Abstract
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 is determined to be 71 319 cm-1 ± 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 a large number of torsions and vibtor levels to be observed for the cation. Vibration-induced modifications of the torsional potential are discussed. Calculated vibrational wavenumbers for the S0, S1, and D0 + states are also presented.
Highlights
Energy flow in molecules is generally accepted as being facilitated by the coupling of both methyl torsion and vibrational motions and so is important for understanding the photophysics of molecules.[1,2] A very recent example highlights the role vibrational excitation has in light harvesting.[3]
It may be seen that the gas phase dispersed fluorescence (DF) values[21] for the S0 state agree well with earlier infrared and Raman values. Both the DF and laser-induced fluorescence (LIF) values for the S0 and S1 states are in good agreement with the calculated values
Vibtor levels involving 301 In Figure 4 we show the ZEKE spectra recorded via 301m1, 301m4, and 301m3(-), and compare to the spectrum obtained via m4
Summary
Energy flow in molecules is generally accepted as being facilitated by the coupling of both methyl torsion and vibrational motions and so is important for understanding the photophysics of molecules.[1,2] A very recent example highlights the role vibrational excitation has in light harvesting.[3] Understanding the processes occurring in complicated molecules is greatly aided by detailed studies on small molecules, and recent examples from our, the Reid and Lawrance groups have looked at toluene,[4,5,6] para-fluorotoluene (pFT)[7,8,9,10,11,12,13,14,15] and para-xylene (pXyl),[10,16,17] using a combination of fluorescence and photoionization spectroscopies These studies have elucidated how vibration-vibration and vibration-torsion coupling can drive the transition to statistical (“dissipative”) intramolecular vibrational redistribution (IVR), underpinning energy dispersal and photostability.[10,12]. The present work on mClT builds upon the work of Ichimura et al.,[21] who recorded laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectra, and of Feldgus et al.[22] who have reported a resonance-enhanced multiphoton ionization (REMPI) spectrum and zero-electron-kinetic-energy (ZEKE) spectra via a handful of the lowest-wavenumber S1 levels
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