G Nienhuis

Coherences between autoionising states of different excitation energies

The authors have observed coherences between the electron impact excitation of autoionising states of helium that are separated in energy up to more than 2 eV. The method is based on the occurrences of interferences between scattered and ejected electrons resulting from the electron impact excitation and subsequent autoionisation of the states concerned. No energy shifting or broadening mechanisms such as those caused by post-collision interactions play a role in the experiment. The authors also present a theoretical description of the observed coherence effects. From this theory a parametrisation for scattered- and ejected-electron profiles is deduced which is fitted to experimental data. This parametrisation appears to describe the observed state-state interference very well.

A scattered-electron cascaded-photon coincidence experiment on the electron impact excitation, of the 31D state of helium

A modified electron-photon coincidence experiment on the excitation of the 31D state of helium is suggested. The idea is to measure the scattered electrons, which have excited the 31D state, in coincidence with 21P to 11S photons resulting from the 31D to 21P to 11S cascade. Due to their large energy the latter can be detected with higher efficiency than the directly emitted 31D to 21P photons. A theory is developed to show that the proposed experiment yields exactly the same information on the 31D excitation as a conventional coincidence experiment between scattered electrons and 31D to 21P photons. Preliminary experiments have been performed, in which the 31D excitation is measured simultaneously with the 31S and 31P excitation. The coincidence signal due to the 31S excitation, with its isotropic radiation field is used to calibrate the 31P and 31D measurements.

Cross sections for simultaneous 21P excitation of two colliding He atoms to various substate combinations

The authors have studied the simultaneous excitation of both collision partners to the 21P state in He-He collisions at 1250 eV. This was done by measuring (in a coincidence experiment) the angular correlations between the two UV photons emitted by the two simultaneously excited atoms in their decay to the ground state. These measurements contain information on the cross sections for simultaneous 21P excitation of the two colliding atoms to the various substate combinations. A theoretical expression for the angular correlation function in terms of the various collision parameters is presented and is used to extract the available information from the experimental data. The results are not in agreement with conclusions which can be drawn from a simple reasoning based on the correlation diagram for the He-He system.

Energy dependence of atomic substate correlation in simultaneous double-atom excitations in He-He collisions

The authors measured the angular correlations of two coincident photons resulting from simultaneous excitation of both atoms to the 21P state in helium-helium collisions; the beam energy ranged from 0.5 to 3.5 keV. A theoretical expression for the angular correlation function has been derived, which contains the various density matrix elements as parameters. The relative cross sections for excitation to the various possible magnetic substate combinations and the coherence between them is determined by fitting this function to the measured data. The authors found a strong dependence of the parameters alpha and beta and a weaker dependence of Re( chi ) on the collision energy. The combined parameters lambda + mu contains such large statistical errors that it is meaningless to determine its energy dependence. The minimal Sigma - Delta model for the collision process, where the authors take into account two successive rotational sigma u- pi u couplings resulting in 1 Sigma g and 1 Delta g molecular states (with respect to the internuclear axis), predicts the correct behaviour of alpha from the measurements of beta . However, the fact that Re( chi ) is positive cannot be understood within this model.

Post-collision interaction in the excitation of helium by electrons

The electron impact excitation of a large number of helium states has been studied in the energy domain where the autoionizing states lie. Apart from the well known negative-ion resonances, several structures are observed which can be explained in terms of a post-collision interaction (PCI) between the inelastically scattered electron and the electron ejected by an autoionizing state. It is found and made plausible that the PCI mechanism preferentially leads to the excitation of S states. From the experimental results it is possible to extract the dependence of the energy exchange between the scattered and the ejected electron on the excess energy of the scattered electron immediately after the excitation of the autoionizing state. From the magnitudes of the PCI structures in the excitation of states with different orbital momenta, conclusions can be drawn about the orbital momentum exchange in the PCI process. A brief discussion follows of the different theoretical approaches that have previously been proposed.

The effect of post-collision interaction in n3S excitation of helium by electrons

The excitation of the 43S, 53S, 63S and 43D states of helium by electron impact has been studied in the energy region around 60 eV. New experimental evidence has been obtained for the existence of an indirect excitation mechanism resulting from a post-collision interaction (PCI) between the scattered electron and the electron ejected from an intermediate autoionizing state. It is found, and made plausible, that the PCI mechanism via the excitation of autoionizing states primarily leads to the excitation of S-states.

A model for post-collision interaction based on an optical-potential description

An optical-potential description is used to study the effects of post-collision interaction (PCI) following the near-threshold electron impact excitation of autoionising states in helium. After some drastic but intuitively plausible approximations to the exact expressions published by Nienhuis and Heideman (1976), a model is obtained which lends itself to practical calculations. Results have been obtained on: (i) angular momentum exchanges during PCI, (ii) angular distributions of ejected electrons and (iii) lineshapes of PCI structures in the excitation of Rydberg states. These results are in accordance with the intuitive picture of the PCI mechanism and are confirmed by the trend in the experimental observations.

Photon Correlations, Atomic State Correlations and Statistical Distributions

First we will describe our experiment where the simultaneous excitation of two colliding helium atoms to various substate combinations is studied by detecting two emitted photons in coincidence. Next we will give the minimal symmetry properties of these two photon correlations. Furthermore the connection between photon correlations and the fundamental collision amplitudes will be treated and finally we will try to interpret the measurements in the context of statistical distributions.