Publisher Summary This chapter discusses resonance and threshold phenomena in low-energy electron collisions with molecules and clusters. Low-energy collisions of electrons with atoms and molecules are among the most important elementary processes in gaseous environments such as discharges, arcs, gas lasers, gaseous dielectrics and the earth's atmosphere. The dynamics behavior of low-energy electron-molecule collisions is discussed. The dynamical behavior of slow electrons traversing gases is to a large extent determined by two effects: the energy dependent evolution of the scattering phases for the relevant partial waves and the influence of temporary negative ion states (resonances). Some aspects of resonance and threshold phenomena are discussed. The theoretical description of electron-molecule collisions generally requires an adequate description of electronic, vibrational and rotational degrees of freedom. However, if the typical collision time is short compared to the rotational period, the molecule can be treated as having a fixed orientation during the collision process, and the result for the cross-section can be averaged over orientations. Treatment of vibrational dynamics is usually more important and more challenging to the theory. In the electron energy region important for applications, many inelastic processes such as vibrational excitation and dissociative electron attachment are driven by negative-ion resonances. The theoretical description of vibrational dynamics in these cases is usually based on the nonlocal complex potential describing the nuclear motion in the intermediate negative-ion state.

/pdf/resonance-and-threshold-phenomena-in-low-energy-electron-42hkyjmsxl.pdf

Resonance and Threshold Phenomena in Low-Energy Electron Collisions with Molecules and Clusters

Compound states of triatomic molecules and simple hydrocarbons are studied using electron transmission spectroscopy. Structures in the derivative of the current transmitted through a gas‐filled collision chamber are interpreted as resonances in the electron‐molecule cross sections. At low electron energies (0–6 eV) we observe, in N2O, H2S, and C2H4, broad and featureless structures which we identify as shape resonances. In the same energy range, the molecules CO2, NO2, C6H6, and SO2 exhibit narrow structures which form vibrational progressions. In CO2, NO2, and C6H6, these vibrational progressions are identified as shape resonances; in SO2, the interpretation is not clear cut. No low‐energy resonances are observed in H2O and in CH4. At higher energies (9–17 eV) we observe sharp structures for H2O, H2S, N2O, CO2, and C2H4 (but not for C6H6 and CH4). These structures form bands, each band consisting of a vibrational progression. The states which are responsible for the bands consist of two Rydberg electrons...

Electron transmission spectroscopy: Resonances in triatomic molecules and hydrocarbons

The theory of electron scattering from polyatomic molecules

We have calculated α and η, the ionization and attachment coefficients, and (E/N) *, the limiting breakdown electric‐field–to–gas‐density ratio, in SF6 and SF6 mixtures by numerically solving the Boltzmann equation for the electron energy distribution. The calculations require a knowledge of several electron collision cross sections. Published momentum transfer and ionization cross sections for SF6 were used. We measured various attachment cross sections for SF6 using electron‐beam techniques with mass spectrometric ion detection. We determined a total cross section for electronic excitation of SF6 by comparing the predicted values of α, η, and (E/N) * with our measured values obtained from spatial current growth experiments in SF6 in uniform fields over an extended range of E/N. With this self‐consistent set of SF6 cross sections, together with published He and N2 cross sections, it was then possible to predict the dielectric properties of SF6‐He and SF6‐N2 mixtures. Published experimental values of α for the SF6‐He mixtures lie between the values of α calculated with and without ionization of SF6 by excited He atoms. Published experimental values of (E/N) * agree with our calculations to within 5% in both the SF6‐He and the SF6‐N2 mixtures.

Dielectric properties for SF6 and SF6 mixtures predicted from basic data

The cross sections for production of negative ions by attachment of low‐energy (<0.2 eV) electrons to selected polyatomic (SF6, CCl4, CFCl3, CH2Br2, CH3I, CHCl3, CF3Br, CH3Br) molecules have been determined as a function of target gas temperature in the range from 300 to 1200°K. For the case of SF6 it is found that the total cross section for negative ion production is independent of gas temperature, although it is known from the experiments of Chen and Chantry that the relative abundances of different species of negative ions produced from SF6 are strongly temperature dependent. In contrast to SF6, the total cross sections for negative ion production in some halogenated hydrocarbons are found to be strongly temperature dependent. It is found that the total cross section for negative ion production in many polyatomic molecules tends towards an absolute maximum as the gas temperature is increased.

Temperature dependence of electron attachment at low energies for polyatomic molecules

The attachment of low‐energy electrons (<4 eV) by the reaction e+N2O → O−+N2 has been studied as a function of gas temperature from approximately 160 to 1040°K. The ions produced by a monoenergetic electron beam are detected by total ion collection or by mass analysis. The kinetic energy distributions of the O− ions have also been measured and found to be relatively insensitive to the electron energy when the latter exceeds 1.5 eV, in which case the most probable ion energy is 0.38 eV. The shape and magnitude of the cross section below 2 eV is found to be sensitive to gas temperature throughout the range studied. The differences in shape and threshold observed by previous workers occur below 2 eV and to a large extent may be reconciled in terms of the differing gas temperatures employed. The temperature insensitive portion of the cross section is ascribed to electron capture into the highest‐energy N2O− state connected to electronic ground state N2 + O−. The O− kinetic energy distributions arising therefr...

P. J. Chantry

Papers

Temperature Dependence of Dissociative Attachment in N2O