Vibrations of the low energy states of toluene (⁠|$\tilde X$|$X̃$ ¹A₁ and |$\tilde A$|$Ã$ ¹B₂) and the toluene cation (⁠|$\tilde X$|$X̃$ ²B₁)

We commence by presenting an overview of the assignment of the vibrational frequencies of the toluene molecule in its ground (S₀) state. The assignment given is in terms of a recently proposed nomenclature, which allows the ring-localized vibrations to be compared straightforwardly across different monosubstituted benzenes. The frequencies and assignments are based not only on a range of previous work, but also on calculated wavenumbers for both the fully hydrogenated (toluene-h₈) and the deuterated-methyl group isotopologue (α₃-toluene-d₃), obtained from density functional theory (DFT), including artificial-isotope shifts. For the S₁ state, one-colour resonance-enhanced multiphoton ionization (REMPI) spectroscopy was employed, with the vibrational assignments also being based on previous work and time-dependent density functional theory (TDDFT) calculated values; but also making use of the activity observed in two-colour zero kinetic energy (ZEKE) spectroscopy. The ZEKE experiments were carried out employing a (1 + 1^′) ionization scheme, using various vibrational levels of the S₁ state with an energy <630 cm^‑1 as intermediates; as such we only discuss in detail the assignment of the REMPI spectra at wavenumbers <700 cm^‑1, referring to the assignment of the ZEKE spectra concurrently. Comparison of the ZEKE spectra for the two toluene isotopologues, as well as with previously reported dispersed-fluorescence spectra, and with the results of DFT calculations, provide insight both into the assignment of the vibrations in the S₁ and D₀⁺ states, as well as the couplings between these vibrations. In particular, insight into the nature of a complicated Fermi resonance feature at ∼460 cm^‑1 in the S₁ state is obtained, and Fermi resonances in the cation are identified. Finally, we compare activity observed in both REMPI and ZEKE spectroscopy for both toluene isotopologues with that for fluorobenzene and chlorobenzene.