Showing papers by "Michael J. Brunger published in 2011"

Electron-collision cross sections for iodine

Abstract: We present results from a joint experimental and theoretical study of elastic electron scattering from atomic iodine. The experimental results were obtained by subtracting known cross sections from the measured data obtained with a pyrolyzed mixed beam containing a variety of atomic and molecular species. The calculations were performed using both a fully relativistic Dirac B-spline R-matrix (close-coupling) method and an optical model potential approach. Given the difficulty of the problem, the agreement between the two sets of theoretical predictions and the experimental data for the angle-differential and the angle-integrated elastic cross sections at 40 eV and 50 eV is satisfactory.

Low-energy elastic electron interactions with pyrimidine

Abstract: We present results of measurements and calculations of elastic electron scattering from pyrimidine in the energy range 3–50 eV. Absolute differential and integral elastic cross sections have been measured using a crossed electron-molecule beam spectrometer and the relative flow technique. The measured cross sections are compared with results of calculations using the well-known Schwinger variational technique and an independent-atom model. Agreement between the measured differential cross sections and the results of the Schwinger calculations is good at lower energies but less satisfactory at higher energies where inelastic channels that should be open are kept closed in the calculations.

Excitation of electronic states in tetrahydrofuran by electron impact.

Abstract: We report on differential and integral cross section measurements for the electron impact excitation of the three lowest lying Rydberg bands of electronic states in tetrahydrofuran. The energy range of the present experiments was 15–50 eV with the angular range of the differential cross section measurements being 15°–90°. The important effects of the long-range target dipole moment and the target dipole polarizability, on the scattering dynamics of this system, are evident from the present results. To the best of our knowledge, there are no other theoretical or experimental data against which we can compare the cross section results from this study.

Positron scattering from the isoelectronic molecules N2, CO and C2H2

Abstract: We report our results on the experimental total cross sections (TCSs) for positron scattering from the isoelectronic molecules N2, CO and C2H2. Where possible, for each species, comparison is made between the present results and those from earlier measurements and calculations. The agreement between the present and earlier experimental results, within the overall uncertainties on the data, is typically satisfactory for energies greater than about 8 eV, but is only marginal at lower positron impact energies. While N2, CO and C2H2 possess 14 electrons each, we find significant differences in the magnitudes of their respective TCSs and subtle differences in the energy dependence of these TCSs. These details are discussed in depth in this paper.

Experimental determination of the scattering length for positron scattering from krypton

Abstract: We report the first experimentally supported determination of the scattering length for positron scattering from krypton. Our result of −10.3 ± 1.5 a.u. compares favourably with that from a convergent close coupling calculation performed as a part of this investigation (−9.5 a.u.), and also with an earlier many body theory calculation of −10.1 a.u. from Gribakin and Ludlow [Phys. Rev. A 70, 032720 (2004)] and a polarized-orbital result of −10.4 a.u. from McEachran et al. [J. Phys. B 13, 1281 (1980)]. The present experimental scattering length supports the existence of a low-lying positron-krypton virtual state (Surko et al. [J. Phys. B 38, R57 (2005)]) at an energy e = 0.13 eV.

A study of electron scattering from benzene: Excitation of the 1B1u, 3E2g, and 1E1u electronic states

Abstract: We report results from measurements for differential and integral cross sections of the unresolved 1B1u and 3E2g electronic states and the 1E1u electronic state in benzene. The energy range of this work was 10–200 eV, while the angular range of the differential cross sections was ∼3°–130°. To the best of our knowledge there are no other corresponding theoretical or experimental data against which we can compare the present results. A generalized oscillator strength analysis was applied to our 100 and 200 eV differential cross section data, for both the 1B1u and 1E1u states, with optical oscillator strengths being derived in each case. The respective optical oscillator strengths were found to be consistent with many, but not all, of the earlier theoretical and experimental determinations. Finally, we present theoretical integral cross sections for both the 1B1u and 1E1u electronic states, as calculated within the BEf-scaling formalism, and compare them against relevant results from our measurements. From that comparison, an integral cross section for the optically forbidden 3E2g state is also derived.

Electron impact vibrational excitation of carbon monoxide in the upper atmospheres of Mars and Venus

Abstract: [1] Infrared emission from CO in the upper atmospheres of Mars, Venus and several other planets is a subject of current theoretical and experimental interest. Electron impact excitation makes a contribution that has not been included in previous studies. Given this, and recent new measurements of absolute cross sections for low-energy electron impact excitation of the vibrational levels of the ground state of CO, results from calculations are presented showing the contribution of electron impact relative to emissions by other mechanisms. It is demonstrated that emissions due to the impact of thermal, photo- and auroral electrons are generally small compared to sunlight-driven (fluorescence and photolysis) emissions, but with some exceptions. It is also shown that thermal-electron emissions may dominate over other processes at nighttime at Mars and that auroral emissions certainly do so. While measurements and other calculations do not appear to be available for Venus, the volume emission rates presented should be valuable in planning such measurements.

