Two-dimensional laser-induced fluorescence (2D-LIF) spectroscopy is a powerful tool allowing overlapped features in an electronic spectrum to be separated, and interactions between vibrations and torsions to be identified. Here the technique is employed to assign the 790-825 cm -1 region above the origin of the S1  S0 transition in para-fluorotoluene (pFT), which provides insight into the unusual time-resolved results of Davies and Reid [Phys. Rev. Lett. 109, 193004 (2012)]. The region is dominated by a pair of bands that arise from a Fermi resonance; however, the assignment is complicated by contributions from a number of overtones and combinations, including vibration-torsion ("vibtor") levels. The activity in the 2D-LIF spectra is compared to recently reported zero-electron-kinetic-energy (ZEKE) spectra [Tuttle et al. J. Chem. Phys. 146, 244310 (2017)] to arrive at a consistent picture of the energy levels in this region of the spectrum.

We investigate Duschinsky rotation/mixing between three vibrations for both m-fluorotoluene (mFT) and m-chlorotoluene (mClT), during electronic excitation and ionization. In the case of mFT, we investigate both the S1  S0 electronic transition and the D0 +  S1 ionization, using two-dimensional laser-induced fluorescence (2D-LIF) and zero-electron-kinetic energy (ZEKE) spectroscopy, respectively; for mClT, only the D0 +  S1 ionization was investigated, using ZEKE spectroscopy. The Duschinsky mixings are different in the two molecules, owing to shifts in vibrational wavenumber and variations in the form of the fundamental vibrations between the different electronic states. There is a very unusual behaviour for two of the mFT vibrations, where apparently different conclusions for the identity of two S1 vibrations arise from the 2D-LIF and ZEKE spectra. We compare the experimental observations to calculated Duschinsky matrices, finding that these successfully pick up the key geometric changes associated with each electronic transition, and so are successful in qualitatively explaining the vibrational activity in the spectra. Experimental values for a number of vibrations across the S0, S1 and D0 + states are reported and found to compare well to those calculated. Assignments are made for the observed vibration-torsion ("vibtor") bands, and the effect of the vibrational motion on the torsional potential is briefly discussed.

Zero-electron-kinetic-energy (ZEKE) spectra are presented for m-chlorotoluene (mClT), employing different low-lying torsional and vibration-torsional ("vibtor") levels of the S1 state as intermediates. The adiabatic ionization energy (AIE) is determined to be 71319 ± 5 cm -1 (8.8424 ± 0.0006 eV). It is found that the activity in the ZEKE spectra varies greatly for different levels and is consistent with the assignments of the S1 levels of m-fluorotoluene (mFT) deduced in the recent fluorescence study of Stewart et al. [J. Chem. Phys. 150, 174303 (2019)] and the ZEKE study from Kemp et al. [J. Chem. Phys. 151, 084311 (2019)]. As with mFT, the intensities in the ZEKE spectra of mClT are consistent with a phase change in the torsional potential upon ionization, allowing large number of torsions and vibtor levels to be observed for the cation. Vibrationinduced modifications of the torsional potential are discussed. Calculated vibrational wavenumbers for the S0, S1 and D0 + states are also presented.