J F Williams

The scattering of electrons from inert gases I. Absolute differential elastic cross sections for argon atoms

Absolute angular differential cross sections for the elastic scattering of electrons from argon atoms have been measured for incident electron energies from 20 to 400 eV over the angular range 20 degrees to 150 degrees . The experimental method is based on the crossed electron and modulated atom beams techniques with an electron energy resolution of 0.095 eV and an angular resolution of 2 degrees for both the incident and scattered electron beams. The experimental cross section values have been calibrated absolutely by a phaseshift analysis over the (3s23p54s2) 2P1/2, 2P3/2 resonance doublet in argon at 11.0 and 11.27 eV. At 11.18 eV, the measured phaseshifts are delta 0=5.063+or-0.050 rad, delta 1=2.533+or-0.020 rad, delta 2=1.092+or-0.020 rad, delta 3=0.112+or-0.018 rad and higher phaseshifts were calculated from an effective range expansion. These values are in good agreement with the measured values of Andrick and Bitsch (1975), but the s and d wave phaseshifts of Weingartshofer et al. (1974) are outside the experimental limits.

The scattering of electrons from inert gases. II. Absolute differential elastic cross sections for neon, krypton and xenon atoms

For pt.I see ibid., vol.8, no.10, p.1670 (1975). Measurements of the absolute differential cross sections for the elastic scattering of electrons from neon, krypton and xenon for a wide range of electron energies and scattering angles and with high angular resolution are presented. The range of electron energies is 20 to 400 eV, the range of scattering angles from 20 to 150 degrees and the angular resolution of both the incident and scattered electron beams is 1.5 degrees. The experimental method is based on the crossed electron and modulated atom beams techniques. Measured angular distributions have been made absolute by carrying out phase shift analyses over the 19.35 eV (1s2s2)2S resonant state of helium and the 11.10 and 11.27 eV (3s23p54s2), 2P1/2, 2P3/2 resonance doublet in argon coupled with accurate determinations of the beam density ratios of neon, krypton and xenon to helium and argon. The absolute differential cross sections are not consistently in agreement with any previous absolute determinations.

Electron scattering from atomic hydrogen. III. Absolute differential cross sections for elastic scattering of electrons of energies from 20 to 680 eV

For pt.II see ibid., vol.8, no.10, p.1683 (1975). Absolute differential elastic cross sections have been measured for the scattering of electrons from atomic hydrogen over the electron energy range from 20 to 680 eV and over the angular range from 10 to 140 degrees. The experimental techniques and method are those associated with crossed electron and modulated atom beams with momentum analysis of the incident and scattered electrons. The measured relative angular distributions were made absolute within 11% by means of the following sequence of measurements, (i) a phaseshift analysis around the (1s2s2)2S resonance elastic scattering in helium, (ii) relative beam densities of helium and hydrogen beams and (iii) absolute calibration of all apparatus functions.

Electron scattering from hydrogen atoms. II. Elastic scattering at low energies from 0.5 to 8.7 eV

For pt.I see ibid., vol.8, no.10, p.1641 (1975). Absolute angular differential cross sections for the elastic scattering of electrons from atomic hydrogen have been measured for incident energies from 0.5 to 8.7 eV and for scattering angles between 10 and 150 degrees. The experimental techniques and method were those normally used with crossed electron and modulated atom beams. The angular behaviour of the cross section is consistent with a partial-wave explanation; that is at the lowest energy of 0.5 eV, the angular distribution is peaked in the backward direction because of dominant p-wave exchange scattering; at 3.4 eV there is an angular minimum around 70 degrees because of interference between s, p and increasing d waves and at 8.7 eV the distribution is peaked in the forward direction because of the increasing effect of higher partial waves.

