School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom.
J Chem Phys. 2017 Mar 28;146(12):124309. doi: 10.1063/1.4977897.
We assign the 0-600 cm region of the S← S transition in p-xylene (p-dimethylbenzene) using resonance-enhanced multiphoton ionization (REMPI) and zero-kinetic-energy (ZEKE) spectroscopy. In the 0-350 cm 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 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. 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.
我们使用共振增强多光子电离（REMPI）和零动能（ZEKE）光谱学研究了对二甲苯（p-二甲苯）的 S←S 跃迁的 0-600 cm 区域。在 0-350 cm 范围内以及强烈的起源带中，存在许多扭转和振动-扭转（vibtor）特征。后者在 Paper I [A. M. Gardner 等人，J. Chem. Phys. 146, 124308 (2017)]中有更详细的讨论。在这里，我们专注于起源和 300-600 cm 区域，在那里观察到振动带和一些 vibtor 活性。从起源 ZEKE 光谱，我们得出对二甲苯的电离能为 68200 ± 5 cm。REMPI 光谱的分配基于在 ZEKE 光谱中观察到的活性，同时结合从量子化学计算获得的振动波数知识。我们分配了几个孤立的振动和一个复杂的费米共振，该共振被发现由振动和 vibtor 能级的贡献组成，我们通过二维 ZEKE 光谱来检查这一点。先前报道的对二甲苯 REMPI 和 ZEKE 光谱中的一些振动特征已被重新分配，现在主要由完全对称的贡献组成。我们简要讨论了非 Franck-Condon 允许跃迁的出现。最后，我们发现与相关的二取代苯（对二氟苯和对氟甲苯）非常相似的光谱活性。
