Variations in Duschinsky rotations in m-fluorotoluene and m-chlorotoluene during excitation and ionization

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, by two-dimensional laser-induced fluorescence (2D-LIF) and zero-electron-kinetic energy (ZEKE) spectroscopy, respectively; for mClT, only the D₀ ⁺ ← S₁ ionization was investigated, by 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 behavior 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 the 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 vibrational motion on the torsional potential is briefly discussed.