Electron-photon Coincidence Experiment Research Articles

Comparison of previously unpublished stepwise excitation alignment and orientation data with recent direct coincidence data for the 61P1 state of mercury

Recent alignment and orientation data from a direct electron-photon coincidence experiment is compared with previously unpublished data from a stepwise excitation coincidence experiment. The stepwise excitation data is also used to further investigate novel radiation trapping effects that can arise in stepwise excitation experiments.

Magnetic angle changer—a new device allowing extension of electron–photon coincidence measurements to arbitrarily large electron scattering angles

The electron–photon coincidence technique has been used as the most sensitive tool for investigating inelastic collisions of atoms with electrons and heavy particles and led to new insights into collision dynamics theory. However, for purely geometrical reasons, due to finite sizes of the electron energy analysers and electron beam sources, the measurements have not been carried out for the largest scattering angles. We present a novel system for electron–photon coincidence experiments on electron inelastic scattering with the magnetic angle changer allowing determination of excitation parameters at arbitrarily large scattering angles. As shown by experimental tests, our device can be successfully used in the electron–photon coincidence experiment and it has unprecedented efficacy—deflection angle up to 60° for 100 eV electrons, without a ferromagnetic core.

Photon polarization and spin asymmetry in the elementary process of bremsstrahlung

Experimental and theoretical results are reviewed concerning the photon polarization and spin asymmetry in the elementary process of bremsstrahlung. In electron–photon coincidence experiments using an unpolarized primary beam (300 keV) the electron–nucleus bremsstrahlung was found to be almost completely linearly polarized. The same behavior was found in electron–electron bremsstrahlung. By using a transversely polarized electron beam the photon emission asymmetry was measured for fixed direction of the outgoing electrons.

Distribution of electron clouds in the excited atomic state in electron-photon coincidence scattering experiment

Based on the general theory of the electron-photon coincidence experiment, the S tokes parameters of an excited atomic state were calculated with the distorted w ave Born approximation. According to the correlation between experiment and theo ry, the distribution of electron clouds in an excited atomic state can be descri bed by their orientation and alignment parameters. The values obtained are in g reement with the results of the experiments.

Solid angle and lens corrections for light-polarization analysis inatomic scattering experiments

Although it is a routine matter to correct for the systematic effects of a finite solid angle and the polarizing effects of a light-collection system in atomic collision studies, the methods for conducting such corrections are rarely (if at all) presented. It is the intention of the present work to present explicit formulae for the general case and explicit results for some popular experiments. No experimental results are presented. Integration over the solid angle of a conical light-collection system and the polarizing effect of a lens will be discussed. Three sample cases for which the solid-angle integration can be performed analytically are presented: a standard J = 1 J = 0 (or P S) electron-photon coincidence experiment; a laser-mediated stepwise coincidence experiment (laser-excited J = 1 J = 2 and spontaneous J = 2 J = 1); and an uncorrelated 3P0,1,2 3S1 light-polarization measurement (such as is commonly used to establish electron spin polarization). For most realistic cases, the total effect is very small and, furthermore, the lens, with some important exceptions, only marginally modifies the result of a pure solid-angle integration. The effect of internal absorption within the lens is considered and shown to be negligible. In-plane Stokes' measurements, such as of P4 (or the stepwise P190 parameter), are most seriously affected by these corrections.

Multipole treatment of radiation trapping in a stepwise-excitation, electron-photon coincidence experiment.

A theory is presented of radiation trapping effects in time-resolved, stepwise-excited, electron-photon coincidence signals. The theory is based on a detailed analysis of laser-induced optical pumping effects in the stepwise excitation process combined with a multipole treatment of the radiation trapping processes. New data from a stepwise excitation experiment on the inelastic excitation of the 6${\mathrm{}}^{1}$${\mathit{P}}_{1}$ state in ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$-Hg collisions and previously reported data are compared with the predictions of the theory. Good agreement is found between experiment and theory, including confirmation that under some experimental conditions, the ${\mathit{P}}_{10}$ Stokes parameter of the coincidentally detected stepwise excited fluorescence increases in magnitude with increasing target gas density. \textcopyright{} 1996 The American Physical Society.

