanced multiphoton ionization and excitation. The femtosecond multiphoton excitation spectra of two different observables are presented for perylene dissolved in hexane. The first of these is the normal S 1 → S 0 fluorescence that arises from relaxation of perylene through the singlet manifold after multiphoton excitation. The second observable is the ion yield that results from multiphoton ionization. A comparison of these two excitation spectra is presented, and their striking similarity indicates that both processes occur via the same initial two-photon resonance in the 500-550 nm region. A mechanism is presented illustrating the predicted excitation and relaxation pathways. The multiphoton-excited emission spectrum of the sample exhibits anomalous emission in the 550-620 nm range. Comparison of this emission with the one-photon-excited emission spectrum of perylene in concentrated sulfuric acid shows that the anomalous emission originates from electronic excitation of cationic species present in sulfuric acid. Previous assignment of emission from perylene dissolved in sulfuric acid to the protonated cation implies that the perylene radical cation in hexane abstracts a hydrogen atom from the solvent on an ultrafast time scale.