Photon angular correlations and pressure-dependent effects of helium 21P and 31P excitation by electrons
- 14 April 1994
- journal article
- Published by IOP Publishing in Journal of Physics B: Atomic, Molecular and Optical Physics
- Vol. 27 (7), 1429-1451
- https://doi.org/10.1088/0953-4075/27/7/018
Abstract
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.This publication has 37 references indexed in Scilit:
- Polarisation correlation measurements on the 31P state of helium for positive and negative scattering anglesJournal of Physics B: Atomic, Molecular and Optical Physics, 1988
- Collisional alignment and orientation of atomic outer shells I. Direct excitation by electron and atom impactPhysics Reports, 1988
- Correlations in Electron-Atom ScatteringPublished by Elsevier ,1988
- Differential cross sections and angular correlation parameters for electron impact excitation of the 33P, 31D and 33D states of heliumJournal of Physics B: Atomic, Molecular and Optical Physics, 1988
- Alignment and orientation in He 21P and 31P excitation by electron impactJournal of Physics B: Atomic and Molecular Physics, 1987
- Angular correlation measurements for the electron-excited 31P state of heliumJournal of Physics B: Atomic and Molecular Physics, 1981
- The 11S to 21S and 11S to 21P excitation of helium by electron impactJournal of Physics B: Atomic and Molecular Physics, 1980
- Electron-photon angular correlation in atomic physicsPhysics Reports, 1979
- Electron-photon angular correlations in electron-helium collisions: measurements of complex excitation amplitudes, atomic orientation and alignmentJournal of Physics B: Atomic and Molecular Physics, 1974
- Excitation processes in heliumProceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences, 1960