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Journal de Physique IV (Proceedings)

Coherence between the excitation of states with different energies

J. A. de Gouw, J. van Eck, S. A. van der Aart, H G M Heideman

Published Nov 1, 1993• 0 citations
Publisher

Abstract

Abstract: In recent experiments we have shown both in electron impact and in photoionisa-tion studies the possibility to determine coherences between the excitation of states of different energies. In this paper the consequences of applying coincident techniques to these experiments are discussed. In a coincidence experiment one has to choose, however, for lower energy resolu-tions in order to reach sufficient count rates. We present an experiment which shows that these lower resolutions do not totally obscure the formerly observed interference effects, thus proving the feasibility of a future coincidence experiment. 1

DOI
10.1051/jp4:1993620

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Abstract: In recent experiments we have shown both in electron impact and in photoionisa-tion studies the possibility to determine coherences between the excitation of states of different energies. In this paper the consequences of applying coincident techniques to these experiments are discussed. In a coincidence experiment one has to choose, however, for lower energy resolu-tions in order to reach sufficient count rates. We present an experiment which shows that these lower resolutions do not totally obscure the formerly observed interference effects, thus proving the feasibility of a future coincidence experiment. 1
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.
The authors present experimental data on the autoionisation of neon and argon excited by electron impact. The energy of the incident electrons is chosen such that the two outgoing electrons in the final state have comparable energies. In this energy region state-state coherences and post-collision interactions (PCI) between the two electrons may be observed. Examples of observations of state-state interferences are presented and the effect of PCI in these experiments is discussed. A detailed analysis of the measurements is complicated due to the presence of overlapping neighbouring states.
A brief review is given on development of electron-photon coincidence studies from electron impact of atoms. The theory of electron-photon correlations is outlined in detail in terms of cross sections of excited atoms and excited state charge clouds. The state of a target atom is determined by the various orientation and alignment parameters and the multipole moments of the excited atom. General considerations, such as coherence effects in impact excitation, the influence of fine and hyperfine structures in correlations are taken into account. The relationship between scattering amplitudes (including their phases) and target parameters is worked out for singlet and doublet transitions.