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Physical Review A

Electron-photon angular correlation measurements for the 2P state of hydrogen at 35 eV

J. A. Slevin, M. Eminyan, J. M. Woolsey, G. Vassilev, H. Q. Porter, C. G. Back, Simon Watkin

Published Sep 1, 1982• 31 citations
Publisher

Abstract

Electron-photon angular correlations have been measured for excitation of the $2P$ 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 $R$. The experimental results are compared with several theories. In general, none of the theories is found to be adequate at this energy.

DOI
10.1103/physreva.26.1344

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Measurements are presented representing a complete set of electron-photon polarization correlation parameters for the excitation of n 1P^{1}P1P states of He at an incident energy of 80 eV and an electron scattering angle of 20\ifmmode^\circ\else\textdegree\fi{}. The data support the predictions of a recent theoretical paper that these parameters should exhibit little variation with n. However, disagreement in absolute values between experiment and theory indicates the need for additional theoretical input into the problem.
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.