Andrew Mikosza

Electron-photon angular correlations from the electron impact excitation of the 2s and 2p electronic configurations of atomic hydrogen

The angular correlation of Lyman-alpha photons detected in coincidence with the 10.2 eV energy-loss electrons have been measured for 54.4 eV incident electrons exciting the n=2 levels of atomic hydrogen. For electron scattering angles in the range from 10 to 140 degrees the in-plane angular correlations were measured for photon angles from 10 to 140 degrees . These data were interpreted in terms of lambda , the ratio of the differential cross sections mod aml mod 2 for magnetic sublevel ml excitation, and R, the real part of the product (a0a1) in which the angle brackets indicate summation over the spin states. The error bars on these values are significantly smaller than on previous measurements of Williams (1975), Hood et al (1979) and Weigold et al. (1980). No existing theory predicts values in agreement with the present data. An angular correlation in which the photon detector axis is perpendicular to the scattering plane is shown to measure directly the 2p state angular differential cross section as well as the ratio of that cross section to the total 2p cross section. Also, by using an appropriate quenching pulse, the 2s state angular differential cross section, sigma (2s), is measured.

The triple coincidence detection of the helium 3 1 D state decay

The first triple coincidence detection of the scattered energy loss electron and the sequential cascading photons, 667·8 nm 31 D → 21P and 58·4 nm 21P → 11S, from the electron impact excitation of the 31D state of helium is reported. An experimental method for identification and determination of the triple coincidences and initial results is reported. The measurement is the final experimental step in determining all the information to define completely the excited 3D state.

Resonance trapping effects in stepwise electron/laser excitation

Abstract This paper considers the relaxation processes in stepwise electron/laser excitations that are affected by radiation trapping. The pressure dependence of the observed linear and circular polarisations is examined and quantified. An extrapolation to zero pressure is found to be significantly different from a linear fitting procedure. This enables the extraction of true values of the state multipoles, which describe the alignment and orientation of the mercury 6 1 P 1 state immediately after electron-impact excitation.

Magnetic sublevel coincidence spectroscopy and characterisation of the 31D state of Helium

Abstract Instrumentation, techniques and a statistical analysis are described for the accurate observation of the two, and three, particle coincidences of the scattered energy loss electrons with the two sequential cascading photons from the 3 1 D state of helium excited by incident electrons. A three-dimensional histogram data accumulation system enables the two, and three, particle true and random coincidences to be analysed. These observations yield information not obtainable in other ways and were used to deduce the scattering amplitudes, relative phases and their various combinations for the excited 3 1 D state. The measured values agree with the best convergent close coupling calculations within one standard deviation experimental uncertainty.

Review of advances in describing the 31D-state excitation of helium

Recent advances in describing the electron impact excitation of the 31D state of helium are reported for an incident energy of 40 eV and scattering angles from 10° to 40°. Measurements of the Stokes parameters have been made using the polarized photon–photon and electron–photon coincidence techniques. The partial cross section σm for the magnetic sublevels in the collision frame indicate that the m = 0 excitation is favoured. For the above scattering kinematics the data are in agreement with convergent close coupling calculations. The deduced scattering amplitudes, in the natural frame, show that the m = 2 excitation becomes dominant at the larger angles. Progress is reported on the complete determination of the scattering amplitudes and their relative phases using the triple-coincident detection of the scattered electron with the two sequential cascading photons.

Photon angular correlations and pressure-dependent effects of helium 21P and 31P excitation by electrons

The orientation and alignment parameters of the 21P and 31P excited states of helium at an incident electron energy of 81.6 eV have been remeasured for electron scattering angles from 25 degrees to 5 degrees and extended down to 2 degrees . The studies investigate a wide range of experimental conditions and measurement techniques, including beam alignment, electron and photon angular distribution asymmetries in both the singles and coincidence detection modes, in order to establish better precision in the measurements. Nevertheless the present coherence lambda , chi and gamma a, parameters for He(21P) and He(31P) state excitation for scattering angles in the range theta e=2 degrees to 25 degrees are collectively not in good agreement with any single distorted wave, first Born or close coupling model predictions, although there are ranges of scattering angles where the individual measured parameters are in good agreement with a particular theory. Radiation trapping effects on the 31P state lifetime enabled the relative relaxation rates for the population, orientation and alignment, i.e. rank 0, 1 and 2 multipoles, of the excited state to be measured. The effects of these relaxation rates on the measured intensities and the coherence parameters are quantified. The lifetime, which can be measured an order of magnitude more quickly than the state parameters, reflects these relaxation rates and provides a reliable guide to radiation trapping effects. Radiation trapping influences the state parameters down to the lowest background operating pressure of 10-7 Torr at which the measurements have been made.