Virtual photon exchange vs electron transfer in interparticle Coulombic electron capture.

We have investigated Interparticle Coulombic Electron Capture (ICEC) using an ab initio approach for two systems, H+ + H2O and H + H2O+. In this work, we have determined the contribution of virtual photon exchange and electron transfer to the total ICEC cross section as a function of the distance between the charged and neutral particles. Furthermore, we have shown that the relative orientation of the electron acceptor and neighbor systems affects the magnitude of the ICEC cross sections by at least two orders at relatively small distances. This geometry dependence, present even for distances as large as 10 a0, is due to the electron transfer contribution. The relative magnitude of each contribution to ICEC seems to depend on the system studied. By replacing the projectile electron with a positron, we have confirmed that electron transfer also takes place in positron collisions and that the charge of the projectile has a noticeable effect on the process, particularly at low scattering energies.

Resonant Fragmentation of the Water Cation by Electron Impact: a Wave-Packet Study.

We have investigated the dissociation of a resonant state that can be formed in low energy electron scattering from H2 O+ . We have chosen the second triplet resonance above the B ˜ 2 A ' ${{{\tilde{\rm {B}}}}\;^2 {\rm{A{^\prime}}}}$ ( B ˜ 2 B 2 ) ${{\rm{(\tilde{B}}}\;^2 {\rm{B}}_2 )}$ state of H2 O+ whose autoionization mainly produces H2 O+ ( X ˜ 2 A ' ' ${{{\tilde{\rm {X}}}}\;^2 {\rm{A{^\prime}{^\prime}}}}$ ). We have considered both dissociation of the resonant state itself, dissociative recombination (DR), or the dissociation of the H2 O+ cation after autodetachment, dissociative excitation (DE). The time-evolution of a wave packet on the potential energy surfaces of the resonance and cationic states shows, for the initial conditions studied, that the probability for DR is about 38 % while the probability for DE is negligible.

A Complete Cross Section Data Set for Electron Scattering by Pyridine: Modelling Electron Transport in the Energy Range 0-100 eV.

Electron scattering cross sections for pyridine in the energy range 0-100 eV, which we previously measured or calculated, have been critically compiled and complemented here with new measurements of electron energy loss spectra and double differential ionization cross sections. Experimental techniques employed in this study include a linear transmission apparatus and a reaction microscope system. To fulfill the transport model requirements, theoretical data have been recalculated within our independent atom model with screening corrected additivity rule and interference effects (IAM-SCAR) method for energies above 10 eV. In addition, results from the R-matrix and Schwinger multichannel with pseudopotential methods, for energies below 15 eV and 20 eV, respectively, are presented here. The reliability of this complete data set has been evaluated by comparing the simulated energy distribution of electrons transmitted through pyridine, with that observed in an electron-gas transmission experiment under magnetic confinement conditions. In addition, our representation of the angular distribution of the inelastically scattered electrons is discussed on the basis of the present double differential cross section experimental results.

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.

Cross sections for electron scattering from thiophene for a broad energy range.

We present cross sections for elastic and inelastic electron scattering from thiophene calculated in the energy range 0.1-1000 eV. The R-matrix and independent atom representation-screening-corrected additivity rule (IAM-SCAR) methods were used for low-energy and intermediate and high scattering energies, respectively. The results provide a consistent picture of the scattering process in the whole energy range. The effect of including an interference term in the IAM-SCAR approach is considered. Agreement with prior theoretical results is also discussed.

Shape and Core-Excited Resonances in Thiophene.

We present a comprehensive study of resonance formation in electron collisions with thiophene. Detailed calculations have been performed using the ab initio R-matrix method. Absolute differential cross sections for electron impact excitation up to 18 eV and for two scattering angles, 90 and 135°, have been measured. Agreement between the calculated and measured experimental cross sections is very good. Three shape resonances previously described, two of π* character and one σ*, as well as a number of resonances of core-excited or mixed character are identified and characterized in the calculations. The measured cross sections provide experimental confirmation for a number of the core-excited resonances. The link between these resonances and prior DEA experiments is discussed.

Theoretical study of resonance formation in microhydrated molecules. II. Thymine-(H2O)n, 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)n with n = 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.

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.

Resonances in low-energy electron scattering from para-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.

On the computations of interatomic Coulombic decay widths with R-matrix method.

Interatomic Coulombic Decay (ICD) is a general mechanism in which an excited atom can transfer its excess energy to a neighbor which is thus ionized. ICD belongs to the family of Feshbach resonance processes, and, as such, states undergoing ICD are characterized by their energy width. In this work, we investigate the computations of ICD widths using the R-matrix method as implemented in the UKRmol package. Helium dimer is used here as a benchmark system. The results are compared with those obtained with the well established Fano-Algebraic Diagrammatic Construction method. It is shown that the R-matrix method in its present implementation provides accurate total and partial widths if the kinetic energy of the ICD electron is lower than 10 eV. Advantages and limitations of the R-matrix method on the computations of ICD widths are discussed.

