Low-energy electron scattering by methane,arsine and phosphine

We present cross sections for the elastic scattering of low-energy electrons by phosphine (PH₃) and arsine (AsH₃), and for electron-impact excitation of the (1t ₂ 3sa ₁)³ T ₂ and¹ T ₂ states in methane (CH₄). These results were calculated using the Schwinger multichannel method as implemented on distributed-memory parallel computers. The PH₃ and AsH₃ cross sections show a pronounced low-energy shape resonance which may provide a pathway to dissociative attachment. The^1,3 T ₂ cross sections for CH₄ correlate fairly well with recent measurements of CH₂ production via electron-impact dissociation of methane.Contribution No. 8587     

Chemical bonding effect in electron scattering by gaseous molecules

The experimental intensity of 30 keV electron small angle scattering by a gaseous molecule is much different from the calculation using usual independent atom model. This is due to the rearrangement of electron distribution in a molecule by the formation of chemical bonds, and is called chemical bonding effect (CBE). The molecules studied are mainly hydrocarbons such as methane, acetylene, ethane, etc. and some non-hydrocarbons. The measurement was carried out on both elastic and total scattering and the effect was found for not only elastic but also inelastic scattering. The effect is relatively large for hydrogen rich molecules as H₂O, NH₃ and hydrocarbons, but is essentially related to the number of atoms contained in molecules. The origin of CBE will attribute mainly to the concentration of inner atomic electrons resulting from chemical bonding.     

To the theory of electron scattering by polyatomic molecules

Within the Born-Oppenheimer adiabatic perturbation theory, an equation was obtained for the intensity of fast electron scattering by polyatomic molecules. All of its parameters are explicitly defined by vibronic interaction in a molecule; due to this, quantum chemical calculations are fully applicable to electron diffraction studies of molecular geometries. Engels Anti-Aircraft Missile Higher Military School. Translated fromZhurmal Strukturnoi Khimii, Vol. 35, No. 2, pp. 40–45, March–April, 1994. Translated by L. Smolina     

Elastic electron scattering from state-selected molecules

Electronically elastic, electron scattering cross sections are calculated for molecules in particular rotational states in order to establish the sensitivity of the scattering pattern to the quantum state. For the examples of diatomic molecules and symmetric tops considered here, the scattering pattern provides a unique fingerprint of the quantum state if one measures the scattered intensity for different orientations of the scattering vectors. The structure in the scattered intensity reflects the anisotropy of the square of the rotational wavefunction. Even for low angular momentum states which have diffuse rotational wavefunctions, very large differences in intensity are the result at certain scattering angles for states whose quantum numbers differ even by only one unit.     

Low-energy electron scattering from DNA and RNA bases: shape resonances and radiation damage

Calculations are carried out to determine elastic-scattering cross sections and resonance energies for low-energy electron impact on uracil and on each of the DNA bases (thymine, cytosine, adenine, and guanine), for isolated molecules in their equilibrium geometry. Our calculations are compared with the available theory and experiment. We also attempt to correlate this information with experimental dissociation patterns through an analysis of the temporary anion structures that are formed by electron capture in shape resonances.     

Low-energy electron scattering by cubane: resonant states and Ramsauer-Townsend features from quantum calculations in the gas phase

Calculations are carried out, using a nonempirical modeling of the interaction potential and solving the quantum scattering coupled channel equations, for low energy electron scattering from cubane (C8H8) molecules in the gas phase. Total integral cross sections are obtained and partial contributions are analyzed for the most important irreducible representations that describe the continuum electron in the Oh molecular symmetry. Several trapping resonances are found and analyzed in terms of the molecular-type features of the resonant electron states associated with them. A Ramsauer-Townsend minimum is also found and its possible behavior related to features of the scattering length as k --> 0.     

Inelastic scattering of fast electrons and the electron density of the scattering molecules

Theoretical and Experimental Chemistry -     

Elastic electron scattering cross sections of oriented and aligned molecules

Elastic differential electron scattering cross sections of oriented methyl iodide are calculated using the independent atom model. Results are presented for two specific orientations of the ICH₃ molecule for the purpose of comparison with the fictitious molecule IC, similarly oriented, at electron energies of 600 eV and 40 keV. Cross sections are also calculated for IC with a large angular momentum. In a comparison of the results for different orientations of the angular momentum vector, including random orientation, large differences between the cross sections are evident. This sensitivity to the plane of rotation of the molecule suggests the possibility of determining the degree of alignment of the angular momenta of a beam of such molecules by electron diffraction.     

Low-energy electron capture by 6-Aza-2-thiothymine: investigations by electron attachment and electron transmission spectroscopies

The interaction of low-energy (0-10 eV) electrons with 6-aza-2-thiothymine is investigated in the gas phase by studies of sharp structure in the total electron scattering cross section and by mass analysis of the stable or long-lived negative ions produced by electron attachment. The most efficient fragmentation process, occurring at 0.15 eV, involves the ejection of a closed-shell neutral molecule (CH3CN). Ab initio calculations support our proposal that this process leads to ring closure to form a stable four-member heterocyclic anion. A long-lived parent anion with an approximate lifetime of 75 microseconds is observed near zero electron energy, and evidence is also seen for the slow decay of this anion by ejection of CH3CN. Near 3.3 eV, an anion of m/e 41 is produced that is likely to be a metastable valence anion of bent CH3CN, but the dipole-bound anion cannot be ruled out.     

