Electron–methane interaction model for the energy range 0.1–10000eV

Positron scattering from structurally related biomolecules

We report the integral elastic, momentum transfer, and inelastic (positronium formation and ionisation) cross sections for positron scattering from structurally related molecules. The molecules chosen for the current investigation are formamide, formylphosphine, formic acid, N-methylformamide, acetone, acetic acid, and formaldehyde. The cross sections were calculated using the optical potential approach and the complex scattering potential-ionisation contribution method for impact energies between 1 and 5 keV. A sizable repository of data is now available for positron scattering from various atoms and molecules; however, data on the impact of positrons on current targets is still scarce and fragmented. While most cross sections are the first of their kind, we analyze our total cross sections (TCSs) with the previous literature available, which has become attractive to researchers trying to model the tracks of charged particles in matter. TCSs have recently seen a resurgence in popularity thanks to their utility in specifying the mean-free path between the collisions of such simulations. We find good qualitative convergence between experimental and theoretical results below and above the positronium formation threshold. However, around the threshold region, a significant discrepancy is encountered, which can be accounted for due to the experiment's lack of forward angle scattering effect discrimination. This level of agreement evolves to become quantitative at intermediate and higher energies.

Origin of resonant character in the electron impact two-body neutral-fragmentation of methane

The observation of peaks in the threshold region of two-body neutral fragmentation of methane molecule, i.e.,  CH 4   ->  CH 3 + H, by low energy electron (LEE) impact has been an enigma. The prevailing explanation that this resonant behavior is due to excitation energy transfer is unsatisfactory since this process is not expected to show peaks in the cross-sections unless there is the involvement of electron-molecule resonances. Our first-principles calculations now reveal that the observed peaks could be explained as due to the formation of negative ion resonances, which dominantly dissociate into two neutral fragments and a free-electron. This case of methane is a pointer to the possibility that such reactions contribute significantly to neutral radical production from molecules by LEE impact in comparison to dissociative electron attachment, and in general could play a significant role in electron-based chemical control.

Elastic Electron Scattering from Methane Molecule in the Energy Range from 50-300 eV

Electron interaction with methane molecule and accurate determination of its elastic cross-section is a demanding task for both experimental and theoretical standpoints and relevant for our better understanding of the processes in Earth's and Solar outer planet atmospheres, the greenhouse effect or in plasma physics applications like vapor deposition, complex plasma-wall interactions and edge plasma regions of Tokamak. Methane can serve as a test molecule for advancing novel electron-molecule collision theories. We present a combined experimental and theoretical study of the elastic electron differential cross-section from methane molecule, as well as integral and momentum transfer cross-sections in the intermediate energy range (50-300 eV). The experimental setup, based on a crossed beam technique, comprising of an electron gun, a single capillary gas needle and detection system with a channeltron is used in the measurements. The absolute values for cross-sections are obtained by relative-flow method, using argon as a reference. Theoretical results are acquired using two approximations: simple sum of individual atomic cross-sections and the other with molecular effect taken into the account.

Fragmentation of tetrahydrofuran molecules by H(+), C(+), and O(+) collisions at the incident energy range of 25-1000 eV

We have studied fragmentation processes of the gas-phase tetrahydrofuran (THF) molecules in collisions with the H(+), C(+), and O(+) cations. The collision energies have been varied between 25 and 1000 eV and thus covered a velocity range from 10 to 440 km/s. The following excited neutral fragments of THF have been observed: the atomic hydrogen H(n), n = 4-9, carbon atoms in the 2p3s (1)P1, 2p4p (1)D2, and 2p4p (3)P states and vibrationally and rotationally excited diatomic CH fragments in the A(2)Δ and B(2)Σ(-) states. Fragmentation yields of these excited fragments have been measured as functions of the projectile energy (velocity). Our results show that the fragmentation mechanism depends on the projectile cations and is dominated by electron transfer from tetrahydrofuran molecules to cations. It has been additionally hypothesized that in the C(+)+THF collisions a [C-C4H8O](+) complex is formed prior to dissociation. The possible reaction channels involved in fragmentation of THF under the H(+), C(+), and O(+) cations impact are also discussed.

