Low-energy Electron Collisions Research Articles

Resonances in collisions of low-energy electrons with ozone: Experimental elastic and vibrationally inelastic differential cross sections and dissociative attachment spectra

A comprehensive study of the elastic and vibrationally inelastic scattering of low-energy electrons on ozone, as well as of the dissociative electron attachment has been carried out. Absolute differential cross sections (DCSs) for the elastic and vibrationally inelastic scattering are presented as a function of electron energy and of scattering angle. The elastic data are compared to existing experimental and theoretical work. Two shape resonances are observed in the vibrational DCSs, peaking at 4.2 and 6.6 eV. We used the Koopmans theorem to assign them as and , with temporary capture of the incident electron into orbitals which are very strongly antibonding with respect to the O - O bond length. Both resonances excite not only the totally symmetric vibrations and , but also the antisymmetric stretch vibration , and we assign the excitation of the latter to vibronic coupling between the resonances. Dissociative electron attachment below 5 eV yields and with considerable intensity. formation through three-body attachment or ion - molecule reactions is also observed. Four resonant peaks are observed in the dissociative attachment spectra, at 0.4 eV, 1.3 eV, 3.2 eV, and 7.5 eV. The ions are formed with moderate kinetic energies for the first, nearly zero kinetic energy for the second, and large kinetic energies for the third resonance. The resonances in the dissociative attachment spectra are assigned as the `high energy tail' of the ground state of , a core excited resonance, the shape resonance, and a Feshbach resonance, respectively. The shape resonance appears at a lower energy in the dissociative attachment channel than in the vibrational excitation channel because of kinetic shift. Angular distribution and approximate absolute values are given for the formation at 3.2 eV.

Analytic Born completion in the calculation of electron-molecule differential cross sections.

In calculating the elastic differential cross section for electron scattering from linear nonpolar molecules, one must include elements of the transition matrix corresponding to a large number of partial waves in order to obtain convergence at small angles. We present an analytic correction to the widely used expression for the differential cross section in terms of body-frame T-matrix elements calculated in the fixed-nuclear-orientation approximation. This correction incorporates contributions from all high-order partial waves via the Born approximation. It efficiently produces accurate differential cross sections and shows rapid convergence even at small angles. We illustrate the importance and accuracy of this procedure for low-energy (less than 10 eV) collisions of electrons with ${\mathrm{H}}_{2}$ and ${\mathrm{N}}_{2}$. \textcopyright{} 1996 The American Physical Society.

Collisions of low-energy electrons with carbonyl-containing molecules in a gas phase

Collisions of low-energy electrons with carbonyl-containing molecules in a gas phase

Low-energy electron collisions with sodium: Scattering of spin-polarized electrons.

The electron-sodium system is a prototype of nonrelativistic electron scattering from a quasi-one-electron atomic target and is tractable both experimentally and theoretically. Recently, this system has been studied in a series of sophisticated measurements that together approach complete experiments for elastic (3s\ensuremath{\rightarrow}3s) and inelastic (3s\ensuremath{\rightarrow}3p) scattering. We apply here the theory of orientation and alignment (OA) in atomic collisions to this system using scattering matrices from coupled channel R-matrix calculations described in the first paper in this series [W. K. Trail et al., Phys. Rev. A 49, 3620 (1994)]. To facilitate the extension of OA theory to other transitions and systems and to clarify its relationship to canonical scattering theory, we present a reformulation in terms of the state spaces identified by a particular scattering event. Following the application of this formulation to paradigmatic OA experiments, we compare our results to those from existing measurements and other theoretical calculations. To contextualize these experiments and aid in identifying promising regions for future measurements, we also present a comprehensive three-dimensional overview of the calculated differential OA parameters for energies from threshold to 8.6 eV.

