Abstract
Evidence is found showing that coupling with vibration-torsion (“vibtor”) levels of both in-plane and out-of-plane vibrations is instrumental in causing dissipative intramolecular vibrational redistribution (IVR). Both zero-electron-kinetic-energy (ZEKE) spectroscopy and two-dimensional laser-induced fluorescence (2D-LIF) spectroscopy are employed to investigate a series of bands located ∼1200 cm−1 above the S1 ← S0 origin in p-fluorotoluene. Transitions in this wavenumber region have been the focus of a number of studies probing IVR. By recording both ZEKE and 2D-LIF spectra, a prepared S1 population is projected onto both the ground state cation and ground state neutral energy states, respectively, giving added confidence to the assignments. The spectral region under discussion is dominated by a pair of fundamental bands, but for the first time, we present explicit evidence that this is complicated by contributions from a number of overtones and combinations, including vibtor levels. We deduce that very different extents of coupling are present across a 60 cm−1 window of the spectrum, even though the density of states is similar; in particular, one of the fundamentals couples efficiently to the increasing bath of levels, while one does not. We explain this by the influence of serendipitous near-coincidences of same-symmetry levels.
Highlights
The ability to describe the making and breaking of chemical bonds and the flow of energy through a molecule necessitates a knowledge of the internal energy level structure; in particular, vibrations and torsions
Note that we shall refer to previously calculated values (B3LYP/aug-cc-pVTZ) of the vibrational wavenumbers in the three electronic states pertinent to the present study, presented in our previous work – those for the S1 and D0+ states are taken from Ref. 23, while those for the S0 state are from Ref. 41
The data in Ref. 35 is in agreement that there is widespread coupling for 51, and it is deduced that the intramolecular vibrational redistribution (IVR) lifetime for 51 is about a quarter of that for 61, consistent with the more significant underlying unresolved structure in the ZEKE, 2D-laserinduced fluorescence (LIF) and dispersed fluorescence (DF) spectra
Summary
The ability to describe the making and breaking of chemical bonds and the flow of energy through a molecule necessitates a knowledge of the internal energy level structure; in particular, vibrations and torsions. Building on work by Parmenter and coworkers,[1,2,3,4,5,6,7] and the group of Weisshaar,[8,9,10] recent work by the Lawrance group and ourselves has identified that vibration-torsional (“vibtor”) coupling is of key importance in the following para-substituted molecules that contain methyl groups: toluene,[11,12,13,14,15,16,17] pFT18,19,20,21,22,23,24,25,26,27 and para-xylene (pXyl).[25,28,29] Some of the most recent work has employed the technique of two-dimensional laser-induced fluorescence (2D-LIF), which has been reviewed recently.[30]. We have examined the bands close to 400 cm-1 (Ref. 26) and those at ~800 cm-1 (Ref. 27) using 2D-LIF, and compared the results to our earlier ZEKE work. 18,21,23 The assignments of the 2D-LIF spectra indicated that both vibrational interactions and vibration-torsional coupling were occurring
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