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.

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Observations of triple coincidence polarization correlations of the sequential cascading photons (667.8 nm 3 1 D ! 2 1 P and 58.4 nm 2 1 P ! 1 1 S transitions) and the scattered electrons from electron impact excitation of the 3 1 D state of helium have determined the amplitudes and relative phases of the magnetic substates. The ambiguity of electron-photon coincidence measurements, which determine only the cosine of the sum of the phases between the m 62 amplitudes, is resolved. The measured amplitudes and phases at 60 eV incident energy and 40 ± scattering angle confirm the convergent close coupling values. [S0031-9007 (97)04277-4] PACS numbers: 34.80.DpFundamental studies of quantum physics frequently concern the symmetry and invariances of dynamical systems. The continuous symmetries of space, time, and rotation lead to the conservation of linear momentum, energy, and angular momentum which lead to the well known structure of atoms and their associated scattering properties. The reflection and rotational symmetries of an electronic charge distribution for a specific electronically excited state of an atom are revealed by choosing a reference axis or frame and determining the state multipoles. For example, the selection of an axis of cylindrical symmetry and detection of a nonisotropic population of the radiated photons determines the polarization of the atomic state. The selection of planar symmetry and detection of the radiated photons in coincidence with the scattered electron leads generally to knowledge of the atomic alignment and orientation. The alignment is defined as the nonisotropic population of magnetic substates of equal M with ͗M͘ 6͗J 2 ͘͞3 and determines the shape and size of the electronic charge cloud. The orientation is defined as the nonisotropic population of the 1M and 2M magnetic substates and determines the angular momentum transferred during the excitation process. Work of this type has been reviewed [1,2] and forms part of the quest for quantum mechanically complete measurements [1][2][3][4][5] to determine the scattering amplitudes and relative phases for a unique state and specific scattering dynamics.Studies of the 2 1 P state characterization with the detection of two particles (the scattered electron and the single 2 1 P to 1 1 S photon) are extensive and essentially complete [2]. However, the complete characterization of the amplitudes and relative phases for higher angular momentum states requires a lowering of the scattering symmetry and hence the detection of an increasing number of photons. For the 3 1 D state of helium studied here, the polarization requires specification of only an axis of symmetry and has been measured frequently since about 1964 [6]. However, angular and polarization correlation analyses, requiring a plane of symmetry and hence the detection of two outgoing particles (the scattered energy loss electron and either the 3 1 D to 2 1 P or 2 1 P to 1 1 S photon), have been made only since about 1987 [7][8][9][10]. Similar measurements of the correlations of th...

New measurements of the three reduced Stokes parameters for 54.4 eV electron impact excitation of the 2p state of atomic hydrogen support convergent close coupling and propagating exterior complex scaling theoretical values within two standard deviations and over a larger angular range of 2°–140° than earlier work. Whereas the present measurements and theoretical values of the circular polarization Stokes parameter P3 agree, they differ by up to about four standard deviations from the measurements by Gradziel and O'Neill (2004 J. Phys. B: At. Mol. Opt. Phys. 37 1893) in the vicinity of 30° electron scattering angle and so do not support their explanation of an increased significance of electron exchange for this excitation process at 54.4 eV. We explore some variations in experimental methods and their implications for the measurements.

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