Multiphoton ionization spectrum of fluorobenzene

Abstract

The multiphoton ionization (MPI) spectrum of fluorobenzene vapor has been recorded in the one-photon wavelength region 3600–5500 Å. The spectrum contains one valence and several Rydberg band systems. A Rydberg transition terminating in a 3s orbital is observed as a two-photon resonance around 3900 Å (origin at 50914 cm −1); three-proton resonances in p and d or f orbitals occur in the region 4100–4600 Å and around 5400 Å. An extensive band system with the origin near 5300 Å represents the 1B 2(L b ← 1A 1(S 0) valence transition as a two-photon resonance v 11, v 12, v 14 and v 16, are observed as coupling modes. The spectrum strongly supports a ground state mechanism for the intensity of the dominant vibronic coupling mode, v 14, in benzene and its derivatives. The spectrum also eliminates Herzberg—Teller schemes involving A 28 excited states as major mechanisms for the activity of the important coupling mode v 12 in substituted benzenes. There is evidence for interference between vibronic and Franck—Condon intensity contributions for this mode. A general conclusion is that σ-electron contributions to the two-photon absorption amplitude in benzene and its derivatives are unimportant compared to those due to π-electrons.