Abstract
Negative ions do not possess Rydberg states, but polar anions may have diffuse dipole-bound states just below the detachment threshold, analogous to Rydberg states of neutral molecules. Excitation to vibrational levels of the dipole-bound state can induce autodetachment via vibronic coupling. The resulting resonant photoelectron spectrum is highly non-Franck-Condon and yields much richer vibrational information than conventional photoelectron spectroscopy. We developed an experimental apparatus integrating an electrospray ionization source with photoelectron spectroscopy, which allows negative ions from solution samples to be studied in the gas phase. Subsequent development of a cryogenically-cooled Paul trap to create cold anions from electrospray has allowed high-resolution photoelectron imaging to be conducted for complex molecular anions,1 opening opportunities to probe dipole-bound excited states using photodetachment spectroscopy and resonant photoelectron imaging.2 I will present recent advances in our investigation of dipole-bound excited states, including the observation of p-type dipole-bound states,3 and polarization-assisted dipole-bound states.4,5 I will also present observation of shape-resonances and bound electronic excited states in large polycyclic aromatic hydrocarbon (PAH) anions.6,7
1. L. S. Wang, Electrospray Photoelectron Spectroscopy: From Multiply-Charged Anions to Ultracold Anions. J. Chem. Phys. 143, 040901 (2015).
2. G. Z. Zhu and L. S. Wang, High-Resolution Photoelectron Imaging and Resonant Photoelectron Spectroscopy via Noncovalent-Bound Excited States of Cryogenically-Cooled Anions. Chem. Sci. 10, 9409-9423 (2019).
3. D. F. Yuan, Y. Liu, C. H. Qian, Y. R. Zhang, B. M. Rubenstein, and L. S. Wang, Observation of a p-Type Dipole-Bound State in Molecular Anions. Phys. Rev. Lett. 125, 073003 (2020).
4. D. F. Yuan, Y. Liu, Y. R. Zhang, and L. S. Wang, Observation of a Polarization-Assisted Dipole-Bound State. J.Am. Chem. Soc. 145, 5512-5522 (2023).
5. Y. R. Zhang, D. F. Yuan, C. H. Qian, G. Z. Zhu, and L. S. Wang, The Role of Polarization Interactions in the Formation of Dipole-Bound States. J. Am. Chem. Soc. 145, 14952-14962 (2023).
6. J. Kang, E. I. Brewer, Y. R. Zhang, D. F. Yuan, G. S. Kocheril, and L. S. Wang, Observation of Bound Valence Excited Electronic States of Deprotonated 2-Hydroxytriphenylene using Photoelectron, Photodetachment, and Resonant Two-Photon Detachment Spectroscopy of Cryogenically-Cooled Anions. J. Chem. Phys. 160, 184301 (2024).
7. J. Kang, E. I. Brewer, Y. R. Zhang, D. F. Yuan, S. C. Tian, W. Roberts, and L. S. Wang, Photoelectron and Photodetachment Spectroscopy of Cryogenically-Cooled 2-Anthrolide Anion. J. Chem. Phys. 162, 194302 (2025).