Electron scattering from atomic hydrogen. I. Differential cross sections for excitation of the n=2 states

Absolute differential cross sections for electrons losing 10.2 eV in exciting the n=2 states of atomic hydrogen have been measured over the angular range 20 to 140 degrees for incident electron energies of 54 to 680 eV. The experimental apparatus, method and techniques are based upon those normally used in crossed electron and modulated atom beams experiments which study electron energy-loss spectra with good angular (2 degrees ) and energy (0.03 eV) resolution of the incident and scattered beams. Absolute cross section values have been determined by a method based on (i) a phase shift analysis of the elastic differential scattering of electrons from resonant states the (1s2s2)2S state in helium and the (3s23p54s2)2P3/2 and 2P1/2 states in argon and (ii) measurement of beam source pressures.

The 2S and 2P state excitation of atomic hydrogen by electron impact

The excitation of the 2P and 2S states of atomic hydrogen by electron impact has been studied by observing separately the prompt ( approximately 10-9 s) Lyman- alpha photons from the decay of the 2P state, the delayed Lyman- alpha photons from the quenched 2S state and the 10.2 eV energy-loss scattered electrons. Observations of these channels indicate the presence of at least four resonant states at 11.74, 11.85, 11.94 and 12.05 eV (+or-0.080 eV). These energies agree within the experimental error with the positions of the 1S, 1D, 1P and 1S(3P) resonance states predicted by six-state close-coupling theory. Over the first 0.5 eV above threshold, the measured values of the absolute cross section for separate excitation of the 2S and 2P states agree, within the experimental errors of +17% and -15%, with the three-state (1s,2s,2p) plus twenty-correlation-term close-coupling predictions of Taylor and Burke (1967). At incident electron energies of 13.87, 16.46 and 19.58 eV the first low-energy measurements are reported of the differential cross sections for exciting the n=2 states.

High-resolution energy and angular correlations of the scattered and ejected electrons in electron impact ionisation of argon atoms

The electron impact ionisation (e, 2e) spectrum of argon has been studied with a resultant energy resolution of 0.06 eV and an angular resolution of two degrees in a crossed electron beam and modulated atom beam apparatus. The previously observed 3s3p6, 3s23p43d and 4d(2S1/2) states were identified; the spin-orbit splitting of 0.18 eV between the 3p3/2 and 3p1/2 states was revealed and the 3s23p44s(2P0) state was seen. Angular correlations between the scattered and ejected electrons in coplanar symmetric scattering were obtained for each of the above six states. The deduced electron momentum distributions for these states are consistent with the factorised distorted-wave impulse approximation model formulated by McCarthy and Weigold (1976). The characteristic electron orbitals are those appropriate for the total angular momentum and parity for each state. The cross sections for production of the 3p3/2 and 3p1/2 states are in the ratio of their statistical weights.

Alignment and orientation of the 21P state of helium by electron impact at 29.6 and 51.2 eV

The electron-photon coincidence technique has been used to deduce the lambda and chi parameters for excitation of the 21P state of helium by electron impact at 29.6 and 51.2 eV incident energies and for electron scattering angles in the range 15 to 130 degrees . The lambda parameter shows a minimum at small and large scattering angles. When combined with the 1979 data of Hollywood and co-workers at 81.2 eV, it can be seen that as the incident energy decreases, the depth of the low-angle minimum decreases and its position moves to larger scattering angles. In contrast, the depth of the second minimum increases with decreasing incident electron energy. The experimental values of lambda and chi are compared with values predicted from the first Born approximation, a distorted-wave polarised-orbital approximation, a first-order many-body theory and an R-matrix theory.

Angular correlation measurements for the electron-excited 31P state of helium

Angular correlations between the scattered electrons resulting from the 11S-31P excitation process in helium and the photons emitted in the decay of the 31P state to the ground state have been measured. Data have been obtained at incident electron energies of 34.6, 45.6 and 75.6 eV for electron scattering angles up to 35 degrees . Analysis of the data gives the ratio of differential cross sections for excitation of the degenerate magnetic sublevels of the state, the magnitude of the phase difference between the corresponding excitation amplitudes and the Fano-Macek orientation and alignment parameters. Reasonable agreement is obtained between the present data at 45.6 and 75.6 eV and the original data of Eminyan et al. (1975) at 50 and 80 eV respectively. Even at these small scattering angles, no theoretical work yields values for these fundamental parameters in agreement with experiment.