Orientation of the 41P state in calcium by electron impact

This paper reports the first study of superelastic electron scattering from optically pumped calcium atoms. The orientation parameter Lperpendicular to has been measured at two energies, 25.7 and 45 eV, referred to the ground state. The angular range of the experiments was 3 degrees to 100 degrees . The present results are compared with those obtained from an electron-photon coincidence experiment and with the predictions of several calculations based on a distorted wave formalism. Poor agreement is found with the previous experiments. Agreement between the present work and theory is better at 45 eV than at the lower energy.

Electron-photon coincidence experiment after polarized-electron impact on xenon

Light-polarization measurements have been performed after polarized-electron impact excitation of the xenon resonance levels. Both electron-photon coincidence experiments as well as measurements of the integrated Stokes parameters have been carried out with a vacuum-ultraviolet analyser of the reflection type. A complete set of integrated Stokes parameters is given, which partly depend on the electron polarization and allow one to draw conclusions on spin effects involved in the scattering process. At a scattering energy of 25 eV and angles between 10 degrees and 20 degrees a spin up-down asymmetry was measured using the coincidence technique. A comparison of our experimental data with a DWBA calculation shows good agreement.

Relative differential cross sections for the electron impact excitation of the 33P, 31D and 33D states of helium

The authors present measurements of relative differential cross sections for the electron impact excitation of the 31D, 33D and 33P states of helium at energies of 40, 45, 60 and 80 eV and for scattering angles ranging from 30' to 120'. The results are extracted from existing data of electron-photon coincidence experiments. The results are in agreement with distorted wave calculations. The authors point out the consistency of the results with the angular behaviour of the coherence parameter rho 00 also obtained from these coincidence experiments.

The effect of a finite interaction region on the measurement of coherence parameters in electron-photon coincidence experiments

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.

Electron impact excitation of the 33P state of helium: coherence parameters

In an electron-photon coincidence experiment the authors have measured the polarization degree of the decay radiation from the electron impact excited 33P to 23S transition in helium. They performed their measurements perpendicular to the scattering plane. To analyse the polarization they used a beam splitting polarizer instead of the usual sheet polarizer. The linear polarization degree was measured at an electron impact energy of 40 eV and the circular polarization degree was measured at 40, 60 and 80 eV. The electron scattering angle ranged from 32.8' to 116'. The experimentally determined degrees of polarization are in good agreement with the theoretical results of the distorted-wave Born approximation (DWBA). The scaling relation of Lin et al. (1989) is in agreement with the DWBA results of the circular polarization for 33P excitation.

Electron Scattering from Optically Pumped Sodium Atoms

Abstract A superelastic electron-scattering technique is described in which electrons are scattered from optically pumped sodium atoms. The scattering parameters derived from these experiments are shown to provide a sensitive and dynamic description of the excited 32P state of the sodium atom. These parameters are compared with those derived from electron-photon coincidence experiments which are the time reversed equivalent of the present experiments. Although the present results agree with the previous data, the much larger kinematic range of the present work indicates that optical pumping will be the technique of choice for future measurements. The parameters are also compared with those predicted by a coupled-channels theory where fair agreement is found.

Electron-laser stepwise excitation coincidence experiment on the 6(1)P1 state of mercury.

A new type of electron-photon coincidence experiment is presented which utilizes stepwise electron and laser excitation of the target atoms. The technique has been applied to the measurement of atomic collision parameters for e/sup -/-Hg(6 /sup 1/P/sub 1/) collisions at an incident electron energy of 16.0 eV.

Effect of a finite angular resolution on the measurement of polarization correlation parameters in electron-heavy-noble-gas collisions.

