Two-photon ionization spectrum of the 1Lb ← S0 transition in toluene

The multiphoton ionization (MPI) spectrum of toluene arising from the ¹B₂ (¹L_b) valence state has been investigated. The state participates as a two-photon resonance. A total of nine excited state fundamentals have been characterized, including three non-totally symmetric vibrations. The toluene MPI spectrum shows a strong resemblance to the two-photon fluorescence excitation spectrum with the strongest transitions taking place to the origin and excited state modes ν₁(a₁), ν₁₂(a₁) and ν₁₄(b)₂). The intensities of the observed fundamentals are rationalized in terms of Franck-Condon and vibronic coupling effects. A major conclusion is, that the primary mechanism for the activity of ν₁₂ is vibronic coupling.