Interaction of low energy electrons with gaseous and condensed perfluoro compounds

Formation of anions induced by collisions of low energy electrons (0–14 eV) with the perfluorinated compounds CF4, C2F6, C2F4, and C6F6 is studied (a) in the gas phase under collision-free conditions in a beam experiment, and (b) in the condensed phase where the electron beam interacts with the molecules deposited under ultrahigh vacuum conditions in definite amounts on a cold (20 K) metallic substrate. In the gas phase different fragment anions [F−, (M–F)−, CF−3] are formed via dissociative attachment (DA). From condensed molecules, desorption of anions is virtually restricted to F−. This can be explained by the polarization interaction of the excess charge and the desorption kinematics preferring desorption of light fragment ions. Below 10 eV F− desorption is induced by DA at or near the surface while above 10 eV a nonresonant signal from dipolar dissociation is observed. The present results indicate that the desorption cross section is essentially controlled by the amount of translational energy released to F− in the dissociation of the precursor anion (M−). In CF4 the precursor ion CF−4 is assigned as the T2 shape resonance with its pronounced σCF* antibonding nature. This results in F− with high kinetic energy release, and, consequently, a high desorption cross section. In C2F6 the decomposition of the transitory anion is less direct resulting in a comparatively low desorption yield. In C2F4 and C6F6, on the other hand, electron capture proceeds via the π* system associated with indirect (statistical) decomposition processes. This results in a very low desorption cross section for both compounds.

Negative ion formation from CH 3I by electron impact

The formation of fragment negative ions from CH 3I on low energy electron collision (0–50 eV) has been measured using a pulsed electron beam and pulsed ion extraction setup in a crossed-beam geometry. In addition to I − we have observed H − and CH − at specific resonant energies due to dissociative electron attachment to CH 3I. The cross-sections were put on an absolute scale using the relative flow technique. We have also carried out ab initio molecular orbital calculations to derive information on the ground state potential energy surfaces of CH 3I, CH 3I − and their various dissociation products.

Low-energy electron collision processes in NF3.

The results of close-coupling calculations are reported for low-energy electron collision processes involving NF{sub 3}. We have used the complex Kohn variational method to calculate the elastic differential and momentum-transfer cross sections, as well as cross sections for (dissociative) excitation of the lowest singlet and triplet electronically excited states. Comparison is made between the results of this study and the limited body of experimental results available for this system.

Relativistic effects in low-energy spin-dependent electron collisions with rare-gas atoms

The relativistic effects in low-energy spin-dependent electron scattering from rare-gas atoms Ar, Kr and Xe are analyzed by comparing the results obtained respectively with Dirac-Fock, Cowan's quasirelativistic Hartree-Fock and non-relativistic Hartree-Fock wave functions for target atoms. It is shown that the intra-target relativistic effects, in particular the explicit spin dependences of the one-electron orbitals of Dirac-Fock atomic wave function, create apparet quantitative changes in the spin polarization parameters at some collision energies and scattering angles.

Low energy electron collisions on OCS: differential vibrational cross sections and S− production around 1 eV

Abstract The low energy vibrational excitation in OCS has been re-examined. The angular dependences of the vibrational cross sections in the 2 Π resonance region are shown to be proportional to 2 - cos 2 θ for π vibrational modes, and to 1 + 7 cos 2 θ for σ modes. The threshold spectrum ( E residual = 0 eV) reveals that, in the bending series below 0.5 eV, the vibrational modes of σ symmetry are more excited than the π modes. Indeed the vibrational excitation cross sections recorded for a few vibrational modes, show a peak at threshold for the σ modes but not for the π ones. We have also observed a new structure in the S − /OCS dissociative attachment cross section.

Low-energy electron-collision processes in molecular chlorine.

