E Horsdal-Pedersen

Ionization and fragmentation of freon-12 molecules in collisions with protons

Mass-to-charge spectra of ions formed in ionization of freon-12 molecules, CF2Cl2, by protons with energies of Ep = 10 keV and 1 MeV are studied for the first time. Relative probabilities of different dissociation channels of the ionized freon-12 molecule, are obtained and compared with similar data for freon-12 ionization by electrons, positrons and photons. Absolute total and partial experimental cross sections for the formation of different fragment ions are evaluated for the case Ep = 10 keV. Neutral chlorine atom detachment is found to be the main fragmentation channel at both energies studied. Calculations of geometry and binding energies of fragment ions as well as energies required for activating various dissociation channels are carried out. A qualitative explanation is given of the observed fragmentation spectra.

Differential cross sections for K-shell ionisation and capture by H+ in C (CH4) and Ne

Cross sections differential in projectile scattering angle for total K-shell-vacancy production and for electron capture from the K shell by 200-600 keV H+ in carbon (CH4) and 500-1500 keV H+ in neon have been measured using scattered-particle, Auger electron coincidence techniques. The collision velocities range from 0.46 to 0.86 in units of the K-shell velocities, and the scattering angles correspond to impact parameters within a region around the K-shell radius. The ratio between the differential cross sections for K-shell capture and for total K-shell-vacancy production depends weakly on the scattering angle at the present collision velocities. The measured differential cross sections are transformed to impact parameter dependent probabilities to facilitate comparison with theoretical predictions. First-order perturbation theory with accurate initial and final electronic wavefunctions reproduces the main characteristics of the experimental ionisation probabilities. Recent progress in the theoretical description of capture in asymmetric collisions brings the theoretical predictions into close agreement with the experimental data for the capture process.

Dynamics of a single Rydberg shell in time dependent external fields

Probabilities for adiabatic or near-adiabatic state transformation within a highly excited shell of Li(n = 25) were studied experimentally and theoretically for a time dependent electric field, (t), and a constant magnetic field, . The fields were sufficiently weak and the time dependence slow enough such that only states belonging to the chosen shell were involved. The studies show that the dynamics are governed by the approximate hydrogenic character of the system in most cases, but for some specific time dependences it is influenced strongly by core interactions as expressed through the quantum defects, δl. The s-state is effectively decoupled from the rest of the n = 25 manifold due to a very large quantum defect. However the quantum defects of the p, d and f states are shown to play a decisive role in the dynamics. The core interactions lead to avoided crossings, non-adiabatic state transformations, and possibly even phase-interference effects. When a resonance condition pertaining to the hydrogenic character of the system is fulfilled, a linear Stark state is transformed completely into a circular Stark state oriented along f.

Angular differential measurements of ionization in fast H+-He collisions

Experimental differential cross sections for single and double ionization of He by impact of fast protons and deuterons are presented. The cross sections are measured at projectile scattering angles in the interval 0.27-4.2 mrad and for energies between 0.1 and 2.0 MeV amu-1. Pure ionization events, in which the electrons that leave the He atom go into continuum states, are experimentally distinguished from ionization reactions associated with charge transfer. The results on pure single ionization by protons show how a structure in the differential cross section at 0.55 mrad, first observed by Kamber et al. (1988) at 3 and 6 MeV, gradually appears as the proton energy is increased beyond about 500 keV. Results on double ionization of He at small impact parameters are presented in the form of ratios between cross sections for double and single ionization. Using reasonable models for the dependence on impact parameter of the probabilities for pure single ionization or for single-electron capture, the authors extract energy-dependent characteristic impact-parameter ranges for the reactions.

Mg+(3p) excitation amplitudes in slow Mg+-Ar collisions

The probabilities and amplitudes for exciting the magnetic sublevels of Mg+(3p) in collisions between accelerated Mg+ ions and Ar atoms have been studied by means of the polarised-photon, scattered-ion coincidence technique. Selected collision energies and scattering angles within the ranges 1.2-11 keV and 0.9-9 degrees were chosen. The excitation probability for the Mg+(3p) configuration shows an apparent angular threshold. A special effort was made to determine the amplitudes in the region of this threshold. Strongly polarised charge distributions are formed in the individual collisions. The direction of this polarisation is parallel to the collision plane and in most cases nearly perpendicular to the beam direction. In the region of the apparent angular excitation threshold it starts turning within the collision plane. In close collisions at high energies some degree of circular polarisation is observed.

