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Measurement Science and Technology

Systematic errors in electron-photon coincidence measurements of the 31P state of helium

Ian Humphrey

Published Feb 6, 2001• 2 citations
Publisher

Abstract

The coherence parameters associated with the electron impact excitation of the He 3 1 P state are studied for a Monte Carlo simulation of an experiment consisting of crossed electron and atom beams. The effects of changes in gas pressure and interaction region geometry are monitored by a fixed polarimeter analysing radiation resulting from the 3 1 P to 1 1 S (53.7 nm) transition. This work extends the simulations of Humphrey with the incorporation of a finite electron detector, a region of background gas and angular dispersion in the electron beam. Humphrey found that with an infinitesimal electron detector, the measured polarizations were not strongly dependent on interaction region geometry. In this work, however, it is found that the coherence parameters are very strongly dependent on interaction region geometry, pressure and the solid angle of the electron detector. Even very small solid angles can adversely affect the measured parameters. While angular correlations are not specifically studied, there are indications from simulations of electron scattering angle integrated polarizations that it is most important that any measurements involving a rotation of the photon detector should rotate about a symmetry axis of the gas source.

DOI
10.1088/0957-0233/12/3/303

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The coherence parameters associated with the electron impact excitation of the He 3 1 P state are studied for a Monte Carlo simulation of an experiment consisting of crossed electron and atom beams. The effects of changes in gas pressure and interaction region geometry are monitored by a fixed polarimeter analysing radiation resulting from the 3 1 P to 1 1 S (53.7 nm) transition. This work extends the simulations of Humphrey with the incorporation of a finite electron detector, a region of background gas and angular dispersion in the electron beam. Humphrey found that with an infinitesimal electron detector, the measured polarizations were not strongly dependent on interaction region geometry. In this work, however, it is found that the coherence parameters are very strongly dependent on interaction region geometry, pressure and the solid angle of the electron detector. Even very small solid angles can adversely affect the measured parameters. While angular correlations are not specifically studied, there are indications from simulations of electron scattering angle integrated polarizations that it is most important that any measurements involving a rotation of the photon detector should rotate about a symmetry axis of the gas source.
Electron-photon coincidence experiments are usually carried out by crossing an electron beam with a beam of atoms emerging from a capillary tube or array. Recently it has been found that the finite dimensions of the interaction region formed by the crossing beams can affect the coherence parameters that are measured in these experiments. The authors have developed models with which these effects and those related to the finite acceptance angles of the detectors can be simulated numerically. This article presents a description of the models and presents results which illustrate the possible magnitude of these effects in electron-rare gas scattering experiments. Other depolarizing effects such as internal atomic interactions are also discussed.
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.
The authors present circular-polarisation measurements of 3 1 P to 2 1 S photons (501.6 nm) detected in delayed coincidence with electrons inelastically scattered upon excitation of the 3 1 P state of helium. They performed these experiments as a function of the electron scattering angle at incident electron energies of 80, 60 and 50 eV in order to study the orbital angular momentum transfer during the collision. The experimental results are compared with previous angular correlation data and with theory. For each incident electron energy they found a sign reversal of the angular momentum transfer as a function of the electron scattering angle.
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.
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.
It is possible to measure electron impact coherence parameters (EICPs) at large scattering angles with application of magnetic angle changing devices, however the presence of magnetic field can strongly influence experimental results for the excited states of longer lifetimes. We consider a new type of MAC device producing very low magnetic field in the scattering region and a new approach to coincidence measurements – application of multiple photon detectors with individual coincidence circuits to reduce the time of data accumulation.