The results of close-coupling calculations using the complex Kohn variational method are reported for a variety of low-energy electron-collision processes involving molecular chlorine. We report cross sections for elastic scattering and momentum transfer, as well as dissociative excitation of the five lowest electronically excited states ([sup 1,3][Pi][sub [ital u]], [sup 1,3][Pi][sub [ital g]], [sup 3][Sigma][sub [ital u]][sup +]) which are formed by promoting an occupied valence electron into an antibonding (5[sigma][sub [ital u]]) orbital. We also report cross sections for the excitation of the lowest bound optically allowed states in Cl[sub 2]. The cross sections, especially at very low energies, are found to depend sensitively on both target polarization and a proper balance of correlation effects in the [ital N]- and ([ital N]+1)-electron systems. Comparison is made between the results of this study and the limited body of experimental results available for this system.

Measurement of absolute differential cross section for the excitation of the lowest triplet state of ethene

Absolute differential cross sections for collisions of low-energy electrons with ethene were measured. Presented are the energy dependence of the elastic cross section at 90° in the range 0.5–16 eV, the energy dependence of the cross section for the excitation of the 3B 1u lowest triplet state at 90° up to 20 eV and the angular dependence of the triplet excitation cross section at energies 1.5, 3.0 and 10 eV above threshold. The angular dependence of the cross section for excitation of the v 2 vibration at 1.95 eV is also given for reference. The electronic excitation data are compared with the results of recent ab initio scattering calculations of Rescigno and Schneider and Sun, Winstead, McKoy and Lima. The calculated cross sections are found to be about two times larger than experiment in absolute magnitude and to reproduce qualitatively correctly the main experimental features both in the energy and the angular dependence.

Anisotropic low energy electron collision cross sections for methane derived from transport coefficients

Based on a comprehensive set of transport parameters of electrons in methane derived from single electron spectra in the range of field 0.01-15 Td the authors determined a set of elastic and inelastic scattering cross sections up to 3 eV. The necessity to include anisotropic elastic scattering in a Boltzmann analysis of methane is demonstrated. In addition the drift velocity in a mixture of methane with helium was used to enhance the reliability of the derived cross sections. The low energy part of the cross sections for elastic scattering is subject to a MERT analysis and is compared with total cross section data from single scattering experiments.

Inelastic processes in e–HCl collisions in the energy range including the dissociation threshold

Inelastic processes in low-energy collisions of electrons with HCl molecules are considered. We present results of calculations of vibrational excitation cross sections from the ground state and vibrationally excited states, dissociative attachment cross sections, and electron-impact dissociation cross sections near threshold. Our approach is based on the resonance R-matrix theory with the quasiclassical treatment of the nuclear motion. We use a rigorous procedure for the inclusion of the vibrational continuum. This allows for calculations of the cross sections in the energy range including dissociation threshold. The results for the dissociation cross sections are analyzed from the point of view of the threshold law for the three-particle break-up process.

Experiments relative to the desorption of negative ions during low-energy electron impact on surfaces

During collisions of low-energy electrons on adsorbed layers, resonant processes may induce intense negative ion desorptions. For such investigations an electrostatic mirror configuration was used to freely decelerate an electron beam just as it reached the investigated surface. The same optical system ensured a very efficient collection for the negative desorbed ions before carrying out their mass and energy analysis. The desorption of H − ions from water vapour over-layers adsorbed on (100) and (111) aluminium crystals was reported here. Additional intensity measurements were performed. They provided useful information relative to the surface work function of the sample and to the variation of both secondary electron emission and electron backscattering during very low-energy electron collisions.

Accurateab initiotreatment of low-energy electron collisions with ethylene

We report the results of a complex Kohn variational calculation of the low-energy electron scattering from ethylene, ${\mathrm{C}}_{2}$${\mathrm{H}}_{4}$. Our calculations are the first to confirm the existence of the Ramsauer-Townsend effect in a molecule which possesses a permanent quadrupole moment and suggest that such minima may be found in electron-scattering cross sections for many nonpolar molecules. The calculations also reveal a low-energy shape resonance in $^{2}$${\mathit{B}}_{2\mathit{g}}$ symmetry at 1.83 eV, in excellent agreement with both beam and swarm measurements.

