A. L. Landers

K-shell photoionization of CO and N2: is there a link between the photoelectron angular distribution and the molecular decay dynamics?

We have used COLTRIMS to measure the angular distribution of electrons released from the K-shell of N2 and the carbon K-shell of CO by absorption of one linear polarized photon. For each ionization event which leads to two charged fragments we determine the angle of the photoelectron with respect to the fragment ion momenta. In addition we determine the charge state and energy of the molecular fragments. We find a breakdown of the axial recoil approximation for CO for kinetic energy releases below 10.2 eV, whereas for N2 that approximation is found to be valid for all fragment energies. Furthermore, the photoelectron emission spectrum for N2 is found to be the same for the molecular breakup channels producing N + N + and N + N ++ . (Some figures in this article are in colour only in the electronic version)

Interference effects in electron emission from H2 by 3 and 5 MeV H+ impact

Abstract Interference effects are expected to occur when electrons are emitted coherently within a multi-center molecular field. This phenomenon is analogous to Young’s two-slit experiment. The present work focuses on ionization of H 2 , a molecule composed of two indistinguishable centers, by 3 and 5 MeV H + impact. A parallel-plate electron spectrometer was used to investigate electron emission in the range ∼3–250 eV at observation angles of 30°, 45°, 90°, 135° and 150° with respect to the incident beam. To help identify interference effects, experimental cross-sections were normalized to CDW-EIS calculations for electron emission from two independent H atoms. A sinusoidal-like variation was observed in the normalized electron emission yield for electron energies less than ∼250 eV in general agreement with model calculations.

Kinematically complete investigation of momentum transfer for single ionization in fast proton-helium collisions

The dynamics of singly ionizing proton-helium collisions have been studied experimentally for several energies of the projectile (0.2, 0.5, 1.0 and 1.3 MeV) with the technique of cold target recoil-ion momentum spectroscopy (COLTRIMS). The complete final-state distribution in momentum space of all three particles was determined by measuring the three momentum components of the emitted electron and the coincident recoiling target ion. The momentum transfer and energy loss of the outgoing projectile was determined by momentum and energy conservation laws. Doubly differential cross sections of the kinematically complete experimental investigation are presented. The present data are compared with results from fast highly charged heavy-ion impact experiments.

Energy sharing and asymmetry parameters for photo double ionization of helium 100 eV above threshold in single-particle and Jacobi coordinates

We present a joint experimental and theoretical study of the double differential cross section for the double ionization of helium by absorption of one 179 eV linear polarized photon (100 eV above threshold). We show the energy sharing and the asymmetry parameters in both single-particle coordinates and in Jacobi coordinates. An asymmetric sharing of the electron excess energy and a breakup of the electron pair preferentially parallel to the polarization axis is found. This marks a deviation from the propensity rule favouring perpendicular relative electron motion which has been observed to hold at energies closer to the double-ionization threshold.

Electron Correlation Leading to Double-K-Shell Vacancy Production in Li-Like Ions Colliding with Helium

Single and double K‐shell vacancies in Be+, B2+, C3+, and O5+ Li‐like ions colliding with neutral helium at intermediate‐to‐high velocities have been investigated. Specific excited states were observed using high‐resolution Auger‐electron spectroscopy. The velocity dependence of the measured cross sections for the formation of doubly vacant K‐shell states was used to determine the mechanisms responsible for producing these so‐called “hollow states”. Electron correlation effects were inferred from the spectral features and the velocity dependence of the observed hollow states. The variation of the correlation strength was investigated as a function of the atomic number of the Li‐like ion.

Photo- and charged-particle-ionization of He and D2 studied with COLTRIMS

The COLTRIMS (COLd Target Recoil Ion Momentum Spectroscopy) approach to final-state momentum imaging is now being widely used in at least a dozen accelerator and synchrotron-radiation laboratories in the world and its use is growing rapidly. The technique combines fast imaging detectors with a supersonically cooled gas target to allow the charged particles from a collision, including both recoil ions and electrons, to be collected with extremely high efficiency and with fully measured vector momenta. It allows the investigation of correlations between ejected momentum fragments and in some cases the identification of collective modes of disintegration. When molecular targets are used, it allows the a posteriori determination of the alignment of the molecule at the time of the collision. We will discuss the use of this approach to study the single and double ionization of He and D2 by the impact of photons and of charged particles over a wide range of velocities.