Fully Differential Cross Sections for the Single Ionization of Helium by Ion Impact

We present experimental and theoretical fully differential cross sections for the single ionization of He by heavy-ion impact for electrons emitted into the scattering plane. Data were obtained for 2 MeV amu-1 C6+ and 3.6 MeV amu-1 AuQ+ (Q = 24, 53) projectiles, corresponding to perturbations (projectile charge to velocity ratio) ranging from 0.7 to 4.4, a regime which is inaccessible for electron-impact ionization. We observe a decreasing recoil peak intensity (relative to the binary peak) and at the same time an increasing peak in the forward direction with increasing perturbations. Large discrepancies between the experimental data and theoretical predictions are found, which can at least be partly attributed to the use of hydrogenic wavefunction

Recoil-Ion and Electron Momentum Spectroscopy: Reaction-Microscopes

Recoil-ion and electron momentum spectroscopy is a rapidly developing technique that allows one to measure the vector momenta of several ions and electrons resulting from atomic or molecular fragmentation. In a unique combination, large solid angles close to 4 and superior momentum resolutions around a few per cent of an atomic unit (a.u.) are typically reached in state-of-the art machines, so-called reaction-microscopes. Evolving from recoil-ion and cold target recoil-ion momentum spectroscopy (COLTRIMS), reaction-microscopes—the `bubble chambers of atomic physics'—mark the decisive step forward to investigate many-particle quantum-dynamics occurring when atomic and molecular systems or even surfaces and solids are exposed to time-dependent external electromagnetic fields. This paper concentrates on just these latest technical developments and on at least four new classes of fragmentation experiments that have emerged within about the last five years. First, multi-dimensional images in momentum space brought unprecedented information on the dynamics of single-photon induced fragmentation of fixed-in-space molecules and on their structure. Second, a break-through in the investigation of high-intensity short-pulse laser induced fragmentation of atoms and molecules has been achieved by using reaction-microscopes. Third, for electron and ion-impact, the investigation of two-electron reactions has matured to a state such that the first fully differential cross sections (FDCSs) are reported. Fourth, comprehensive sets of FDCSs for single ionization of atoms by ion-impact, the most basic atomic fragmentation reaction, brought new insight, a couple of surprises and unexpected challenges to theory at keV to GeV collision energies. In addition, a brief summary on the kinematics is provided at the beginning. Finally, the rich future potential of the method is briefly envisaged

Reconciliation of Measured Fully Differential Single Ionization Data with the First Born Approximation Convoluted with Elastic Scattering

An analysis of experimental fully differential data for single ionization in 100 MeV/amu C6++He collisions is reported. We present a convolution of the first Born approximation with elastic scattering by using an event generator technique. Furthermore, the calculation is convoluted with all known experimental resolutions. Our analysis shows that elastic scattering is a viable explanation for surprising structures observed in the fully differential cross sections outside the scattering plane. Furthermore, it may even explain discrepancies in the “recoil peak” frequently observed for both ion and electron impact

Penning ionization of doped helium nanodroplets following EUV excitation

Helium nanodroplets are widely used as a cold, weakly interacting matrix for spectroscopy of embedded species. In this work we excite or ionize doped He droplets using synchrotron radiation and study the effect onto the dopant atoms depending on their location inside the droplets (rare gases) or outside at the droplet surface (alkali metals). Using photoelectron-photoion coincidence imaging spectroscopy at variable photon energies (20-25 eV), we compare the rates of charge-transfer to Penning ionization of the dopants in the two cases. The surprising finding is that alkali metals, in contrast to the rare gases, are efficiently Penning ionized upon excitation of the (n=2)-bands of the host droplets. This indicates rapid migration of the excitation to the droplet surface, followed by relaxation, and eventually energy transfer to the alkali dopants

Theoretical Description of Two- and Three-Particle Interactions in Single Ionization of Helium by Ion Impact

In this work we calculate doubly differential cross sections (DDCS) for single ionization of helium by highly charged ion impact. We study the importance of two-particle interactions in these processes by considering the cross sections as a function of all two-particle subsystems momenta. Experimental DDCSs were obtained recently from kinematically complete experiments on single ionization of He by 100 MeV/amu C6+ and 3.6 MeV/amu Au24,53+ impact. Furthermore, we evaluated the importance of three-particle interactions by plotting the squared momenta of all three collision fragments simultaneously in a Dalitz plot. Using the first Born and distorted-wave approximations for fully differential cross sections, together with Monte Carlo integration techniques, we were able to reproduce the main features observed in experimental data and to assess the quality of the models implied by the different employed approximations

Multiple Scattering Mechanisms in Simultaneous Projectile-Electron and Target-Electron Ejection in H⁻ + He Collisions

We studied simultaneous electron ejection from both collision partners in 200-keV H-+He collisions in a kinematically complete experiment by measuring the fully momentum-analyzed recoil ions and both active electrons in coincidence. The data were analyzed in terms of Dalitz spectra, in which the momentum exchange between three particles is plotted simultaneously in a single spectrum. We found that the energy transfer occurs predominantly between the active electrons, but most of the momentum is exchanged in elastic scattering between the cores of the collision partners

Comparison of theoretical and absolute experimental fully differential cross sections for ion-atom impact ionization

We report fully differential cross section (FDCS) calculations and absolute measurements for ion-atom impact ionization. Using the COLTRIMS (cold target recoil ion momentum spectroscopy) method we have obtained absolute FDCS both in the scattering plane as well as out of the scattering plane for 100 MeV amu(- 1) C6+ ionization of helium FDCS results are presented for different projectile scattering angles and ejected electron energies. The measurements are compared with a theoretical calculation employing an asymptotically exact three body final state wavefunction that contains all active two particle subsystem interactions to infinite order in perturbation theory. For the active electron a Hartree-Fock (HF) bound state wavefunction is used for the initial state and numerical continuum state eigenfunctions of a HF potential for the ion are used for the final state In the scattering plane these theoretical results are in very good agreement with experiment for small and intermediate momentum transfer. However some significant discrepancies are found for large momentum transfer and outside the scattering plane. These discrepancies disappear upon comparison with successively less differential cross sections

Steering proton migration in hydrocarbons using intense few-cycle laser fields

Proton migration is a ubiquitous process in chemical reactions related to biology, combustion, and catalysis. Thus, the ability to control the movement of nuclei with tailored light, within a hydrocarbon molecule holds promise for far-reaching applications. Here, we demonstrate the steering of hydrogen migration in simple hydrocarbons, namely acetylene and allene, using waveform-controlled, few-cycle laser pulses. The rearrangement dynamics are monitored using coincident 3D momentum imaging spectroscopy, and described with a quantum-dynamical model. Our observations reveal that the underlying control mechanism is due to the manipulation of the phases in a vibrational wavepacket by the intense off-resonant laser field.Comment: 5 pages, 4 figure

EUV ionization of pure He nanodroplets: Mass-correlated photoelectron imaging, Penning ionization and electron energy-loss spectra

The ionization dynamics of pure He nanodroplets irradiated by EUV radiation is studied using Velocity-Map Imaging PhotoElectron-PhotoIon COincidence (VMI-PEPICO) spectroscopy. We present photoelectron energy spectra and angular distributions measured in coincidence with the most abundant ions He+, He2+, and He3+. Surprisingly, below the autoionization threshold of He droplets we find indications for multiple excitation and subsequent ionization of the droplets by a Penning-like process. At high photon energies we evidence inelastic collisions of photoelectrons with the surrounding He atoms in the droplets