A J Duncan

Threshold behaviour of Ar K and Xe L3 ionisation by electron impact

Ionisation of the Ar K and Xe L3 shells has been investigated by electron impact with energies from about 10 eV to 1 keV above threshold. The experimental data are compared with the theoretical predictions by Wannier for ionisation near threshold and with more recent plane-wave Born and Coulomb-Born exchange calculations.

The two-photon decay of metastable atomic hydrogen

The two-photon decay of metastable atomic hydrogen has been observed for the first time. A series of tests which confirm the observations are described, including preliminary measurements of the angular distributions and spectral distribution for the two-photon decay.

Doubly differential cross sections for the ionization of the molecule by electron impact

The partial doubly differential cross sections (PDDCS) for the ions , , , , (, ), , , and resulting from the dissociative ionization of the molecule by electron impact employing the ejected-electron and produced-ion coincidence technique have been measured. Measurements have been made for incident electron energies of 100 and 200 eV and for ejected electron energies of 30, 50, 65, 100 and 150 eV. The angular range for the detection of the ejected electron is between and . No trace of ions is found at any energy used. No previous data or theoretical predictions exist with which to make comparisons.

Two-Photon Polarization Fourier Spectroscopy of Metastable Atomic Hydrogen

For many years the theoretical and experimental study of atomic hydrogen has been used to improve our understanding and extend our knowledge of the fundamental properties and behaviour of atoms. The states with principal quantum number n=2 are and have been of special interest and importance, in particular with regard to the determination of the fine structure constant and measurement of the Lamb shift. It was, of course, the observations of the Lamb shift in 1947 and 1950 by Lamb and Retherford [1] which, by demonstrating the nondegeneracy of the 22S½ and 22P½ states, confirmed that the 22S½ state would be metastable in experimentally realisable situations, and showed that it should be possible to observe the two-photon emission which is the main mode of decay of this state. However, Goppert-Mayer, in 1931, in a paper [2] which pioneered the field of multiphoton transitions, was the first to predict the possibility of the spontaneous two-photon decay process and, in 1940, Breit and Teller [3] applied this theory to the 22S½ − 12S½ transition in atomic hydrogen. Improved calculations of the characteristics of the two-photon decay process were carried out by Spitzer and Greenstein [4], Shapiro and Breit [5], Zon and Rapaport [6], Klarsfeld [7] and Johnson [8]. Further refinements to the theory have been made for example by Goldman and Drake [9], Parpia and Johnson [10], Tung et al [11], Florescu [12], Costescu [13] and Drake [14]. A comprehensive review concerning the metastability of atomic hydrogen up to 1969 was given by Novick [15] in which he emphasised the various controversies with regard to the metastability or otherwise of the 2S state during the first part of this century.

The effect of optically active devices on the polarisation properties of ultrashort pulses

The interaction of linearly polarised ultrashort laser pulses with optically active devices such as half- or multi-order wave plates is considered. It is shown that, for pulses having coherence lengths comparable to their centre wavelength, such devices do not act in the usual manner and a method of measuring the duration of the pulses is proposed. It is pointed out that radiation of sufficiently broad bandwidth is capable of exhibiting a polarisation state not possible for quasi-monochromatic radiation.

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.

Circular polarization correlation of the two photons emitted in the decay of metastable atomic deuterium

The circular polarization correlation of the two photons emitted simultaneously in the decay of metastable atomic deuterium has been measured for the first time. Taking full account of the imperfections of the achromatic quarter-wave plates and linear polarizers used in the experiments, it is concluded that the results are most probably consistent with the predictions of the quantum mechanical formalism but do not provide a test of Bells inequality.

Excitation of spin-polarized sodium and potassium atoms by electron impact

A beam of unpolarized electrons is crossed at right angles with a beam of partially polarized sodium or potassium atoms (degree of polarization PA approximately=21%) and resonance lines from the excited 2P3/2, 1/2 states are observed in a direction perpendicular to both the atom and electron beams for electron energies from threshold to 200 eV. The polarization of the emitted photons is measured in the form of the Stokes parameters P1, P2 and P3. The present measurements for sodium atoms are in better agreement with the theoretical values than the previously published experimental results. No other experimental or theoretical results are available for potassium atoms.

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.