B. Wallbank

Laser-assisted elastic electron-atom collisions: low electron energy and small scattering angle

The authors report experimental measurements of electron spectra resulting from the scattering of low energy (6.2-32 eV) electrons by helium atoms through an angle of 9 degrees in the presence of a high intensity ( approximately 108 W cm-2)CO2 laser. The intensities of the additional peaks caused by the presence of the laser which occur separated from the elastic scattering peak by multiples of the photon energy are much greater than expected on the basis of calculations, using the Kroll-Watson approximation (1973). The intensity of these free-free transitions also increases with decreasing electron energy, again in disagreement with the calculations. The authors propose that the disagreement between the experimental results and those calculated is due to the electrons scattering from a helium target polarized by the laser field.

Differential cross sections for laser-assisted elastic electron scattering from argon

We report experimental measurements of the differential cross sections for scattering of electrons in the range 8-20 eV from helium and argon through scattering angles of 10 degrees -140 degrees in the presence of a high intensity ( approximately 108 W cm-2) carbon dioxide laser. The majority of the results reported are scattering from argon under conditions for which the Kroll-Watson approximation, usually applied to these types of measurements, predicts zero cross sections. The cross sections reported range up to approximately 12% of the appropriate field-free elastic scattering cross sections in strong disagreement with the theoretical predictions.

Laser-induced free?free transitions in elastic electron scattering from CO2

This paper presents measurements of laser-induced free–free transitions of electrons scattered from CO2 molecules in the ground electronic state at incident electron energies of 3.8 and 5.8 eV under a pulsed CO2 laser field. The differential cross section of free–free transitions involving absorption and emission of up to two photons was measured at various scattering angles with the polarization of the laser either parallel with or perpendicular to the momentum change vector of the scattered electrons. The results of the parallel geometry are found to be in qualitative agreement with the predictions of the Kroll–Watson approximation within the experimental uncertainty whereas those of the perpendicular geometry show marked discrepancy with the Kroll–Watson predictions.

Low-energy electron - helium scattering in a laser field

We report measurements for the scattering of electrons by helium atoms through in the presence of a laser. The incident energy of the electrons was in the range 0.2 - 2.5 eV and the polarization of the laser was arranged to be parallel to the direction of the electron momentum transfer. The data were compared with the results of simple calculations assuming the Kroll - Watson approximation.

Simultaneous electron-photon excitation of He 23S: an experimental investigation of the effects of laser intensity and polarisation

Laser assisted electron excitation of the 23S state of helium is examined at electron energies near to the threshold for excitation in the field of a pulsed CO2 laser delivering approximately 108 W cm-2 in the scattering region. The simultaneous electron-photon excitation cross section of helium is studied at various laser intensities and at different angles between the laser polarisation and the incident electron beam. These data are a first step towards elucidating the influence of various scattering parameters on this complex process and are compared with the results of calculations.

Absolute cross sections for near-threshold electron-impact excitation of Be-like C2+, N3+, and O4+ ions

Abstract Absolute total cross sections for electron-impact excitation of the Be-like C2+, N3+, and O4+ ions have been measured near threshold using the merged electron-ion beams energy-loss (MEIBEL) technique and calculated using a close-coupling R-matrix (CCR) approach. The 2s2 1S → 2s2p 1P0 dipole-allowed transition from the ground state was investigated for all three ions and satisfactory agreement between experiment and theory is found. In addition, calculations and measurements for the 2s2p 3P0 → 2p2 3P allowed transition from the metastable level of C2+ are in excellent agreement. The sum of the cross sections for the 2s2 1S → 2s2p 3P0 and 2s2p 3P0 → 2s2p 1P0 spin-forbidden transitions, not completely resolved by the MEIBEL technique, are measured for C2+ and O4+ and compared to CCR calculations scaled to account for the ground state and metastable fractions of the target ion beams. The results for these unresolved transitions are in reasonable agreement except for a resonance feature measured in the 2s2p 3P0 → 2s2p 1P0 transition in C2+ that is not predicted by theory.

Simultaneous electron-photon excitation of metastable states of rare-gas atoms

The authors report data demonstrating the laser-assisted electron excitation of the 21S state of helium and the 23P states of neon and argon in the field of a pulsed carbon dioxide laser. These are the first measurements of the simultaneous electron-photon excitation cross sections of atomic states other than the 23S state of helium. Comparison of the relative cross sections for these excitations with that for the He 23S state is made.

Merged-beam measurements of absolute cross sections for electron-impact excitation of S4+ (3s2 1S→3s3p1P) and S5+(3s2S→3p2P)

Absolute cross sections for electron-impact excitation of the dipole-allowed transitions S4+ (3s(2) S-1 -> 3s3p P-1) and S5+ (3s S-2 -> 3p P-2) were measured near threshold using the merged electron-ion beams energy-loss technique. Although the magnitudes of the measured cross sections are in reasonable agreement with available theoretical data, the experimental data indicate that the contributions of dielectronic resonances in the near-threshold region are underestimated by these calculations.

Laser‐assisted electron‐atom collisions

Recent experimental measurements of laser‐assisted elastic scattering of electrons from atoms in the presence of a carbon dioxide laser are reported. These data are discussed in terms of the Kroll‐Watson approximation which is normally applied to such measurements. Large discrepancies between the predictions of such a theoretical treatment and the experimental data are observed.

Recent experiments on near-threshold electron-impact excitation of multiply charged ions

Some recent measurements of excitation of multiply charged ions by electrons studied in beam-beam experiments are highlighted. The emphasis is on absolute total cross sections measured with the merged electron-ion beams energy-loss (MEIBEL) technique, although some results obtained with the crossed-beams fluorescence method are also presented. The MEIBEL technique allows the investigation of optically-allowed and forbidden transitions with sufficient energy resolution, typically about 0.2 eV, to resolve resonance structures in the cross sections. Results from the JILA/ORNL MEIBEL experiment on dipole-allowed transitions in several ions demonstrate the success of various theoretical methods in predicting cross sections in the absence of resonances. Comparisons of R-matrix calculations and measured cross sections for spin-forbidden transitions in Mg-like Si{sup 2+} and Ar{sup 6+}, however, show that further refinements to the theory are needed in order to more accurately predict cross sections involving significant contributions from dielectric resonance`s and interactions between neighboring resonances.