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1. Spectroscopic signatures of states in the continuum characterized by a joint experimental and theoretical study of pyrrole

   https://doi.org/10.1063/5.0123603  

   Mukherjee, Madhubani; Ragesh Kumar, T. P.; Ranković, Miloš; Nag, Pamir; Fedor, Juraj; Krylov, Anna I. (November 2022, The Journal of Chemical Physics)

   We report a combined experimental and theoretical investigation of electron–molecule interactions using pyrrole as a model system. Experimental two-dimensional electron energy loss spectra (EELS) encode information about the vibrational states of the molecule as well as the position and structure of electronic resonances. The calculations using complex-valued extensions of equation-of-motion coupled-cluster theory (based on non-Hermitian quantum mechanics) facilitate the assignment of all major EELS features. We confirm the two previously described π resonances at about 2.5 and 3.5 eV (the calculations place these two states at 2.92 and 3.53 eV vertically and 2.63 and 3.27 eV adiabatically). The calculations also predict a low-lying resonance at 0.46 eV, which has a mixed character—of a dipole-bound state and σ* type. This resonance becomes stabilized at one quanta of the NH excitation, giving rise to the sharp feature at 0.9 eV in the corresponding EELS. Calculations of Franck–Condon factors explain the observed variations in the vibrational excitation patterns. The ability of theory to describe EELS provides a concrete illustration of the utility of non-Hermitian quantum chemistry, which extends such important concepts as potential energy surfaces and molecular orbitals to states embedded in the continuum. 

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2. Electron-induced vibrational excitation and dissociative electron attachment in methyl formate

   https://doi.org/10.1039/c9cp05165a  

   Kumar T. P., Ragesh; Kočišek, J.; Bravaya, K.; Fedor, J. (January 2020, Physical Chemistry Chemical Physics)

   null (Ed.)

   We probe the low-energy electron collisions with methyl formate HCOOCH 3 , focusing on its resonant states. Experimentally, we (i) use two-dimensional electron energy loss spectroscopy to gain information about the vibrational excitation and (ii) report the absolute dissociative electron attachment cross sections. The electron scattering spectra reveal both the threshold effects due to the long-range electron–molecule interaction and a pronounced π* resonance centered around 2.1 eV. This resonance gives rise to dissociative electron attachment into three different anionic channels, the strongest one being the production of the formate anion. Theoretically, we characterize this resonant state using the complex absorbing potential approach combined with multistate multireference perturbation theory, which predicts its position and width in excellent agreement with the experiment. 

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3. Exploring electronic resonances in pyridine: Insights from orbital stabilization techniques

   https://doi.org/10.1063/5.0232581  

   Pyla, Maneesh; Matsika, Spiridoula (October 2024, The Journal of Chemical Physics)

   Electron attachment to pyridine results in electronic resonances, metastable states that can decay through electronic or nuclear degrees of freedom. This study uses orbital stabilization techniques combined with bound electronic structure methods, based on equation of motion coupled cluster or multi-reference methods, to calculate positions and widths of electronic resonances in pyridine that exist below 10 eV. We report four 2B1 and four 2A2 resonances, including one 2B1 not previously reported experimentally and two 2A2 resonances not reported at all in the literature. The two lower energy resonances are one-particle shape resonances, while the remaining are mixed or primarily core-excited resonances. Multi-reference perturbation theory provides the best description of these resonances, especially when their character is mixed. We describe the character of these resonances qualitatively and calculate Dyson orbitals, which provide information about their decay channels. 

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4. Influence of hydrogen and oxygen surface termination on the mechanism of sub-bandgap photoelectron emission from diamond(111) into vacuum and into water

   https://doi.org/10.1016/j.diamond.2025.112011  

   Saucedo, Cesar; Rieders, Nathaniel; Hamers, Robert J (January 2025, Diamond and Related Materials)

   Wide-bandgap semiconductors have unique electron emission properties by virtue of having high-lying conduction bands. Among these, diamond stands out because of its chemical stability, allowing it to serve as a solid-state electron source in vacuum and non-vacuum environments, including water. However, the underlying mechanisms of electron emission are not well understood. Here, we report investigations of the mechanisms of electron emission from H-terminated and oxidized surfaces of single-crystal boron-doped diamond(111) in vacuum and in water using both sub-bandgap (4.75 eV and 3.05 eV) and above-bandgap (21.2 eV) excitation. Energy-resolved photoemission spectra in vacuum using different incident photon energies reveal two distinct energy distributions, reflecting different emission pathways. While oxidation greatly reduces electron emission into vacuum using both sub-bandgap and above-bandgap sources, facile electron emission into water persists on the oxidized samples using sub-bandgap excitation and is directly observed through transient optical absorption measurements using sub-bandgap excitation. Low-energy inverse photoemission spectroscopy shows that oxidation leads to broad distribution of surface states throughout the diamond bandgap. Our studies highlight 

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5. On the electronic structure and spin–orbit coupling of BiB from photoelectron imaging of cryogenically-cooled BiB− anion

   https://doi.org/10.1063/5.0170325  

   Gao, Han-Wen; Choi, Hyun Wook; Hui, Jie; Chen, Wei-Jia; Kocheril, G. Stephen; Wang, Lai-Sheng (September 2023, The Journal of Chemical Physics)

   We report a study on the electronic structure and chemical bonding of the BiB molecule using high-resolution photoelectron imaging of cryogenically cooled BiB− anion. By eliminating all the vibrational hot bands, we can resolve the complicated detachment transitions due to the open-shell nature of BiB and the strong spin–orbit coupling. The electron affinity of BiB is measured to be 2.010(1) eV. The ground state of BiB− is determined to be 2Π(3/2) with a σ2π3 valence electron configuration, while the ground state of BiB is found to be 3Σ−(0+) with a σ2π2 electron configuration. Eight low-lying spin–orbit excited states [3Σ−(1), 1Δ(2), 1Σ+(0+), 3Π(2), 3Π(1), 1Π(1)], including two forbidden transitions, [3Π(0−) and 3Π(0+)], are observed for BiB as a result of electron detachment from the σ and π orbitals of BiB−. The angular distribution information from the photoelectron imaging is found to be critical to distinguish detachment transitions from the σ or π orbital for the spectral assignment. This study provides a wealth of information about the low-lying electronic states and spin–orbit coupling of BiB, demonstrating the importance of cryogenic cooling for obtaining well-resolved photoelectron spectra for size-selected clusters produced from a laser vaporization cluster source. 

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