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Physical Review Letters

Relaxation rates of the monopole, magnetic dipole, and electric quadrupole moments of the 31 Pstate of helium

Jim Williams, Andrew G. Mikosza, Jingbo B. Wang, A. Bruce Wedding

Published Aug 3, 1992• 15 citations
Publisher

Abstract

The relaxation rates of the monopole, magnetic dipole, and electric quadrupole moments have been determined for the ${3}^{1}$P state of helium excited by electron impact, The data are determined from atomic lifetimes using linear and circular polarization analyzed radiation with the electron-photon delayed coincidence technique. This has permitted the direct observation of the time dependence of all the off-diagonal elements of the density matrix of the excited state. The implications for atomic state information determined from polarization and angular correlation methods are indicated.

DOI
10.1103/physrevlett.69.757

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The delayed coincidence technique was applied to investigate angular and polarization correlations between electrons scattered inelastically from helium atoms and photons emitted in the decay of n1P states. \n \nElectron photon angular correlations were studied for the excitation of the 21P and 31P states of helium at an incident electron energy of 50 eV and for electron scattering angles in the range of 20° - 102° and 35° - 55o respectively. From the analysis of these measurements the excitation parameters λ = (|a¬o| 2)/(2| a1| 2+|ao| 2) = σo/σ and the magnitude |x| of the phase difference between the excitation amplitudes a_ML for the M_L = 1 and M_L = 0 states are obtained. \n \nUsing the 3¬¬1P - 21S decay light the electron-photon polarization correlation were measured for the 31P state of helium at electron impact energies 50, 80, 120 and 160 eV and for electron scattering angles between 27.5° and 108°. The Stokes parameters n1, n2 and n3 were derived and the sign change of the angular momentum transfer < Ly > to the atom was investigated using the circular polarization results (n2). A sign change was confirmed for all energies studied and the scattering angle of the sign change was found to increase with decreasing electron energy. The excitation parameters λ and x have been derived and the present results are compared with theoretical calculations and with previous experimental values obtained from angular and polarization correlation measurements.
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We present linear and circular polarization measurements of $3^{1}P\ensuremath{-}2^{1}S$ (501.6 nm) photons detected in delayed coincidence with electrons inelastically scattered from the 31P3^{1}P31P level of helium. Vector polarization and coherence parameters of the coincident radiation provide direct experimental evidence of the coherent nature of the excitation process. Atomic orientation is obtained directly from the circular polarization measurements. Analysis of the data also yields $\ensuremath{\lambda}$ and $\ensuremath{\chi}$ parameters for 31P3^{1}P31P excitations, which are compared with previous data and theory.
Abstract Influence Using impact excitation, the measurement of alignment relaxation rates of atomic states is influenced by cascading transitions. The two methods for measuring the collisional depolarization, the Hanle-effect and the direct measurement of the polarization of the emitted radiation, were investigated theoretically and compared. The results were checked by measuring the depolarization of helium states excited by electron impact. The depolarization cross sections of the 3,4,5 3P-states were determined from Hanle-effect measurements as 1.8(4), 3.9(5), and 5.6(6) · 10-14 cm2 respectively.
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