Low-energy electron collisions with glycine

We report cross sections for elastic electron scattering by gas phase glycine (neutral form), obtained with the Schwinger multichannel method. The present results are the first obtained with a new implementation that combines parallelization with OpenMP directives and pseudopotentials. The position of the well known π* shape resonance ranged from 2.3 eV to 2.8 eV depending on the polarization model and conformer. For the most stable isomer, the present result (2.4 eV) is in fair agreement with electron transmission spectroscopy assignments (1.93 ± 0.05 eV) and available calculations. Our results also point out a shape resonance around 9.5 eV in the A' symmetry that would be weakly coupled to vibrations of the hydroxyl group. Since electron attachment to a broad and lower lying σ* orbital located on the OH bond has been suggested the underlying mechanism leading to dissociative electron attachment at low energies, we sought for a shape resonance around ~4 eV. Though we obtained cross sections with the target molecule at the equilibrium geometry and with stretched OH bond lengths, least-squares fits to the calculated eigenphase sums did not point out signatures of this anion state (though, in principle, it could be hidden in the large background). The low energy (~1 eV) integral cross section strongly scales as the bond length is stretched, and this could indicate a virtual state pole, since dipole supported bound states are not expected at the geometries addressed here.     

Low-energy electron scattering with the purine bases of DNA/RNA using the R-matrix method

R-matrix calculations on electron collisions with the purine bases found in DNA and RNA (i.e., adenine and guanine) are presented. Resonant anion states of these systems are identified by employing different approximation levels of ab initio theoretical methods, such as the static exchange, the static exchange plus polarization, and the close-coupling methods. The results are compared with other available calculations and experiments. All of these ab initio approximations, which we refer to as a scattering "model," give four shape resonances of (2)A' (π) symmetry within the energy range of 10 eV for both molecules. For adenine, the most sophisticated method, the close-coupling model, gives two very narrow (2)A' (σ) symmetry Feshbach-type resonances at energies above 5 eV. Quantitative results for the total elastic and electronic excitation cross sections are also presented.     

Low-energy resonance states upon electron capture by five-membered heterocycles and cyclopentadiene

New resonance states were discovered for the negative molecular ions of thiophene and selenophene and a series of resonances was found for various heterocyclic compounds in the region 3.0–3.6 eV. The low-energy resonances at 1.65–2.3 eV are formed by a resonance mechanism of a form of the molecular ground state, while an electronically excited Feschbach resonance is responsible for the series of resonance states at 3.0–3.6 eV. The mother state for the latter resonance states is the first triplet state of these molecules. The first triplet state of selenophene is at 3.6±0.15 eV.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 925–927, April, 1990.     

Low-energy electron collisions with gas-phase uracil

We have studied gas-phase collisions between slow electrons and uracil molecules with a view to understanding the resonance structure of the scattering cross section. Our symmetry-resolved results for elastic scattering, computed in the fixed-nuclei, static-exchange and static-exchange-plus-polarization approximations, provide locations for the expected pi* shape resonances and indicate the possible presence of a low-energy sigma* resonance as well. Electron-impact excitation calculations were carried out for low-lying triplet and singlet excitation channels and yield a very large singlet cross section. We discuss the connection between the resonances found in our elastic cross section and features observed in dissociative attachment.     

Electron scattering by water and alcohol molecules

Broad resonances in electron scattering by H₂O, aliphatic alcohols and phenol are reported.     

Low-energy electron scattering cross section for the production of CO within solid films of carbon dioxide

We report absolute electron scattering cross sections sigma(p) for the production of CO within thin solid film of carbon dioxide (CO(2)) condensed on a solid Ar substrate. The CO fragments, which remain trapped within the bulk of the carbon dioxide film, are detected in situ by recording energy losses to their lowest triplet electronic state a (3)Pi using high-resolution electron-energy-loss spectroscopy. The production of CO is studied as a function of the electron exposure, film thickness, and incident electron energy between 2 and 30 eV, a range within which most of the secondary electrons are created in systems irradiated by high-energy particles. The energy dependence is characterized by a feature around 4 eV with sigma(p)=(7.0+/-4.0)x10(-18) cm(2), a minimum around 7 eV, a strong rise up to a large and broad maximum around 15 eV with sigma(p)=(5.4+/-2.5)x10(-17) cm(2), a decrease to a minimum around 18.5 eV, and finally a monotonous increase up to 30 eV. The CO production is discussed in terms of the formation of electron resonances or transient anion states, which may lead directly to the fragmentation of the molecule via dissociative electron attachment or indirectly by decaying into an entirely repulsive part of the corresponding excited neutral and positive ion states.     

Low-energy dissociative recombination in small polyatomic molecules

Indirect dissociative recombination of low-energy electrons and molecular ions often occurs through capture into vibrationally excited Rydberg states. Properties of vibrational autoionization, the inverse of this capture mechanism, are used to develop some general ideas about the indirect recombination process, and these ideas are illustrated by examples from the literature. In particular, the Δv = -1 propensity rule for vibrational autoionization, i.e., that vibrational autoionization occurs by the minimum energetically allowed change in vibrational quantum numbers, leads to the prediction of thresholds in the dissociative recombination cross sections and rates at the corresponding vibrational thresholds. Capture into rotationally excited Rydberg states is also discussed in terms of recent low-temperature studies of the dissociative recombination of H(3)(+).