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Journal of Physics B Atomic and Molecular Physics

Time-dependent Stark-perturbed Stokes parameters in atomic hydrogen

E L Heck, Jim Williams

Published Jun 28, 1987• 14 citations
Publisher

Abstract

The 2S and 2P states of atomic hydrogen are excited by 350 eV electrons in the presence of a parallel electric field of 250 V cm -1 . The time decay of the Stark-perturbed states has been observed through the detection of the 10.2 eV energy loss scattered electrons in coincidence with the 121.6 nm circularly polarised photons. The time development of the eta 2 Stokes parameter shows well defined Lamb shift beats with a period of 0.7 ns; the sign of eta 2 changes as the sign of the electric field changes. With a negative electric field eta 2 changes from negative to positive values after 3 ns indicating a relative change in the decay of the 2S and 2P states. This behaviour is explained in terms of the time development coefficients and their dependence on the electric field. These measurements are the most sensitive test of scattering theory and are in good agreement with the prediction of van Wyngaarden and Walters (1985) at 350 eV and 3 degrees scattering angle.

DOI
10.1088/0022-3700/20/12/031

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•Jan 1, 1986•0 citations
The 2S and 2P states of atomic hydrogen are excited by 350 eV electrons in the presence of a parallel electric field of 250 V cm -1 . The time decay of the Stark-perturbed states has been observed through the detection of the 10.2 eV energy loss scattered electrons in coincidence with the 121.6 nm circularly polarised photons. The time development of the eta 2 Stokes parameter shows well defined Lamb shift beats with a period of 0.7 ns; the sign of eta 2 changes as the sign of the electric field changes. With a negative electric field eta 2 changes from negative to positive values after 3 ns indicating a relative change in the decay of the 2S and 2P states. This behaviour is explained in terms of the time development coefficients and their dependence on the electric field. These measurements are the most sensitive test of scattering theory and are in good agreement with the prediction of van Wyngaarden and Walters (1985) at 350 eV and 3 degrees scattering angle.
A determination of all the state multipoles of the n=2 excited state of atomic hydrogen has been made. The kinematical conditions were a crossed electron-atom-beam geometry in which the incident electron energy was 350 eV and the scattering angle 3'. The experimental technique measures the scattered 10.2 eV energy loss electrons in coincidence with the 10.2 eV photons from either the 2P state or the delayed Stark-quenched 2S-state decay. An external electrostatic field mixes the S and P states, permitting the observation of their coherence by measuring the linear and circular photon polarisations, while in-plane angular correlations for a zero electric field determine the P-state multipoles. The state multipoles were deduced from a least-squares fit of the measured data to the quantum mechanical expressions determined for a model of an S-P (m j =1/2) superposition excited state with averages over the singlet and triplet spin states.
This thesis presents the results of experimental investigation of coherence in the excitation of the 2p states of atomic hydrogen through a collision with an unpolarised electron. The polarisation correlation variation of the delayed coincidence method has been employed to simultaneously measure the reduced Stokes parameters P1,P2 and P3, which are needed to fully characterise the state of the atom immediately after the collision. This has also allowed the excitation coherence parameter P+ to be determined directly. In the case of the excitation of 2p states of hydrogen, a deviation of P+ from unity is indicative of the importance of spin exchange during the collision. A new Lyman- polarisation analyser, that permits independent rotation of a retarder and a linear polariser, was developed for this work. A novel approach to the analysis of data from electron-photon coincidence measurements has been implemented to improve the quality of the final results. The reduced Stokes parameters P1,P2 and P3 were measured at 54.4 eV electron energy, and in the 4-40 range of electron scattering angles. The P1 and P2 results are consistent with the earlier measurements, and recent theoretical predictions. The new P3 results, however, deviate significantly from the predictions of all modern theories, and also differ from previous experimental results. The newly measured values of P+ also suggest less coherent excitation than predicted by the theory, even at a relatively low scattering angle of 30. Several possible sources of systematic error including resonance radiation trapping, Stark mixing and erroneous phase-shift of the retarder have been considered, but are insufficient to explain the observed deviations from theory. Therefore, we are forced to conclude that spin exchange may play a more significant role during the collision than current theoretical methods predict.
Electron-photon angular correlations have been measured for excitation of the 2P2P2P states of hydrogen at an incident energy of 35 eV. The data presented relate to electron-scattering angles from ${\ensuremath{\theta}}_{e}=8\ifmmode^\circ\else\textdegree\fi{}$ to ${\ensuremath{\theta}}_{e}=120\ifmmode^\circ\else\textdegree\fi{}$ and yield values for the parameters $\ensuremath{\lambda}$ and RRR. The experimental results are compared with several theories. In general, none of the theories is found to be adequate at this energy.
The evolution of a (n=2) hydrogen atom passing through a linearly increasing or decreasing electric field is calculated in a three-state approximation. The observable fraction of 2p 1/2 , 2s 1/2 and 2p 3/2 quantum state in the resulting coherent mixture is calculated. The amplitude of the oscillations, which appear when the field parameters are varied, is shown to depend only on the rate of change of the field, as seen by the atom. Maps of oscillation amplitudes and average values are given for the metastable atom. Comparison is made with the sudden and adiabatic approximations.
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.