We present the results of a simple model calculation that highlights the effect of the finite (i.e., nonzero) angular resolution of electrostatic energy analyzers on the measurement of polarization correlation parameters in electron-photon coincidence experiments. It is demonstrated that measurements can yield drastically reduced ``apparent'' parameters in cases where the coincidence parameters vary rapidly as a function of electron scattering angle. For example, a measurement of the ${P}_{1}$ parameter for Ar${(}^{1}$P) excited by 100-eV electrons which are scattered in the forward direction and detected by an analyzer with an angular resolution (full width at half maximum) of 3\ifmmode^\circ\else\textdegree\fi{} will yield an apparent ${P}_{1}$ value of 0.83 compared to the nominal value of 1.00. Since the measurement of ${P}_{1}$ in the forward direction reveals important information about the relative contributions of direct excitation and exchange excitation, it is imperative that an experiment can distinguish a ${P}_{1}$ value truly smaller than unity from an ``apparent'' value which is reduced merely by instrumental effects.

Angular correlation parameters for the electron impact excitation of the 21 P and 31 P states of helium using first-order many-body theory

A simplified form of the first-order many-body theory was used for the calculation of the coherence and correlation parameters for the excitation of the 21P and 31P states of helium. The excited state wave functions were calculated numerically in the fixed core Hartree-Fock(HF) approximation. The ground state wave function was used in the HF approximation. Scattering orbitals were calculated numerically in the static exchange approximation. Calculations were performed forE=29.6, 40, 50, 60 and 80 eV impact energies for the 21P state, and forE=50 eV and 80 eV for the 31P state. Results for the coherence and correlation parameters are compared with the experimental values obtained from electronphoton coincidence experiments, and with other theoretical results.

A polarisation correlation study of the excitation of the H2d3Πu(ν1=0, N1=1) level by electron impact

Full details are given of an experiment in which the three normalized Stokes' parameters characterizing the decay of the electron impact excited d3Piu (v1 = 0, N1 = 1) rotational level of H2 were determined in an electron-photon coincidence experiment at 25 eV impact energy and over the angular range of 20-60 deg. An analysis shows how these parameters are related to the state multipoles characterizing the excited state. The rather low degree of coherence which was observed is consistent with various depolarizing effects, namely the averaging over magnetic sublevels which occurred in both the excitation and decay channels and also the perturbations due to fine and hyperfine structure relaxations.

Rotationally resolved electron-photon coincidence study of H2(d3Πu) exciton

The three normalised Stokes' parameters characterising the decay of the electron impact excited d3 pi u ( nu 1=0, N1=1) rotational level of H2 were determined in an electron-photon coincidence experiment at 25 eV impact energy and 60 degrees scattering angle. The low degree of polarisation (coherence) which was observed could be caused by both the averaging over the magnetic sublevels of the ground state of the target and the fine and hyperfine relaxation process. The average angular momentum transfer perpendicular to the scattering plane from the electrons to the molecules was found to be near zero in this excitation process.

Excitation and decay of Stark-mixed n=2 states of hydrogen observed in an electron-photon coincidence experiment

Measurements have been made of the Lyman- alpha intensity from Stark-mixed n=2 states of hydrogen, using the electron-photon coincidence technique at an incident electron energy of 350 eV. The data are related to a combination of excitation amplitudes, including a term which depends on the relative phase of S and P amplitudes. Field-free measurements of lambda and R have also been made at this energy. Comparison is made with several theoretical results.

Photon linear polarization in the elementary process of atomic-field bremsstrahlung

In an electron-photon coincidence experiment the angular dependence of the photon linear polarization in the elementary process of atomic-field bremsstrahlung was measured for fixed direction and energy of the outgoing electron and for coplanar geometry. An electron beam of 300 keV was used, incident on copper and gold targets. It was found that for photon-emission angles with large cross section the radiation is almost completely polarized with the electric vector in the reaction plane. A decrease of the polarization caused by electron spin-flip radiation was observed for a photon-emission angle near the minimum of the cross section. The measurements are compared with theoretical calculations.

Orbital angular momentum transfer in the excitation of the 21P state of helium by electrons

The authors have studied the orbital angular momentum transfer (as a function of the electron scattering angle) in the excitation of the 21P state of helium by electrons at incident electron energies of 50, 60 and 80 eV. This was done in an electron-photon coincidence experiment. At 80 eV the orbital angular momentum transferred by the electron to the atom appears to change sign at a certain scattering angle. At lower energies of 50 and 60 eV no sign reversal is observed. These results can be explained qualitatively by the semiclassical model of Steph and Golden (1980).