Resonance effects in elastic cross sections for electron scattering on pyrimidine: Experiment and theory.

We measured differential cross sections for elastic (rotationally integrated) electron scattering on pyrimidine, both as a function of angle up to 180(∘) at electron energies of 1, 5, 10, and 20 eV and as a function of electron energy in the range 0.1-14 eV. The experimental results are compared to the results of the fixed-nuclei Schwinger variational and R-matrix theoretical methods, which reproduce satisfactorily the magnitudes and shapes of the experimental cross sections. The emphasis of the present work is on recording detailed excitation functions revealing resonances in the excitation process. Resonant structures are observed at 0.2, 0.7, and 4.35 eV and calculations for different symmetries confirm their assignment as the X̃(2)A2, Ã(2)B1, and B̃(2)B1 shape resonances. As a consequence of superposition of coherent resonant amplitudes with background scattering the B̃(2)B1 shape resonance appears as a peak, a dip, or a step function in the cross sections recorded as a function of energy at different scattering angles and this effect is satisfactorily reproduced by theory. The dip and peak contributions at different scattering angles partially compensate, making the resonance nearly invisible in the integral cross section. Vibrationally integrated cross sections were also measured at 1, 5, 10 and 20 eV and the question of whether the fixed-nuclei cross sections should be compared to vibrationally elastic or vibrationally integrated cross section is discussed.

Absolute cross sections for electronic excitation of pyrimidine by electron impact.

We measured differential cross sections for electron-impact electronic excitation of pyrimidine, both as a function of electron energy up to 18 eV, and of scattering angle up to 180°. The emphasis of the present work is on recording detailed excitation functions revealing resonances in the excitation process. The differential cross sections were summed to obtain integral cross sections. These are compared to results of R-matrix calculations, which successfully reproduce both the magnitude of the cross section and the major resonant features. Comparison of the experiment to the calculated contributions of different symmetries to the integral cross section permitted assignment of several features to specific core-excited resonances. Comparison of the resonant structure of pyrimidine with that of benzene revealed pronounced similarities and thus a dominant role of π-π(∗) excited states and resonances. Electron energy loss spectra were measured as a preparation for the cross section measurements and vibrational structure was observed for some of the triplet states. A detailed analysis of the electronic excited states of pyrimidine is also presented.

Electron scattering cross section calculations for polar molecules over a broad energy range.

We report computational integral and differential cross sections for electron scattering by two different polar molecules, HCN and pyrimidine, over a broad energy range. We employ, for low energies, either the single-centre expansion (ePOLYSCAT) or the R-matrix method, while for the higher energies we select a corrected form of the independent-atom representation (IAM-SCAR). We provide complete sets of integral electron scattering cross sections from low energies up to 10,000 eV. Our present calculated data agree well with prior experimental results.

Shape and core excited resonances in electron collisions with diazines.

We present a comprehensive ab-initio study of electron collisions with pyrazine, pyrimidine, and pyridazine. The emphasis is placed on the identification and characterization of electron resonances in these systems. We use the R-matrix method and show that analysing the time-delay reveals resonances whose signature is not visible in the eigenphase sums. In addition to the well known π∗ resonances below 5 eV, we find three core-excited shape resonances in the energy range 5.5-8.5 eV and a few Feshbach resonances in the dipolar molecules. Additionally, 11 resonances with little effect on the elastic scattering from ground state diazines (but significant effect in elastic collisions with the molecules in an excited state) are found and characterized. We correlate these resonances across the three molecules and discuss their possible correspondence to resonances described in earlier studies on uracil.

Elastic and inelastic cross sections for low-energy electron collisions with pyrimidine.

We present theoretical elastic and electronic excitation cross sections and experimental electronic excitation cross sections for electron collisions with pyrimidine. We use the R-matrix method to determine elastic integral and differential cross sections and integral inelastic cross sections for energies up to 15 eV. The experimental inelastic cross sections have been determined in the 15-50 eV impact energy range. Typically, there is quite reasonable agreement between the theoretical and experimental integral inelastic cross sections. Calculated elastic cross sections agree very well with prior results.

Elastic and inelastic low-energy electron collisions with pyrazine.

We present results of ab-initio scattering calculations for electron collisions with pyrazine using the R-matrix method, carried out at various levels of approximation. We confirm the existing experimental and theoretical understanding of the three well-known π∗ shape resonances. In addition, we find numerous core-excited resonances (above 4.8 eV) and identify their most likely parent states. We also present differential cross sections, showing high sensitivity to the scattering model chosen at low energies. We make recommendations regarding the selection of models for scattering calculations with this type of targets.