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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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This content will become publicly available on July 14, 2026
Potential energy surfaces of bound and metastable electron-attached states of N2O characterized by a joint experimental and theoretical study
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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- Award ID(s):
- 2154482
- PAR ID:
- 10647925
- Publisher / Repository:
- The Journal of Chemical Physics
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 163
- Issue:
- 2
- ISSN:
- 0021-9606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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