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

Abstract

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 vibrational states of the molecule as well as position and structure of electronic resonances. The calculations using non-Hermitian extensions of equation-of-motion coupled-cluster theory 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 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 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.

Electron Energy Loss Processes in Methyl Methacrylate: Excitation and Bond Breaking

“…Resonances are states embedded in a continuum, and their proper theoretical characterization requires either scattering calculations 50 or extensions of quantum chemistry methods. 54 To assign the peaks in Figure 4 , we used the empirical scaling method of Gallup and Chen, 55 which approximately relates energies of virtual orbitals with the expected positions of corresponding resonances (vertical attachment energies (VAEs)). Figure 5 shows VAEs of MMA obtained by such scaling.…”

Analytic Evaluation of Nonadiabatic Couplings within the Complex Absorbing Potential Equation-of-Motion Coupled-Cluster Method

“…An important advantage of CAP-EOM-CC for the study of anionic resonances is that the ground and excited states of the neutral molecule and the bound and resonance states of the corresponding anion are treated on the same footing. CAP-EOM-CC analytic gradients have been used to compute adiabatic electron affinities of temporary anions, to locate minimum-energy crossing points between anionic and neutral PESs, to characterize exceptional points, and most recently to investigate the electron-energy loss spectrum of pyrrole …”

Irradiation of Plutonium Tributyl Phosphate Complexes by Ionizing Alpha Particles: A Computational Study

“…The application of complex boundary conditions was extended to the Schrödinger equation, , and then to the time-dependent Schrödinger equation applied on molecular systems to deal with diffuse reactions such as collisions between atoms. , In this context, CAP have been introduced as an alternative to mask functions . By adding a complex component to the system’s Hamiltonian, the electron density is gradually absorbed beyond a threshold distance. ,,− CAPs have been shown to be very effective when coupled with various electronic structures methods such as multiconfigurational methods, − coupled-cluster methods, − Lanczos-based approach, and Electron Attachment Algebraic-Diagrammatic Construction (EA-ADC) method in the study of metastable resonance states for anionic molecular systems. Its applications are extended in the generation of high harmonic generation (HHG) spectra with RT-TD-DFT, other Time-Dependent methods (TD-CIS, TD-SE, TD-CI), − or in non-Hermitian exceptional point (EPs) degeneracies .…”