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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. Potential energy surfaces of bound and metastable electron-attached states of N2O characterized by a joint experimental and theoretical study

   https://doi.org/10.1063/5.0273585  

   Mukherjee, Madhubani; Baker, Carson; Ranković, Miloš; Nag, Pamir; Fedor, Juraj; Krylov, Anna I (July 2025, The Journal of Chemical Physics)

   We report a combined experimental and theoretical investigation of electron scattering from nitrous oxide (N2O). Experimental two-dimensional electron energy loss spectra (EELS) provide information about vibrational states of a molecule and about potential energy surfaces of anionic resonances. This study reports the EELS measured at 2.5–2.6 eV incident energy. The calculations using complex-valued extensions of equation-of-motion coupled-cluster theory (based on the non-Hermitian quantum mechanics) facilitate the assignment of all major EELS features. Our simulations identified two broad and partially overlapping resonances—one of π* and another of σ* character—located at ∼2.8 and 2.3 eV vertically at the equilibrium geometry of the neutral. Due to the Renner–Teller effect, the π* resonance splits upon bending. The upper state, 2Π, remains linear. The lower state mixes with the σ* configuration, giving rise to the 2A′ resonance, which becomes strongly stabilized at bent geometries (αNNO = 134°), resulting in very low adiabatic electron attachment energy. The calculations estimate the electron affinity of N2O to be −0.140 eV. The 2A′ state is predissociative, with the barrier for the N–O bond dissociation of 0.183 eV. The measured EELS feature sharp vibrational structures at low energy losses, followed by a linear (in logarithmic scale) tail extending to the maximum energy loss. The simulations attribute the sharp features at the low energy loss to the non-resonant excitations and contributions from the cold 2Π resonance. The tail is attributed to the vibrationally hot 2A′ state, and its slope is determined by the excess energy available in this state. 

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3. 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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4. The Electronic Structure of Zirconium and Hafnium Monochalcogenides

   https://doi.org/10.1021/acs.jpca.5c05402  

   F_Romeu, João G; Joyner, Nickolas A; Kaye, Elliot; Peterson, Kirk A; Kawagoe, Thomas T; Stinson, Samantha L; Morse, Michael D; Dixon, David A (October 2025, The Journal of Physical Chemistry A)

   High-level ab initio CCSD(T) and spin-orbit icMRCI+Q calculations were used to predict potential energy curves (PECs) for the lowest-lying states of ZrO, ZrS, HfO, and HfS. The prediction of the ground state is basis set dependent at the icMRCI+Q level for ZrO and ZrS due to the small singlet-triplet splitting between the lowest 1Σ+ and 3Δ states. CCSD(T) with a spin orbit correction predicted the 1Σ+ ground state in agreement with experiment. New all-electron basis sets were developed for Hf to improve the results over those predicted by use of effective core potentials (ECPs) that subsume the 4f electrons into the definition of the core. The use of the new DK-4f basis sets rather than ECPs became more important for HfO and HfS where there is a lack of a good core-valence separation. icMRCI+Q, CCSD(T), and DFT calculations for the spectroscopic parameters of ZrO, ZrS, HfO, and HfS were benchmarked with available experimental data. Bond dissociation energies (BDEs) of these four systems were calculated at the Feller-Peterson-Dixon (FPD) level to be 762.1 (ZrO), 543.5 (ZrS), 803.8 (HfO), and 575.1 kJ/mol (HfS), in excellent agreement with experiment. The HfS BDE was remeasured using the R3PI method, providing an updated experimental measurement of D0(HfS) = 5.978 ± 0.002 eV = 576.8 ± 0.2 kJ/mol. This experimental value, combined with experimental measurements of the ionization energies of Hf and HfS, gives the cationic BDE of D0(Hf+-S) = 5.124 ± 0.002 eV = 494.4 ± 0.2 kJ/mol. 

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5. 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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