Marc Pieksma

The velocity distribution of saddle-point electrons

An approximate, analytic expression is derived for the distribution of saddle-point electrons emitted in ionizing one-electron atomic collision systems. The electron distributions of two other relevant ionization mechanisms, the direct S and the radial decoupling mechanisms, are also taken into account. The electron velocity distributions of 1-6 keV amu-1 collisions of H+ and He2+ with atomic hydrogen are presented.

Saddle point electrons in slow ion‐atom collisions

Ionization in atomic collisions at adiabatic impact energies is discussed. Special attention is given to the saddle point ionization mechanism. This mechanism can be described by a quantum mechanical theory based on the concept of hidden crossings. The fundamental H+‐H collision system is studied as a test case. For 1–6 keV collisions both the experimental velocity distributions and the relative total ionization cross sections of this system can be explained by the theory in a consistent way. At collision energies of 4 keV and higher there is strong evidence for the existence of a saddle point ionization mechanism. Saddle point effects in the He2+‐H (theory) and H+‐He (experiment) collision systems are briefly commented upon.

Radial-decoupling excitation mechanism in slow atomic collisions studied using zero-range model potentials

One-dimensional zero-range model potentials are employed to investigate the recently proposed radial-decoupling excitation mechanism in ion-atom collisions. The expression for the total (i.e. excitation plus ionization) radial-decoupling probability, obtained in the adiabatic limit, agrees with the result of our earlier, tentative approach (Pieksma and Ovchinnikov 1994 J. Phys. B: At. Mol. Opt. Phys. 27 4573) to second order in the collision velocity v. In fact, at low collision energies the cross section just scales with . This is in accordance with the adiabatic theorem as formulated by Born and Fock (1928 Z. Phys. 51 165), which also implies a power law for the cross section as the collision velocity becomes sufficiently low. The dependence is a purely quantal effect, and it is argued that it represents the correct low-energy cross section behaviour, even though it seems to contradict the exponential law that follows from the conventional theory of adiabatic transitions.

Saddle-point effects in adiabatic to intermediate-energy ion-atom collisions

Abstract An overview will be given of the many experimental and theoretical studies of saddle-point phenomena, with special emphasis on adiabatic collisions and simple systems. For such cases, experimental evidence for the actual existence of saddle-point electrons has been obtained by different groups using different techniques. At intermediate-energy collisions, the observation of saddle-point effects is still a controversial issue. It will be argued that even though for these relatively fast collisions the underlying saddle-point mechanism may still be present, no pronounced saddle-point features should be expected.

SUB-EV ELECTRON SPECTROSCOPY IN ION-ATOM COLLISIONS

A newly designed spectrometer is presented, which is eminently suited for the measurement of electron velocity distributions in the sub‐eV electron energy region. The application of this spectrometer is demonstrated in an ion‐atom collision experiment, using a time‐of‐flight technique. As an example, the low‐energy electron spectrum of 4 keV ionizing H+−H2 collisions is shown.

Low energy electrons in slow ion-atom collisions

Abstract We have investigated velocity distributions of electrons ejected in adiabatic atomic collisions, both experimentally and theoretically, in search for saddle point electrons. These electrons reside on/near the top of the internuclear potential barrier on the outgoing way of the collision. The so-called theory of hidden crossings is used to treat this unusual and seemingly unstable ionization mechanism. We will discuss the experimental set-up, in particular our newly designed “magnetic time-of-flight electron spectrometer”. For the H + H system, at collision energies of 4 and 6 keV/amu, the good agreement between theory and experiment, for both electron velocity distributions and integrated cross sections, strongly suggests the actual existence of a saddle point ionization mechanism. At collision energies below 2 keV/amu the experimental data suggest that the recently proposed radial decoupling ionization mechanism might be effective. Besides the H + H system we have also studied the He 2+ H (theory) and H + He (experimental) collision systems.

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