(2+2) Resonance enhanced multiphoton ionization spectra of jet-cooled ortho-, meta- and para-xylene

Two-photon spectra of the S1(Ã)–S0(X˜) transition of jet-cooled ortho-, meta- and para-xylene have been recorded in the region 43,000–37,000 cm−1 using resonance enhanced multiphoton ionization excitation. A full vibronic analysis for all three isomers has been carried out enabling the identification of several excited state vibrational frequencies for the first time. In particular, for ortho and para substitutions, a new, strong two-photon vibronic coupling mode ν19b has been identified, and the significance of this result is discussed. Some anomalies in the assignment of previously published spectra of the xylenes, and related disubstituted benzenes, have been observed and reassignments are suggested. In addition we show that the comparison of one and two-photon spectra provides a method for determining the mechanism by which totally symmetric fundamentals gain their intensity in one-photon spectra, namely either from Herzberg–Teller intensity stealing or due to favourable Franck–Condon overlap.