Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States.
J Am Chem Soc. 2020 Nov 25;142(47):20240-20246. doi: 10.1021/jacs.0c10552. Epub 2020 Nov 13.
We report the observation of a symmetry-forbidden excited quadrupole-bound state (QBS) in the tetracyanobenzene anion (TCNB) using both photoelectron and photodetachment spectroscopies of cryogenically-cooled anions. The electron affinity of TCNB is accurately measured as 2.4695 eV. Photodetachment spectroscopy of TCNB reveals selected symmetry-allowed vibronic transitions to the QBS, but the ground vibrational state was not observed because the transition from the ground state of TCNB ( symmetry) to the QBS ( symmetry) is triply forbidden by the electric and magnetic dipoles and the electric quadrupole. The binding energy of the QBS is found to be 0.2206 eV, which is unusually large due to strong correlation and polarization effects. A centrifugal barrier is observed for near-threshold autodetachment, as well as relaxations from the QBS vibronic levels to the ground and a valence excited state of TCNB. The current study shows a rare example where symmetry selection rules, rather than the Franck-Condon principle, govern vibronic transitions to a nonvalence state in an anion.
我们使用低温冷却阴离子的光电离和光电子谱观察到了四氰基苯阴离子（TCNB）中一个对称禁戒的激发四极束缚态（QBS）。TCNB 的电子亲和能被准确地测量为 2.4695 eV。TCNB 的光电子谱揭示了选定的对称允许的振动态跃迁到 QBS，但由于从 TCNB（对称）的基态跃迁到 QBS（对称）的跃迁被电偶极和磁偶极以及电四极矩三重禁戒，因此没有观察到基态振动。QBS 的结合能被发现为 0.2206 eV，由于强关联和极化效应，这是异常大的。在近阈自动离解时观察到离心势垒，以及从 QBS 振动态到 TCNB 的基态和价激发态的弛豫。当前的研究表明了一个罕见的例子，其中对称选择规则而不是 Franck-Condon 原理控制了阴离子中非价态的振动态跃迁。
