J.-P. Ziesel

Very low energy electron scattering from benzene and deuterated benzenes

Measurements are reported of the absolute total cross sections for scattering of electrons between and 2 eV by and . Data are also recorded for scattering in the presence of an axial magnetic field over an energy range which extends from between 5 and 10 meV to 2.5 eV. Absolute values of the total scattering cross sections agree well with those of other groups down to 500 meV below which no other data are available. No differences in scattering cross sections are found within experimental error between and its deuterated derivatives. The shape resonance around 1.1 eV in is found at an energy above that for . This shift in energy is ascribed to the different zero-point energies of and , with a contribution from Jahn-Teller distortion in the temporary negative ion. Data at low electron collision energies show that the scattering cross section rises rapidly below 100 meV to values of 170- at the lowest energies. A combination of data obtained in the absence and presence of the axial magnetic field leads to the conclusion that attaches electrons at very low energy, in agreement with earlier high-pressure attachment studies. The lifetime of the moiety, however, appears to be of the order of microseconds - very much greater than had previously been suggested.

An undulator-based spherical grating monochromator beamline for low energy electron-molecule scattering experiments

A new synchrotron radiation undulator beamline, SGM2, with an energy range of 12–20 eV, has been commissioned on the ASTRID storage ring at the University of Aarhus. Using a spherical grating monochromator, the beamline is presently optimized for an energy of 15.76 eV (78.65 nm), for the 3p5(2P1/2)11s resonance in argon, for use in electron-molecule scattering experiments. Using this beamline in conjunction with an electron-molecule scattering apparatus, a beam of electrons down to kinetic energies of a few meV with a resolution of ∼1 meV full width at half maximum is routinely produced.

Mechanism for resonant O- electron stimulated desorption from condensed O2. Non-adiabatic transitions and dynamics

Absolute kinetic energy distributions and yields associated with ground state 3P and excited state 1D oxygen atoms have been obtained for O− anion electron stimulated desorption from condensed O2 in the electron energy range 6–15 eV. The observed yields are understood as resulting essentially from dissociative electron attachment reactions via the two lowest 2Σ+g O−2 resonance states through adiabatic and non-adiabatic transitions to the limits O−(2P) + O(3P) and O−(2P) + O(1D). The kinetic energy distributions show the prominent role of electron multiple collision processes and post-dissociation interactions of the O− anions in the condensed phase.

Low energy electron scattering in H2, N2 and O2

Electron beam transmission experiments have been performed, in the energy range 10 meV to 175 meV, with a magnetically collimated electron beam formed in a synchrotron radiation photoionization source using SuperACO, LURE. The apparatus has been calibrated with He and absolute backward scattering cross sections have been measured for the target gases H2, N2 and O2. A relationship, involving s and p partial waves, has been established between the backward scattering cross sections ( sigma B) and the momentum transfer cross sections ( sigma M). This has been used to check the accuracy of experimental data and the consistency of values of sigma B, sigma M and total scattering cross sections. Experimental data and theory for H2 are in good agreement, whereas for N2 experimental values of sigma B and sigma M conflict below 80 meV and agreement with theories is mixed. For O2, discrepancies are greater than for N2 both in experimental data and between theory and experiment, which may differ by up to a factor of 5 for the total scattering cross section at the lowest energies.

Very Low Energy Electron Scattering in C6H5F, C6H5Cl, C6H5Br and C6H5I

Integral and backward scattering cross sections are reported for the scattering of electrons from C6H5F, C6H5Cl, C6H5Br and C6H5I over the energy range between 10 and 20 meV to 10 eV. The measurements were made in transmission experiments using a synchrotron radiation photoionization apparatus with an energy resolution in the incident electron beam of ~3.5 meV (FWHM). The absolute accuracy of the cross sections is limited by experimental difficulties with respect to strong rotationally inelastic forward scattering, due to the dipolar nature of the target molecules. Integral scattering cross sections are found to rise to more than 1500 ?2 at the lowest energies in C6H5F, C6H5Cl and C6H5Br and to more than 2000 ?2 in C6H5I due to an additional contribution from electron attachment. At low energy, both integral and backward scattering cross sections are very similar in C6H5F, C6H5Cl and C6H5Br. Data are interpreted in terms of rotationally inelastic scattering, with values of cross sections essentially dictated by the correspondingly similar dipole moments (1.6-1.7 D) of these three species. The Born model for rotationally inelastic scattering consistently underestimates the experimental cross sections and overestimates the degree of forward scattering. Our data illustrate the limitations of the Born model for the estimation of low-energy scattering cross sections for plasma modelling.

