A. Niehaus

Angular dependent post-collision interaction in auger processes

We have reformulated the theory of post-collision interaction (PCI) for Auger-decay following inner-shell photoionisation in order to take the time into account with the Auger-electron need to overtake the slow electron. The energy-shift of the Auger-electron due to PCI is calculated by solving in a reasonable approximation the classical equation of motion for the Auger electron. In contrast to the theory of Russek and Mehlhorn we derive analytical expressions for the transition amplitude, the line shape and the line shift of the Auger-electrons. If in our model the Auger electron and the slow electron are treated uncorrelated in direction our analytical expressions agree well with the numerical results of Russek and Mehlhorn. However if we account for directional electron-electron correlations, we show that deviations from the theory of Russek and Mehlhorn are to be expected. The possibility of detecting these deviations is discussed.

Analysis of electron spectra from grazing-incidence collisions of Ar2+ with Pb(111) single-crystal surfaces

Electron spectra are reported for the scattering of Ar2+ ions by a well defined Pb(111) single-crystal surface. The measurements are performed for beam energies in the range 0.6–10 keV at a grazing angle of 2°. The spectra at these conditions are found to be dominated by contributions from “spontaneous” electron emission processes, i.e., from Auger-type transitions in which two electrons are involved. They show a rather rich structure, especially at the lowest beam energies. A rather detailed analysis in terms of a model is possible. The model describes the time evolution of the ion-surface system during the slow approach. The processes involved are resonant one-electron exchange processes and Auger-type processes. The first processes lead to the distance-dependent population of various singly and doubly excited projectile states, the second lead to electron emission with distance dependent probability and kinetic energy. The analysis in terms of this model yields: (1) an identification of the main electronic states and processes that play a role during the slow surface approach of the projectile, (2) the absolute distance-dependent probabilities for the main resonant electron exchange and Auger-type processes, (3) the distance-dependent population probabilities for the electronic states involved. For the Ar2+-Pb(111) system it is found that at the low primary energies, autoionization of Ar∗∗ and Penning ionization of Ar+∗ dominate the first neutralization step leading to the ground state of Ar+, whereas with increasing energy the direct Auger-capture process of Ar2+ becomes more important. The second neutralization step leading to the ground state of Ar is found to be always due to direct Auger-capture. In the mentioned primary beam energy range the neutralization to the ground state of Ar is found to be completed at the turning point of the trajectory.

Electron emission in slow collisions of protons with a LiF-surface

Abstract Electron spectra have been measured for collisions of protons with surfaces of LiF grown on a tungsten substrate. The measurements were carried out for 5° grazing incidence collisions at energies from 50 eV up to 1 keV. The experimental spectra are compared with theoretical spectra obtained from model calculations. The model is based on the assumption that electron emission is dominated by the mechanism of electron promotion via the 3dσ molecular orbital of the HF − quasi-molecular system. Using theoretically obtained functions for the scattering potential and for the 3dσ orbital energy, and calculating the spectra in semiclassical approximation for a distribution of impact parameters obtained from Monte Carlo simulations for the relevant scattering geometry, good agreement between experimental and theoretical spectra is obtained. This agreement is taken as strong evidence for the assumed ionization mechanism.

Construction of a low velocity metastable helium atomic beam

We have constructed an atomic beam of metastable helium atoms He(2u200a3S) with a mean velocity of 300 m/s (15 K) and a yield of 3×1012u200aatoms/su200asr. The metastable atoms are produced in a dc discharge in a cryogenic environment cooled by liquid helium. Using a hexapole magnetic lens, we have increased further the beam intensity by focusing the metastable atoms. Initial studies show a factor of 2.5 increase in the beam flux but more is expected when the hexapole is constructed from permanent hexapole magnets. The He(2u200a3S) atoms are subsequently loaded into a magneto-optical trap.

A study of double excitation in He+-He collisions

The authors have studied the double excitation of He in He+-He collisions at collision energies between 1.2 and 4 keV and at laboratory scattering angles between 6 degrees and 10 degrees . Under these conditions the (2p2)1D state is excited for more than 95%. The electrons emitted by the doubly excited He after the collision are measured in coincidence with the scattered ions. Both electrons and scattered ions are selected with respect to energy and angle, but the electron intensity is integrated over a certain energy range in order to detect all electrons due to decay of the (2p2)1D state, for emission by both the slow target atom or-after charge exchange-the fast projectile. From the angular distribution for a well-defined collision event the sublevel population amplitudes of the 1D state are obtained. Amplitudes are presented for eight different such events. This complete information on the respective events is analysed in terms of coupling mechanisms operating during the course of the collision. It is shown that, for all eight cases, an independent electron two-step rotational 2p sigma u=2p pi u coupling model can reproduce the main features of the observed 1D distributions. Finer details are interpreted in terms of an asymptotic Sigma - Pi coupling at large distances between the collision partners, and in terms of an additional inner Sigma - Pi coupling which becomes important in the more violent events.

Electronic processes in slow ion-metal surface collisions

Abstract Measured and calculated electron spectra are shown for collisions of Ar 2+ on Pb(111) at grazing incidence in the 1 keV range. The spectra consist of contributions of spontaneous Auger-type transitions in which two electrons are involved. They show a rich structure, especially at the lowest vertical energies (

COLLISIONS BETWEEN ULTRACOLD METASTABLE HE ATOMS

We present experimental data on collisions between excited He-atoms occurring in a magneto-optical trap (MOT) at a temperature of 1.1 mK. He(2 3 S)-atoms produced in a discharge are pre-cooled and trapped using the He(2 3 S)‐ He(2 3 P2) transition for laser manipulation. Measurements of the Penning ionization rate as a function of the MOT-laser frequency are presented and theoretically analyzed. The analysis, based on a model which is presented in detail for the first time, leads to a good understanding of the complex nature of optical collisions. Further, first and preliminary measurements of the kinetic energy distributions of He a - and He a -ions formed by Penning ionization in optical col