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IEEE Transactions on Nuclear Science

Applications of Atom-Photon Coincidence Measurements to Charge Transfer

D. H. Jaecks, F. J. Eriksen, L. Fornari, A. Goldberger

Published Feb 1, 1979• 0 citations
Publisher

Abstract

Measurements have been made of the polarization of characteristic radiation of excited states, formed in charge exchange collisions, in delayed time coincidence with the neutral particles formed and scattered during the collision. Such information directly provides ratios of populations of orbital magnetic sublevels and phase differences between scattering amplitudes. This relative phase data, in conjunction with an appropriate model of the collision process, has provided insights into the evolution of the electronic charge distribution during the collision process. These techniques are also being applied to ion-molecule charge transfer collisions.

DOI
10.1109/tns.1979.4330315

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Measurements have been made of the polarization of characteristic radiation of excited states, formed in charge exchange collisions, in delayed time coincidence with the neutral particles formed and scattered during the collision. Such information directly provides ratios of populations of orbital magnetic sublevels and phase differences between scattering amplitudes. This relative phase data, in conjunction with an appropriate model of the collision process, has provided insights into the evolution of the electronic charge distribution during the collision process. These techniques are also being applied to ion-molecule charge transfer collisions.
The theory of measurements in which photons are detected in delayed coincidence with a scattered particle is developed in a form specifically applicable to atomic collisions. Equations are obtained which relate the measured coincidence rate to excitation amplitudes. These equations incorporate the polarization of the radiation, the fine and hyperfine structure of the atomic levels, the coherence of the radiation, and the time dependence of the radiation intensity. A semiclassical model for certain transitions is introduced to illustrate new features of coincidence measurements. The $\mathrm{Ly}\ensuremath{-}\ensuremath{\alpha}$ transitions in hydrogen and $^{1}P\ensuremath{\rightarrow}^{1}S$ transitions in He are treated in detail to illustrate the general theory.
Electron-photon coincidences with initially polarized electrons have been measured for the first time. Results for linear and circular polarization components of the 254-nm line (Hg $6^{3}P_{1}\ensuremath{-}6^{1}S_{0}$ transition) are presented as a function of electron energy for electrons which are scattered inelastically (63P16^{3}P_{1}63P1​ excitation) in the forward direction. The analysis of these polarization components allows one to determine the role of spin-dependent interactions during collision as well as to determine the relative phase of the two scattering amplitudes involved.
Three further types of collision between atomic systems in which no change of internal energy takes place are considered. These are collisions in which transfer of excitation occurs between two similar atoms, spin exchange collisions, and collisions in which the direction but not the magnitude of the orbital angular momentum of an excited atom is changed on impact. An account is given of the theory of these processes in relation to that for other processes, elastic scattering and symmetrical charge exchange, discussed in the earlier articles, in which no change of internal energy occurs. Methods for measuring the relevant cross sections are described. These make use of optical pumping and nuclear magnetic resonance techniques. Comparison is made between observed and calculated cross sections.
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.
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.
By solving the three-dimensional, time-dependent Schr\"odinger equation, we have demonstrated that the ultrafast charge-transfer process in ion-atom collisions can be mapped out with attosecond extreme uv (xuv) pulses. During the dynamic-charge transfer from the target atom to the projectile ion, the electron coherently populates the two sites of both nuclei, which can be viewed as a ``short-lived'' molecular state. A probing attosecond xuv pulse can instantly unleash the delocalized electron from such a ``transient molecule,'' so that the resulting photoelectron may exhibit a ``double-slit'' interference. On the contrary, either reduced or no photoelectron interference will occur if the attosecond xuv pulse strikes well before or after the collision. Therefore, by monitoring the photoelectron interference visibility, one can precisely time the ultrafast charge-transfer process in atomic collisions with time-delayed attosecond xuv pulses.