Negative ion resonances in CF4 probed by dissociative electron attachment

An electrostatic impact spectrometer is used to measure, between 3 and 10 eV, the kinetic energy, the angular distributions and the yield of negative fragment ions formed by dissociative attachment in CF4. The energy range and the symmetry of the resonances are determined as well as the dissociation processes and their dynamics. The first resonance state, observed around 6.8 eV, is attributed to the CF4- 3T2 shape resonance adiabatically dissociating in the two channels F-+CF3 and CF3-+F. The excess energy is shared between the fragment kinetic energies and the internal (vibrational) excitation of the trifluoromethyl fragment. The next resonance, located around 8 eV, is assumed to be of T1 symmetry. The authors suggest that this Feshbach resonance dissociates in F- ions by the sequential breaking of two C-F bonds through an intermediate CF3-* electronically excited negative molecular ion. The 2A1 shape resonance has not been observed in the dissociative channel.

Very low energy electron scattering in some hydrocarbons and perfluorocarbons

Low-energy scattering has been studied in , , , , , and in a transmission experiment using a synchrotron radiation photoionization source. Backward scattering cross-sections have been determined over the energy range 10-175 meV. The variation of these cross-sections with electron impact energy has been analysed for non-polar and weakly polar species using modified effective range theory, yielding scattering lengths and low-energy limiting cross-sections. Rotationally inelastic scattering cross-sections have been calculated for and using the first Born point-dipole approximation. Results suggest that rotationally inelastic events contribute strongly to low energy scattering in but only weakly in , reflecting the larger dipole moment in .

Very low energy electron scattering in nitromethane, nitroethane, and nitrobenzene

Abstract Absolute total integral and total backward scattering cross sections are reported for CH3NO2, C2H5NO2, and C6H5NO2 at electron-impact energies from 30 meV to several eV, with some additional data at higher energies. These experimental data extend the range of species studied in low-energy electron scattering to include target molecules with dipole moments large enough to support dipole-bound states. Data provide a further test of the validity of the Born point-dipole approximation for the calculation of rotationally inelastic scattering cross sections for polar molecules, which are of importance in modeling the chemical and physical characteristics of industrial and natural plasmas. For CH3NO2 and C2H5NO2, large cross sections are found, monotonically increasing with decreasing electron energy in qualitative and to some degree quantitative agreement with theory based on the Born model. As in earlier studies other polar species, the Born model is found to underestimate significantly backward-scattering cross sections. C6H5NO2 shows a powerful dip in the total integral scattering cross sections

Electron scattering in chlorine dioxide

Cross-sections are reported for total integral and backward scattering of electrons from chlorine dioxide (OClO) for energies in the range 20-500 meV. The measurements were made in transmission experiments using a synchrotron radiation photoionization apparatus with an energy resolution in the incident electron beam of ~1.0 meV (FWHM). Integral and backward scattering cross-sections are reported. The present integral cross-section values are a correction to those reported in an earlier publication (Gulley et al 1998a J. Phys. B: At. Mol. Opt. Phys. 31 5197) which have been found to be too low due to the presence of an impurity in the original OClO samples. The absolute accuracy of the integral cross-sections is limited by experimental difficulties with respect to strong rotationally inelastic forward scattering due to the dipolar nature of the target molecules. Backward scattering data provide good evidence that dissociative attachment occurs through p-wave attachment. Comparison is made with predictions of the first Born pure dipole approximation for both total and backward scattering with excellent agreement above 100 meV with integral scattering data, but showing a significant underestimate for backward scattering cross-sections. Behaviour at energies below 100 meV represents competition between inelastic scattering and attachment and is not well understood.

Low energy electron scattering by CF3Cl, CF2Cl2, CFCl3 and CCl4

We report the results of electron transmission experiments in the chlorofluoromethanes and CCl4 in the energy range from less than 10 meV to a few hundred meV, using a magnetically confined electron beam, generated from a high resolution, synchrotron photoionization source on SuperACO, LURE. Scattering spectra show evidence of electron attachment, Ramsauer-Townsend effects, and pure rotational and rovibrational inelastic scattering. For CF3Cl and CF2Cl2 we analyse the inelastic scattering phenomena in terms of the first Born approximation, showing that this gives a good fit to the form of the variation of the pure rotationally inelastic cross section with electron kinetic energy.