Multiphoton ionization and two-photon excitation of aromatic molecules in liquids

The two-photon excitation of benzene and toluene in a number of different nonpolar liquids (n-pentane, tetramethylsi1ane, perfluoro-n-hexane, perfluoromethylcyclohexane, and perfluorodimethylcyclobutane) was studied over the 360-560 nm laser excitation wavelength range. The effect of the solvent on the upper excited states was investigated and a peak, observed in the two-photon excitation spectra of benzene, was assigned to the (3s)¹E_1g Rydberg state based on the unusual solvent dependence of its relative intensity and spectral shift. Polarization studies and signal vs. laser intensity measurements were also carried out.

Vibrational analyses of the entire range of the ¹B_2u state of benzene and of the ¹B₂ state of toluene, including the hot bands, were performed based on the spectra measured in perfluoro-n-hexane. The 0-0 transitions of these states in perfluoro-n-hexane and n-pentane were observed at 38050 cm^-1 and 37853 cm^-1 for benzene and at 37411 cm^-1 and 37220 cm^-1 for toluene, respectively.

The two-photon excitation spectrum of fluoranthene in a solution of n-pentane over the 420-860 nm laser excitation range was also measured. The 0-0 transition of the first singlet state was observed at 24800 cm^-1, resolving thus the ambiguity about the exact position of that state.

The multiphoton ionization of benzene in dilute solutions of n-pentane and tetramethylsilane was also studied, over the 360-570 nm laser excitation range. Polarization studies and signal vs. laser intensity measurements were performed and the dependence of the spectral features and of the polarization ratio on the laser intensity was studied.

The multiphoton ionization mechanism, the location of the ionization threshold of benzene in n-pentane and in tetramethylsilane and the effect of the medium on the ionization threshold are discussed. A particular ionization behavior was observed in a wavelength region corresponding to the channel 3 relaxation which resulted in a dependence of the apparent order of the ionization process on the laser intensity.

The ionization threshold of benzene in n-pentane and tetramethylsilane, based on the results of the signal vs. intensity measurements, was estimated to lie between 7.35 and 6.8 eV and 6.6 and 5.9 eV, respectively.