School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom.
J Chem Phys. 2018 Sep 7;149(9):094301. doi: 10.1063/1.5045544.
The vibrations of the ground state cation ( ) of -difluorobenzene (DFB) have been investigated using zero-electron-kinetic-energy (ZEKE) spectroscopy. A comprehensive set of ZEKE spectra were recorded via different vibrational levels of the S state (<0 + 1300 cm). The adiabatic ionization energy for DFB was measured as 73 869 ± 5 cm. Use of different intermediate levels allows different cationic vibrational activity to be obtained via the modification of the Franck-Condon factors for the ionization step, allowing the wavenumbers of different vibrational levels in the cation to be established. In addition, assignment of the vibrational structure in the ZEKE spectra allowed interrogation of the assignments of the S ← S transition put forward by Knight and Kable [J. Chem. Phys. , 7139 (1988)]. Assignment of the vibrational structure has been aided by quantum chemical calculations. In this way, it was possible to assign seventeen of the thirty vibrational modes of the ground state DFB cation. Evidence for complex Fermi resonances in the S state, i.e., those that involve more than two vibrations, was established. One of these was investigated using picosecond time-resolved photoelectron spectroscopy. In addition, we discuss the appearance of several symmetry-forbidden bands in the ZEKE spectra, attributing their appearance to a Rydberg state variation of an intrachannel vibronic coupling mechanism.
采用零电子动能（ZEKE）光谱法研究了-difluorobenzene（DFB）基态阳离子（）的振动。通过 S 态的不同振动能级（<0 + 1300 cm）记录了一套完整的 ZEKE 光谱。DFB 的绝热电离能测量值为 73 869 ± 5 cm。通过修改电离步骤的 Franck-Condon 因子，使用不同的中间能级，可以获得不同的阳离子振动活性，从而确定阳离子中不同振动能级的波数。此外，ZEKE 光谱中振动结构的分配允许对 Knight 和 Kable [J. Chem. Phys., 7139 (1988)]提出的 S ← S 跃迁的分配进行质疑。通过量子化学计算，有助于振动结构的分配。通过这种方式，有可能对基态 DFB 阳离子的三十种振动模式中的十七种进行分配。证实了 S 态存在复杂的费米共振，即涉及两个以上振动的共振。其中一个通过皮秒时间分辨光电子能谱进行了研究。此外，我们还讨论了 ZEKE 光谱中出现的几个对称禁带，将其出现归因于同信道振子耦合机制的里德堡态变化。
