© 2026 Aqora Quantum S.A.S.

Terms Privacy Legal Notice

Electron impact excitation of 2p states in atomic hydrogen

Marcin L. Gradziel

Published Jul 1, 2003• 0 citations

Abstract

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.

Related papers

Coherence in inelastic low-energy electron scattering

J. A. Slevin

The results of electron-photon coincidence experiments have given a new insight into the excitation of atoms by electron impact. This article reviews the way in which this new experimental technique has provided an exacting and fundamental test for the theory of the excitation process. The basic experimental method is first described, and a comprehensive treatment of excitation of the n 1 P states of helium is given. The extension of the technique to the study of excitation of the n=2 states of atomic hydrogen is then presented. Coincidence measurements also provide information on the role of spin in the collision, and this subject is treated in detail for excitation of heavy atoms by both unpolarised and spin-polarised beams of electrons. Finally, the electron-photon coincidence method permits observation of the time evolution of excited atomic states, and this review describes the attempts made to observe quantum beats in the radiative decay.

Reports on Progress in Physics•May 1, 1984•89 citations

On the coherence of the excitation of the 32P state in sodium by 100 eV electrons

P. J. O. Teubner, J L Riley, J. E. Furst +1 more

An electron-photon coincidence technique has been used to study the coherence of the excitation of the 3 2 P state in sodium. The total polarisation of the decay photons has been determined by measuring the linear and circular components of the Stokes's vector. Analysis of the reduced degree of polarisation mod P mod shows that the excitation is coherent for electrons which have been scattered through the angular range from 3 to 10 degrees . The concept of reduced polarisation assumes that the Percival-Seaton hypothesis applies to the collision. Thus the authors deduce that the nuclear spin plays no role in the excitation process at this energy and over these scattering angles. Scattering parameters have been deduced from the individual components of the Stokes's vector. These have been compared with those predicted by calculations based on the first Born and distorted-wave polarised orbital approximations. It is found that neither theory adequately describes the interaction.

Journal of Physics B Atomic and Molecular Physics•Jan 28, 1985•11 citations

Photon-scattered-atom coincidence study on K-He with vector polarisation analysis

L Zehnle, E. Clemens, P J Martin +2 more

The linear and circular polarisation of the K(4 2 P to 4 2 S) photons detected in delayed coincidence with the inelastically scattered K atoms are presented for K-He collisions at 93 eV as a function of the scattering angle. Data are analysed in terms of the Stokes parameters, and the coherence properties of the excitation process are examined. It is concluded that spin dependent forces do not play an important role in the excitation process. The lambda and chi parameters are also derived. Simple models for the excitation process are discussed in order to explain the impact parameter dependence of chi .

Journal of Physics B Atomic and Molecular Physics•Aug 28, 1978•27 citations

Angular and polarisation correlation measurements on the 21P and 31P states of helium

Khalid Salman Ibraheim

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.

•Jan 1, 1986•0 citations

The state multipoles of the n=2 state of atomic hydrogen

Jim Williams, E L Heck

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.

Journal of Physics B Atomic Molecular and Optical Physics•May 14, 1988•12 citations

Coherence study of S→D excitation: Li(2s→3d) excitation of Li-He collisions

N. Andersen, T. Andersen, John S. Dahler +3 more

Collision-induced coherent S to D excitation is investigated. A polarisation analysis of the subsequent D to P transition by the photon-particle coincidence method is not sufficient to completely determine the excitation amplitudes for the D state. Two distinct solutions are obtained. The electron distributions associated with these two are mirror images of one another in the plane determined by the normal to the scattering plane and the direction of maximum intensity of linearly polarised light, as measured from the direction of the normal. The Li(2s to 3d) excitation induced by collisions with helium is studied both theoretically and experimentally in the keV energy range. The measurements confirm the existence of a theoretically predicted low-energy structure in the circular polarisation. The results are interpreted by generalising concepts previously developed for S to P excitation in quasi-one-electron systems.

Journal of Physics B Atomic and Molecular Physics•Mar 14, 1983•43 citations

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.

The results of electron-photon coincidence experiments have given a new insight into the excitation of atoms by electron impact. This article reviews the way in which this new experimental technique has provided an exacting and fundamental test for the theory of the excitation process. The basic experimental method is first described, and a comprehensive treatment of excitation of the n 1 P states of helium is given. The extension of the technique to the study of excitation of the n=2 states of atomic hydrogen is then presented. Coincidence measurements also provide information on the role of spin in the collision, and this subject is treated in detail for excitation of heavy atoms by both unpolarised and spin-polarised beams of electrons. Finally, the electron-photon coincidence method permits observation of the time evolution of excited atomic states, and this review describes the attempts made to observe quantum beats in the radiative decay.

An electron-photon coincidence technique has been used to study the coherence of the excitation of the 3 2 P state in sodium. The total polarisation of the decay photons has been determined by measuring the linear and circular components of the Stokes's vector. Analysis of the reduced degree of polarisation mod P mod shows that the excitation is coherent for electrons which have been scattered through the angular range from 3 to 10 degrees . The concept of reduced polarisation assumes that the Percival-Seaton hypothesis applies to the collision. Thus the authors deduce that the nuclear spin plays no role in the excitation process at this energy and over these scattering angles. Scattering parameters have been deduced from the individual components of the Stokes's vector. These have been compared with those predicted by calculations based on the first Born and distorted-wave polarised orbital approximations. It is found that neither theory adequately describes the interaction.

The linear and circular polarisation of the K(4 2 P to 4 2 S) photons detected in delayed coincidence with the inelastically scattered K atoms are presented for K-He collisions at 93 eV as a function of the scattering angle. Data are analysed in terms of the Stokes parameters, and the coherence properties of the excitation process are examined. It is concluded that spin dependent forces do not play an important role in the excitation process. The lambda and chi parameters are also derived. Simple models for the excitation process are discussed in order to explain the impact parameter dependence of chi .

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.

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.

Collision-induced coherent S to D excitation is investigated. A polarisation analysis of the subsequent D to P transition by the photon-particle coincidence method is not sufficient to completely determine the excitation amplitudes for the D state. Two distinct solutions are obtained. The electron distributions associated with these two are mirror images of one another in the plane determined by the normal to the scattering plane and the direction of maximum intensity of linearly polarised light, as measured from the direction of the normal. The Li(2s to 3d) excitation induced by collisions with helium is studied both theoretically and experimentally in the keV energy range. The measurements confirm the existence of a theoretically predicted low-energy structure in the circular polarisation. The results are interpreted by generalising concepts previously developed for S to P excitation in quasi-one-electron systems.