Resonances in low-energy electron scattering from <i>para</i>-benzoquinone

We present detailed ab initio scattering calculations for electron collisions with para-benzoquinone. The R-matrix method has been used to study elastic and electronically inelastic scattering. We have identified 25 resonances of shape, Feshbach, core-excited shape and mixed character between 0 and 8 eV. Agreement of our resonance spectrum with existing literature is discussed, in particular that of the low-lying resonances that participate in the photodetachment process. Integral elastic and total inelastic cross sections are also presented

Electron Collisions with Molecules and Molecular Clusters

State-of-the art computational studies of electron collisions with molecules and small molecular clusters are illustrated with results obtained from the application of the R-matrix method and the UKRMol/UKRMol+ suites. High-level calculations of electronic excitation cross sections and core-excited resonances, mainly of core-excited shape character, show excellent agreement with experiment for mid-size molecules like pyrimidine and thiophene. Simpler calculations are paving the way for an in-depth understanding of the effect of hydration on resonance formation: how the shift in resonance energy depends on the characteristics of the hydrogen bond and the resonance being studied. Finally, applications of the software to a little studied process, interatomic coulombic electron capture are also illustrated

Resonances in molecules and molecular clusters

Resonances play an important role in a number of atomic and molecular processes. Identifying and characterising resonances in electron scattering is essential as they can both enhance a number of processes (e.g. electronic and vibrational excitation) and are crucial in others like dissociative electron attachment and dissociative recombination. We discuss recent theoretical studies of shape and core-excited resonances, both in isolated molecules of biological relevance and in small molecular clusters. The latter are investigated to understand the effect of the environment, in particular hydration, in electron collisions in biological matter

Theoretical study of resonance formation in microhydrated molecules. I. Pyridine-(H₂O)_n, n = 1,2,3,5

We present R-matrix calculations for electron scattering from microhydrated pyridine. We studied the pyridine-H2O cluster at static-exchange (SE), SE + polarization, and close-coupling levels, and pyridine-(H2O)n n = 2, 3, and 5 at SE level only in order to investigate the effect of hydrogen bonding on the resonances of pyridine. We analyse the results in terms of direct and indirect effects. We observe that the total (direct plus indirect) effect of microhydration leads to the stabilization of all resonances studied, both shape and core-excited. The size of the shift is different for different resonances and seems to be linked to the dipole moment of the cluster

Shape and core-excited resonances in electron scattering from alanine

We present detailed ab initio scattering calculations using the R-matrix method for electron collisions with the most stable conformer of α-alanine. The shape resonances that we identify are in good agreement with earlier calculations and experiments. Core-excited and mixed-character resonances are identified and characterized computationally for the first time. Dissociative electron attachment results are discussed in relation to the resonances identified

Electron collisions with BeH₂ below 20 eV

We present calculations of electron scattering from BeH2 using the R-matrix method. Integral and differential elastic and inelastic cross sections for energies up to 20 eV have been determined and the position and width (when possible) of shape and core-excited resonances is reported. The use of Gaussian, B-spline, and mixed bases has been investigated. Results are compared with earlier calculations: it is shown that inclusion of both more target states and higher partial waves affects both the resonant behavior at lower energies and the high-energy behavior of the inelastic cross sections

Effect of the third π∗ resonance on the angular distributions for electron-pyrimidine scattering

We present a detailed analysis of the effect of the well known third π ∗ resonance on the angular behaviour of the elastic cross section in electron scattering from pyrimidine. This resonance, occurring approximately at 4.7 eV, is of mixed shape and core-excited character. Experimental and theoretical results show the presence of a peak/dip behaviour in this energy range, that is absent for other resonances. Our investigations show that the cause of the peak/dip is an interference of background p-wave to p-wave scattering amplitudes with the amplitudes for resonant scattering. The equivalent resonance in pyrazine shows the same behaviour and the effect is therefore likely to appear in other benzene-like molecules

Theoretical study of resonance formation in microhydrated molecules. II. Thymine-(H₂O)_<i>n</i>, n = 1,2,3,5

We have investigated the effect of microsolvation on the low-lying pure shape π* resonances of thymine. Static-exchange R-matrix calculations for elastic electron scattering from microhydated thymine, i.e., Thy-(H2O)π with π = 1,2,3,5 are discussed. We look at the additive effect of water molecules hydrogen-bonding to thymine. The results for Thy-(H2O)5 show that both π* resonances appear at lower energy in the cluster than in isolated thymine, but that the energy shift is different for each resonance. We discuss how our results could help explain the quenching of hydrogen loss in dissociative electron attachment of microhydrated thymine recently recorded experimentally

reskit: a toolkit to determine the poles of an S-matrix

We present the python package reskit, developed to calculate the coefficients of an analytical continuation of the S-matrix of a physical system by means of Padé approximants and, from this fit, determine the complex poles of this S-matrix. The current implementation of the program is restricted to elastic scattering, i.e. cases in which all channels have the same energy. It has been tested using as input ab initio scattering data for electron-molecule collisions obtained for energies along the real axis. The identification and characterisation of the resonances present in the systems using reskit is described and discussed

Interatomic Coulombic electron capture from first principles

Interatomic Coulombic electron capture (ICEC) is an environment-assisted process in which a free electron can efficiently attach to an ion, atom or molecule by transferring the excess energy to a neighboring species. Absolute cross sections are necessary to evaluate the relative importance of this process. In this work, we employ the R-matrix method to compute ab initio these cross sections for a singly charged neon ion embedded in small helium clusters. Our results show that the ICEC cross sections are several orders of magnitude higher than anticipated and dominate other competing processes. Electron energy loss spectra on an absolute scale are provided for the Ne+@He20 cluster. Such spectra exhibit an unambiguous signature of the ICEC process. The finding is expected to stimulate experimental observations