H. Kleinpoppen

Fundamental processes in energetic atomic collisions

This book starts with a brief review of the formal scattering theory by Massey. The chapters that follow survey a coherent selection of interesting topics in energetic ion-atom collisions. The last third of the book contains papers on coherence and correlation in atomic collisions and on new aspects in the study of atomic collisions. The papers on coherence and correlation report progress toward complete experiments where scattering amplitudes and phases are determined in coincidence experiments. The papers on new aspects cover the use of polarized particles in atomic experiments.

Lifetime Measurement of the 33P state of helium

The electron-photon delayed coincidence technique has been used to measure the lifetime of the 33P excited state in helium from exponential decay of the time coincidence spectrum. The present results are in good agreement with corresponding results of other workers.

Electron-impact coherence parameters for the excitation of the 51P state of Sr

Electron-photon polarization correlation measurements have been carried out for the excitation of the 51P state of Sr at electron impact energies of 30.3 and 58.4 eV and electron scattering angles of 20°–130° for 30.3 eV and 20°–100° for 58.4 eV. The resulting Stokes parametersP1,P2,P3 are used to derive the usual complete scattering parameter sets λ, χ and γ,L⊥,Pl. New FOMBT calculations for these parameters are reported alongside the measured data and show substantial agreement with the experiment and with recent calculations by Srivastava et al.

Polarization Correlation in Simultaneous Two-Photon Decay Processes and Tests of Bell’s Inequality

This lecture considers the correlation in the polarization of two photons emitted simultaneously from a common source. The only two processes of this kind on which experiments have been carried out, are the annihilation of para-positronium, in which an electron and positron in a singlet state are converted into two identical photons each with energy 0.511 MeV, and the decay of metastable atomic hydrogen. The former process has been studied experimentally quite extensively and the results are well documented1,2 so we shall concentrate our attention on the decay of metastable atomic hydrogen, for which the polarization correlation has only recently been observed at Stirling3, to illustrate the main features of this type of process and its application to tests of so called “hidden variable” theories and Bell’s inequality. We shall not consider the extensive work4 which has been carried out using atomic cascades in which the two photons are not emitted simultaneously, although many of the conclusions we shall reach also apply to these cascades.

The collision-induced electric dipole moment of H(n=2) in H-He, Ne and Ar collisions

The collision-induced electric dipole moment of H(n=2) in H(1s)-Ne and H(1s)-Ar collisions was measured in an energy range of 1 to 25 keV. For these systems we observe a positive electric dipole moment which corresponds to an electron lagging behind the proton. This behaviour is in contrast to recent measurements for the H-He systems, where a negative dipole moment corresponding to an electron moving in front of the proton was observed. A simple explanation for this difference is given.

Depolarization of atomic two-photon radiation

A technique of depolarization is used to investigate the coherence and spectral properties of the radiation emitted in the two-photon decay of metastable atomic deuterium. The results agree with the quantum mechanical predictions but may also be interpreted to show that a single photon of a two-photon pair can be considered to have a very short coherence length, a very short coherence time and a broad bandwidth, all limited, in practice, by the transmission properties of the detection system.

Polarization and Coherence Analysis of the Optical Two-Photon Radiation from the metastable2 2 S 1/2 State of Atomic Hydrogen

The Paper first summarizes fundamental aspects and results of the quantum electrodynamical theory of the two-photon radiationfromthe decay of the metastable 2 2 S 1/2 atomic hydrogen state. After a brief description of the second improved Stirling two-photon coincidence experiment polarization correlations of the two-photon decay are described in which both two or three linear polarizers are applied in order to test predictions of such correlations based upon quantum mechanics and local realistic theories (i.e., Einstein-Podolsky-Rosen type experiments). It is particularly noticeable that the three-polarizer coincidence measurement provided the largest difference (about 40%) between the Bell limits of local realistic theories and quantum mechanics so far. Apart from confirming in addition the correlations of right-right and left-left circularly polarized two-photon correlations a new type of coherence analysis of the two-photon radiation has been carried out experimentally and theoretically. A result of it is the measured coherence time of τ coh = 1.2 · 10 −15 s and coherence length of l coh = c · τ coh = 350 nm of the two-photon emission. By applying a theoretical model of the two-photon radiation linked to cascade transitions the coherence length can be estimated to l coh ≈ 100 nm in agreement by order of magnitude with the experimental data.

Coherent Excitation of H(n=2) in Few-Electron Collision Systems

Excitation processes in simply-structured one- and two-electron systems are both of fundamental importance and of practical relevance for a number of fields. Valuable information about the dynamics of such collisions can be extracted from a measurement of so-called coherence parameters. These parameters relate to the population of energetically degenerate states and are sensitive to the relative phases between the corresponding excitation amplitudes. In collisions involving hydrogenic atoms, not only the coherence between the energetically degenerate magnetic substates (magnetic quantum number m) but also between the near-degenerate angular momentum states (angular momentum quantum number l) with the same principle quantum number n becomes accessible1.