Zhang Yue-Rou, Yuan Dao-Fu, Qian Chen-Hui, Wang Lai-Sheng
Department of Chemistry, Brown University, Providence, Rhode Island 02912, USA.
J Chem Phys. 2021 Sep 28;155(12):124305. doi: 10.1063/5.0065510.
Negative ions do not possess Rydberg states but can have Rydberg-like nonvalence excited states near the electron detachment threshold, including dipole-bound states (DBSs) and quadrupole-bound states (QBSs). While DBSs have been studied extensively, quadrupole-bound excited states have been more rarely observed. 4-cyanophenoxide (4CP) was the first anion observed to possess a quadrupole-bound exited state 20 cm below its detachment threshold. Here, we report the observation of a DBS in the isoelectronic 4-ethynylphenoxide anion (4EP), providing a rare opportunity to compare the behaviors of a dipole-bound and a quadrupole-bound excited state in a pair of very similar anions. Photodetachment spectroscopy (PDS) of cryogenically cooled 4EP reveals a DBS 76 cm below its detachment threshold. Photoelectron spectroscopy (PES) at 266 nm shows that the electronic structure of 4EP and 4CP is nearly identical. The observed vibrational features in both the PDS and PES, as well as autodetachment from the nonvalence excited states, are also found to be similar for both anions. However, resonant two-photon detachment (R2PD) from the bound vibrational ground state is observed to be very different for the DBS in 4EP and the QBS in 4CP. The R2PD spectra reveal that decays take place from both the DBS and QBS to the respective anion ground electronic states within the 5 ns detachment laser pulse due to internal conversion followed by intramolecular vibrational redistribution and relaxation, but the decay mechanisms appear to be very different. In the R2PD spectrum of 4EP, we observe strong threshold electron signals, which are due to detachment, by the second photon, of highly rotationally excited anions resulted from the decay of the DBS. On the other hand, in the R2PD spectrum of 4CP, we observe well-resolved vibrational peaks due to the three lowest-frequency vibrational modes of 4CP, which are populated from the decay of the QBS. The different behaviors of the R2PD spectra suggest unexpected differences between the relaxation mechanisms of the dipole-bound and quadrupole-bound excited states.
负离子不具有里德堡态，但在电子脱离阈值附近可具有类里德堡非价激发态，包括偶极束缚态（DBSs）和四极束缚态（QBSs）。虽然DBSs已得到广泛研究，但四极束缚激发态却较少被观测到。4-氰基苯氧基（4CP）是首个被观测到在其脱离阈值下方20厘米处具有四极束缚激发态的阴离子。在此，我们报告了在等电子的4-乙炔基苯氧基阴离子（4EP）中观测到一个DBS，这提供了一个难得的机会来比较一对非常相似的阴离子中偶极束缚激发态和四极束缚激发态的行为。对低温冷却的4EP进行光电子能谱（PDS）分析，发现其在脱离阈值下方76厘米处有一个DBS。266纳米处的光电子能谱（PES）表明4EP和4CP的电子结构几乎相同。在PDS和PES中观测到的振动特征，以及从非价激发态的自脱离，在这两种阴离子中也被发现是相似的。然而，观测到4EP中的DBS和4CP中的QBS从束缚振动基态的共振双光子脱离（R2PD）有很大不同。R2PD光谱显示，由于内转换，随后是分子内振动再分布和弛豫，在5纳秒的脱离激光脉冲内，DBS和QBS都会衰变到各自的阴离子基态电子态，但衰变机制似乎非常不同。在4EP的R2PD光谱中，我们观测到强烈的阈值电子信号，这是由于第二个光子使DBS衰变产生的高度旋转激发的阴离子发生脱离所致。另一方面，在4CP的R2PD光谱中，我们观测到由4CP的三个最低频率振动模式产生的分辨良好的振动峰，这些振动模式是由QBS的衰变所填充的。R2PD光谱的不同行为表明偶极束缚激发态和四极束缚激发态的弛豫机制存在意想不到的差异。
