美国布朗大学Lai-Sheng Wang团队近日取得一项新成果。经过不懈努力，他们观察到对称性禁阻的激发态四极束缚态。 这一研究成果于2020年11月13日发表在国际顶尖学术期刊《美国化学会志》上。

研究人员报道了使用光电子和光分离光谱，在低温冷却的四苯阴离子(TCNB^–)中观察到对称性禁阻的激发四极束缚态(QBS)。TCNB的电子亲和力精确测量为2.4695 eV。TCNB^–的光分离光谱揭示了选定的对称性允许的向QBS的振动跃迁，但未观察到基态振动态，因为从TCNB^–（Au对称）的基态到QBS（Ag对称）的跃迁是电偶极子、磁偶极子和电四极子三重禁阻的。QBS的结合能为0.2206 eV，由于强相关和极化效应，结合能异常大。

在接近阈值的自动分离时观察到一个离心势垒，以及从QBS振动电子水平到地面的弛豫和TCNB^–的价态激发态。目前的研究显示了一个罕见的例子，对称选择规则，而不是Franck-Condon原则，支配振动跃迁到一个阴离子的非价态。

附：英文原文

Title: Observation of a Symmetry-Forbidden Excited Quadrupole-Bound State

Author: Yuan Liu, Guo-Zhu Zhu, Dao-Fu Yuan, Chen-Hui Qian, Yue-Rou Zhang, Brenda M. Rubenstein, Lai-Sheng Wang

Issue&Volume: November 13, 2020

Abstract: 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– (Au symmetry) to the QBS (Ag 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.

DOI: 10.1021/jacs.0c10552

Source: https://pubs.acs.org/doi/10.1021/jacs.0c10552