Vibrational and vibrational-torsional interactions in the 0–600 cm−1 region of the S1 ← S spectrum of p-xylene investigated with resonance-enhanced multiphoton ionization (REMPI) and zero-kinetic-energy (ZEKE) spectroscopy

Abstract

We assign the 0–600 cm−1 region of the S1 ← S0 transition in p-xylene (p-dimethylbenzene) using resonance-enhanced multiphoton ionization (REMPI) and zero-kinetic-energy (ZEKE) spectroscopy. In the 0–350 cm−1 range as well as the intense origin band, there are a number of torsional and vibration-torsion (vibtor) features. The latter are discussed in more detail in Paper I [A. M. Gardner et al., J. Chem. Phys. 146, 124308 (2017)]. Here we focus on the origin and the 300–600 cm−1 region, where vibrational bands and some vibtor activity are observed. From the origin ZEKE spectrum, we derive the ionization energy of p-xylene as 68200 ± 5 cm−1. The assignment of the REMPI spectrum is based on the activity observed in the ZEKE spectra coupled with knowledge of the vibrational wavenumbers obtained from quantum chemical calculations. We assign several isolated vibrations and a complex Fermi resonance that is found to comprise contributions from both vibrations and vibtor levels, and we examine this via a two-dimensional ZEKE spectrum. A number of the vibrational features in the REMPI and ZEKE spectra of p-xylene that have been reported previously are reassigned and now largely consist of totally symmetric contributions. We briefly discuss the appearance of non-Franck-Condon allowed transitions. Finally, we find remarkably similar spectral activity to that in the related disubstituted benzenes, para-difluorobenzene, and para-fluorotoluene.

Resonance-enhanced multiphoton ionization (REMPI) spectroscopy of p–chlorofluorobenzene

“…In Figure 1 we show an overview of the REMPI spectra of p 35 ClFB together with the corresponding spectra of pFT and pDFB for comparison -regions of the latter two spectra have been presented in previous work [6], [7], [8]. The origin of the S1  S0 transition is measured as 36275±2 cm -1 , and is the same for both isotopologues, within 1 cm -1 as judged from the position and band profiles.…”

“…However, the observed isotopic shifting of the bands suggests the higher wavenumber, weaker contribution at~347 cm -1 is the 11 1 transition for p 35 ClFB, and the lower one, whose maximum is uncertain because of the overlap, is due to 14 2 . Of course, there is the possibility that D11 and 2D14 are interacting, but evidence from other studies suggests that this is weak [4], [6], [8]. Since the D11 vibration shifts, while the D14 overtone is not expected to, it seem that in p 37 ClFB these two transitions are now coincident (and the fact that they are also supports the very weak interaction suggested.)…”

“…The intensity of the 11 1 transition in the S1  S0 transition of substituted benzenes appears to be affected by HT-coupling (see, for example, Ref. [8]) and so it is difficult to be sure of its expected intensity, and similar comments could be made about 14 2 . However, the observed isotopic shifting of the bands suggests the higher wavenumber, weaker contribution at~347 cm -1 is the 11 1 transition for p 35 ClFB, and the lower one, whose maximum is uncertain because of the overlap, is due to 14 2 .…”

“…We note that the calculated wavenumbers have been scaled by 0.97 to correct both for anharmonicity and other weaknesses in the method; additionally, as we have commented on previously [3], [6], [8] the lowest wavenumber a2 vibration (D14 here) does not appear to be described well for substituted benzenes and we do not report values for this vibration, instead relying on the appearance of the spectrum to assign transitions involving this vibration. Further comments on the calculations will be made below.…”

“…Thus, we have recorded resonance-enhanced multiphoton ionization (REMPI) spectra of the three monohalobenzenes, fluorobenzene (FBz), chlorobenzene (ClBz) and bromobenzene (BrBz) [1], [2], [3], toluene (Tol) [4], [5], para-fluorotoluene (pFT) [6], [7] and para-xylene (pXyl) [8], [9]. Central to establishing the common activity, is the use of consistent labelling schemes, which we have described for the monosubstituted benzenes [10] and the para-disubstituted benzenes [11].…”