Electron attenuation in free, neutral ethane clusters

The electron effective attenuation length (EAL) in free, neutral ethane clusters has been determined at 40 eV kinetic energy by combining carbon 1s x-ray photoelectron spectroscopy and theoretical lineshape modeling. More specifically, theory is employed to form model spectra on a grid in cluster size (N) and EAL (λ), allowing N and λ to be determined by optimizing the goodness-of-fit χ(2)(N, λ) between model and observed spectra. Experimentally, the clusters were produced in an adiabatic-expansion setup using helium as the driving gas, spanning a range of 100-600 molecules in mean cluster size. The effective attenuation length was determined to be 8.4 ± 1.9 Å, in good agreement with an independent estimate of 10 Å formed on the basis of molecular electron-scattering data and Monte Carlo simulations. The aggregation state of the clusters as well as the cluster temperature and its importance to the derived EAL value are discussed in some depth.

Electron scattering cross sections from anthracene over a broad energy range (0.00001-10,000 eV)

We report a computational investigation of electron scattering by anthracene (C14H10) in the gas phase. Integral and differential cross sections have been calculated by employing two distinct ab-initio quantum scattering methods: the symmetry adapted-single centre expansion method (ePOLYSCAT) and a screening corrected form of the independent atom model (IAM-SCAR) at low and high energies, respectively. After a detailed evaluation of the current results, we present a complete set of integral scattering cross sections from 0.00001 to 10,000 eV.

Positron scattering from the cyclic ethers oxirane, 1,4-dioxane, and tetrahydropyran

In this paper we report original measurements of total cross sections (TCSs) for positron scattering from the cyclic ethers oxirane (C(2)H(4)O), 1,4-dioxane (C(4)H(8)O(2)), and tetrahydropyran (C(5)H(10)O). The present experiments focus on the low energy range from ∼0.2  to  50 eV, with an energy resolution smaller than 300 meV. This study concludes our systematic investigation into TCSs for a class of organic compounds that can be thought of as sub-units or moieties to the nucleotides in living matter, and which as a consequence have become topical for scientists seeking to simulate particle tracks in matter. Note that as TCSs specify the mean free path between collisions in such simulations, they have enjoyed something of a recent renaissance in interest because of that application. For oxirane, we also report original Schwinger multichannel elastic integral cross section (ICS) calculations at the static and static plus polarisation levels, and with and without Born-closure that attempts to account for the permanent dipole moment of C(2)H(4)O. Those elastic ICSs are computed for the energy range 0.5-10 eV. To the best of our knowledge, there are no other experimental results or theoretical calculations against which we can compare the present positron TCSs. However, electron TCSs for oxirane (also known as ethylene oxide) and tetrahydropyran do currently exist in the literature and a comparison to them for each species will be presented.

Modelling low energy electron and positron tracks for biomedical applications

PURPOSE

To incorporate the effects of low energy electrons and positrons into radiation interaction models.

MATERIALS AND METHODS

The simulation method proposed here was based on experimental and theoretical cross section data and energy loss spectra we have previously derived. After a summary of the main techniques used to obtain reliable input data, the basis of a Low Energy Particle Track Simulation (LEPTS) procedure was established. The programme is specifically designed to describe electron and positron interactions below 10 keV, down to thermal energies.

RESULTS

Single electron and positron tracks in water are presented and the possibility of using these results to develop tools for nanodosimetry is discussed.

CONCLUSIONS

Standard approximations based on high incident energies, such as the Born-Bethe theory, are not suitable to simulate electron and positron tracks below 10 keV. Prior to the inclusion of low-energy effects in a radiation model, an appropriate study is required to determine both the interaction cross sections and the energy loss spectra.

Structure of neutral nanosized clusters produced by coexpansion of CF4 and CH4

Mixed CH(4)/CF(4) clusters as well as pure clusters of CF(4) were produced by adiabatic expansion and studied by carbon 1s (C1s) X-ray photoelectron spectroscopy. Evidence is presented that CH(4) and CF(4) do indeed form binary clusters in CH(4)/CF(4) coexpansion experiments and that these clusters exhibit radial structure; i.e., CF(4) is primarily found in the bulk. The interpretation of the photoelectron spectra is supported by calculations of C1s ionization energies based on theoretical clusters models.