A model describing inelastic processes in low-energy electron collisions with methyl chloride

Dissociative attachment and vibrational excitation processes in low-energy electron collisions with methyl chloride are considered. A theory based on a combination of the resonance R-matrix approach and the non-local-complex-potential approach is applied for the calculation of the cross sections. For the input data of the theory the author uses Falcetta and Jordan's calculation of the adiabatic potential curve for the CH3Cl- anion. The calculated dissociative attachment cross sections averaged over the thermal vibrational distribution are about three orders of magnitude less than those obtained in a beam experiment of Chu et al. (1990). Possible reasons for this discrepancy are analysed. Vibrational excitation cross sections exhibit a sharp threshold peak and a broad resonance with the position corresponding to the vertical attachment energy. The author also investigates the sensitivity of the results to the input parameters.

Low energy electron collisions with He 2+ molecules

The R-matrix method is used in an ab initio study of low energy elastic scattering by He 2 + molecular ions. SCF and correlated CI wavefunctions have been used in a two state approximation, which retains the lowest two states, 2∑ u + and 2∑ g +, of the He 2 + molecular ion in the scattering basis. Cross section calculations have been performed at two fixed internuclear separations, 1.8 and 2.0625 a 0. At the equilibrium bond separation we have detected and fitted the lowest lying n pσ 3Σ u + resonances in the 3Σ + u scattering symmetry. In addition, bound state calculations have also been carried out for the He 2 (1 σ g +1 σ u2s a 3 Σ u +) state at these two internuclear separations and are seen to compare favorably with MCSCF and SOCI results.

Low-energy electron collisions with dichloromethane and butadiene: Selectivity of resonant vibrational excitation

Vibrational excitation by electron impact via low-lying energy resonances has been investigated in butadiene and dichloromethane, focusing on the problem of the selectivity of this excitation. In dichloromethane, for the lowest resonances around 1.2 and 3.4 eV, the excitation of the CCl stretch mode ν 3(a 1) is dominant, in agreement with the selection rules. For trans-1,3-butadiene, CCC skeleton modes - particularly the CC ν 4 stretch mode - are strongly excited via the 2A u resonance. On the other hand, resonant vibrational excitation of these modes via the 2B g resonance around 3 eV appears only weakly due to the short lifetime of the anion state. Noticeable excitation of CH stretch modes is also observed and attributed to higher resonances. The major excitation cross sections versus electron energy along with their angular dependences are presented for both molecules.

Incident Angle Dependence of Secondary Electron Emission from Copper with Multiple Elastic Scattering of Primary Electron in the Muffin-Tin Potential

A Monte Carlo simulation of the secondary electron emission from a copper metal bombarded with low-energy electrons is carried out for analyzing its dependence on the angle of incidence. For the simulation, the differential cross section and the mean free path of the elastic collision of low-energy electrons in bulk copper are calculated with the muffintin approximation for a crystal potential. The mean free path (m.f.p.) is larger than the nearest-neighbor distance even at very low energy (10 eV), while the m.f.p. with an isolated atom potential is smaller than the distance at any energy of less than 1120 eV. The calculated angular dependence of the secondary electron yield is more consistent with an experimental dependence than that for the isolated atom potential.

HgCl B2Σ+1/2 formation by low-energy electron impact dissociation of HgCl2 and(or) CH3HgCl molecules: applications to HgCl (B → X) laser

An investigation was made of the process of dissociative excitation of a HgCl radical in the B2Σ+1/2 state due to collisions of low-energy electrons with HgCl2 and CH3HgCl molecules. Using the most intense band of the B2Σ+1/2 – X2Σ+1/2 system of the HgCl radical at 557 nm that corresponds to the ν′ = 0 to ν″ = 22 transition, emission cross sections were measured in the electron energy range 1–100 eV. The threshold electron energy for the observation of the B2Σ+1/2 – X2Σ+1/2 band system has been determined to be 7.0 and 8.0 eV for HgCl2 and CH3HgCl molecules, respectively.