Electron capture from elliptic Rydberg states: impact perpendicular to the minor axis

The total cross section, , for electron capture by Na+ ions from oriented coherent elliptic states (CES) of Li with principal quantum number n = 25 was studied experimentally for impact perpendicular to the minor axis of the elliptic orbit. The remaining geometrical parameters of the CES, which are the eccentricity, e , and the angle, , between the major axis and the beam direction, were varied in the course of the experiments, as was the reduced impact velocity v r = nv , where v is the projectile velocity in atomic units. Several representative cuts were chosen within the parameter space (v r ,e , ) = (0.74-2.09, 0-1, 0-2 ). The velocity range includes the region of matching velocities (v r 1) where for given geometrical parameters attains its maximum value and the high-velocity region where is a strongly decreasing function of v r . The cross section depends sensitively on each geometrical parameter, and the dependences change dramatically as v r is varied. The spatial distribution of the CES apparently governs in some region of parameter space (v r ,e , ) and in another the momentum distribution prevails.

Coherent elliptic states of atoms in non-orthogonal E and B fields

The dynamics of coherent elliptic states in time-dependent electric and magnetic fields has been studied and it is shown that a linear Stark state may be transformed adiabatically into a circular Zeeman state through a continuous range of coherent elliptic states provided the fields vary slowly and are not parallel. The orientation vectors of the elliptic states rotate relative to the fields during the adiabatic transformation except when the fields are orthogonal. The condition for adiabatic behaviour is established for an easily realizable choice of field variation.

Electron capture in close collisions between protons and carbon (CH4)

The total probability for electron capture from CH4 by 200-2000 keV protons scattered through 15 or 20 degrees in the laboratory system by the carbon nucleus has been measured. The probability decreases strongly with increasing proton energy, and a clear structure is seen around 400 keV. Complementary differential coincidence measurements of capture by protons from the K shell of carbon (CH4) show that the structure is caused by the K-shell capture component of the total capture probability, which becomes significant in the energy range 200-500 keV. Above 1 MeV, the K-shell capture component dominates the total capture probability. The angular dependence (10-50 degrees ) of the total capture probability was determined at 500 keV, where the K-shell capture component at 20 degrees contributes with (80+or-15)%. An experimental measure of the widths of the impact parameter distributions for K-shell ionisation and capture, respectively, as functions of the collision energy, was obtained on the basis of the pertinent experimental total cross sections and measured probabilities for small impact parameter collisions with nearly straight-line trajectories. The width of the K-shell ionisation distribution increases with increasing projectile kinetic energy, whereas the width of the K-shell capture distribution exhibits a maximum. Particular attention has been paid to the capture probabilities in an energy region around the elastic scattering resonances 12C(p,p)12C near 1.7 MeV.

The n-dependence of electron capture from Rydberg states

The cross section, σn, for electron capture by Na+ ions from coherent elliptic states (CES) of Li was studied experimentally. The dependences on the principal quantum number, n, the eccentricity, e, and the spatial orientation of the CES were determined at the two reduced ion velocities vr = 1.20 and 1.68 (vr≡v/ve, where v is the ion speed and ve = 1/n the mean electron speed of the CES in atomic units). The ion velocity, , was perpendicular to the minor axis of the CES throughout the experiment, so the orientation is fully specified by the angle, , between and the electric dipole moment, , of the CES. According to a generalized correspondence principle, the reduced cross section, = lim n→∞σn/n4, is given accurately by classical mechanics, and it is a universal function of the scaled parameters of the collision, which in the present experiment are vr, e and . It is assumed that this result can be extrapolated to the range of n-values, 20-35, covered in this paper, i.e. σn = np where and p4 depend only weakly on n. The experimental data support the assumed existence of a universal cross section function and the expected n4-dependence is also confirmed at vr = 1.20, but at vr = 1.68 the n-dependence is closer to n3.

Dynamics of Rydberg states in crossed E and B fields: coherent elliptic states

The dynamics of the n = 25 shell of Li atoms in weak electric, E, and magnetic, B, fields was studied experimentally as a function of the rate of change of the electric field strength, , the strength, B, of a constant magnetic field and the angle between the two fixed field directions, . Prior to the variation of E, it was held constant at with strong enough to dominate the Stark-Zeeman manifold for the shell. The uppermost state, which is a coherent elliptic state (CES), was populated selectively by pulsed laser excitation. The field was subsequently varied without rotation from to at the constant rate . It is shown experimentally, in accordance with theory, that the dynamics is described by a single dimensionless parameter given by in atomic units. When the variation of E is so slow that the electron has time to adjust and the population then remains in the uppermost state of the manifold (adiabatic transformation), for smaller the population shifts to an interval of states centred near the middle of the manifold and at the wavefunction is almost frozen and the lowermost part of the manifold populated (diabatic transformation). The experimental findings for near-adiabatic evolution of the CES agree with theoretical results obtained in a non-relativistic, hydrogenic model but discrepancies are seen when non-hydrogenic states are populated for .