Total cross-sections for positron and electron scattering from α-tetrahydrofurfuryl alcohol

Abstract: In this paper, we report original measurements of total cross-sections (TCSs) for positron scattering from an important biomolecule, α-tetrahydrofurfuryl alcohol (THFA). The energy range of these measurements was 0.15–50.15 eV, whereas the energy resolution was ~260 meV. In addition, we report theoretical results, calculated within the independent-screened additivity rule (IAM-SCAR) formalism, on the corresponding electron impact total cross-sections. In this case, the energy range is 1–10 000 eV. With the advent of new particle track simulation codes, which incorporate accurate atomic and molecular data in order to provide interaction details at the nanoscale, interest in positron and electron TCSs has enjoyed something of a recent renaissance as they specify the mean free path between collisions in such codes. Because the present data are, to the best of our knowledge, the first TCSs to be reported for positron scattering from THFA, they fill an important void in the knowledge available to us from the literature.

Electron excitation of the Schumann-Runge continuum, longest band, and second band electronic states in O2.

Abstract: We report measurements of differential and integral cross sections for electron excitation of the Schumann–Runge continuum, longest band, and second band electronic states in molecular oxygen. The energy range of the present study is 15–200 eV, with the angular range of the differential cross section (DCS) measurements from 2 to 130°. A generalized oscillator strength analysis is then employed in order to derive integral cross sections (ICSs) from the corresponding DCSs, and these ICSs are compared with relevant energy and oscillator strength scaled Born cross section (BEf-scaling [Y.-K. Kim, J. Chem. Phys. 126, 064305 (2007)]) results determined as a part of this investigation. Interestingly, while the present Schumann–Runge continuum and second band ICSs were in reasonable agreement with the respective BEf-scaling results, agreement for the longest band was poor below 100 eV with a possible reason for this apparently anomalous behavior being canvassed here. Finally, where possible all present data are c...

An experimental and theoretical investigation into positron and electron scattering from formaldehyde

Abstract: We report on measurements of total cross sections (TCSs) for positron scattering from the fundamental organic molecule formaldehyde (CH2O). The energy range of these measurements was 0.26–50.3 eV, whereas the energy resolution was ~260 meV. To assist us in interpreting these data, Schwinger multichannel level calculations for positron elastic scattering from CH2O were also undertaken (0.5–50 eV). These calculations, incorporating an accurate model for the target polarization, are found to be in good qualitative agreement with our measured data. In addition, in order to compare the behaviour of positron and electron scattering from this species, independent atom model-screened additivity rule theoretical electron TCSs, now for energies in the range 1–10 000 eV, are also reported.

Elastic cross sections for electron scattering from iodomethane

Abstract: Experimental results are reported for elastic differential and integral cross sections for electrons scattering from iodomethane (CH3I). These measurements were made at seven incident electron energies in the range 5–50 eV, with a scattered electron angular range of 20°–135°. Corresponding calculations using the independent atom method plus screened additivity rule (IAM-SCAR), both with and without a dipole correction, are also reported as a part of this study. Where possible, comparison is made to the only other set of experimental results available in the literature (Kato et al 2010 J. Chem. Phys. 132 074309) and to calculated cross sections from the Schwinger multichannel approach at the static exchange level (Natalense et al 2001 Braz. J. Phys. 31 15). In general, good agreement is found between the present measurements and IAM-SCAR computations, and between our results and the earlier investigations.

Effects of alkyl side chains on properties of aliphatic amino acids probed using quantum chemical calculations

Abstract: Effects of alkyl side chains (R-) on the electronic structural properties of aliphatic amino acids are investigated using quantum mechanical approaches. The carbon (C 1s) binding energy spectra of the aliphatic amino acids are derived from the C 1s spectrum of glycine (the parent spectrum) by the addition of spectral peaks, depending on the alkyl side chains, appearing in the lower energy region IP < 290 eV (where IP is the ionization potential). The two glycyl parent spectral peaks of the amide 291.0 eV [C(2)] and carboxylic 293.5 eV [C(1)] C atoms are shifted in the aliphatic amino acids owing to perturbations depending on the size and structure of the alkyl chains. The pattern of the N 1s and O 1s spectra in glycine is retained in the spectra of the other amino acids with small shifts to lower energy, again depending on the alkyl side chain. The Hirshfeld charge analyses confirm the observations. The alkyl effects on the valence binding energy spectra of the amino acids are concentrated in the middle valence energy region of 12–16 eV, and hence this energy region of 12–16 eV is considered as the `fingerprint' of the alkyl side chains. Selected valence orbitals, either inside or outside of the alkyl fingerprint region, are presented using both density distributions and orbital momentum distributions, in order to understand the chemical bonding of the amino acids. It is also observed that the HOMO–LUMO energy gaps of the aliphatic amino acids are reduced with the growth of the alkyl side chain.