V. Mergel

Correlated electron emission in multiphoton double ionization

Electronic correlations govern the dynamics of many phenomena in nature, such as chemical reactions and solid state effects, including superconductivity. Such correlation effects can be most clearly investigated in processes involving single atoms. In particular, the emission of two electrons from an atom—induced by the impact of a single photon, a charged particle or by a short laser pulse—has become the standard process for studies of dynamical electron correlations. Atoms and molecules exposed to laser fields that are comparable in intensity to the nuclear fields have extremely high probabilities for double ionization; this has been attributed to electron–electron interaction. Here we report a strong correlation between the magnitude and the direction of the momentum of two electrons that are emitted from an argon atom, driven by a femtosecond laser pulse (at 38 TW cm-2). Increasing the laser intensity causes the momentum correlation between the electrons to be lost, implying that a transition in the laser–atom coupling mechanism takes place.

Multi-hit detector system for complete momentum balance in spectroscopy in molecular fragmentation processes

A multi-hit detector system has been developed capable of measuring the complete momentum vectors of all ionic fragments after the dissociation of complex molecules induced by photon, electron or ion impact. The fragments are collected in an electrostatic field and detected with a position-sensitive micro-channel plate detector using a fast timing delay-line readout. The detector has a position resolution better than 0.2 mm and can resolve fragments with arrival times separated by at least 5 ns in time. We illustrate the features of this new detector with first measurements for the collision

Complete photo-fragmentation of the deuterium molecule

All properties of molecules—from binding and excitation energies to their geometry—are determined by the highly correlated initial-state wavefunction of the electrons and nuclei. Details of these correlations can be revealed by studying the break-up of these systems into their constituents. The fragmentation might be initiated by the absorption of a single photon, by collision with a charged particle or by exposure to a strong laser pulse: if the interaction causing the excitation is sufficiently understood, the fragmentation process can then be used as a tool to investigate the bound initial state. The interaction and resulting fragment motions therefore pose formidable challenges to quantum theory. Here we report the coincident measurement of the momenta of both nuclei and both electrons from the single-photon-induced fragmentation of the deuterium molecule. The results reveal that the correlated motion of the electrons is strongly dependent on the inter-nuclear separation in the molecular ground state at the instant of photon absorption.

Fast position and time-resolved read-out of micro-channelplates with the delay-line technique for single-particle and photon-detection

Based on delay-line read-out methods of micro-channelplate (MCP) stacks we develop imaging system for single particle and photon spectroscopy. A complete system consists of an open MCP-detector with helical wire anode, specially designed front-end electronics and a stand alone PC-based TDC-system. We achieve a position resolution better than 0.1 mm and excellent linearity for open dimensions up to 100 mm, multi-hit operation, and detection rates up to 20 kiloEvents/sec in an event-listing mode or over 1 MegaCount/sec in a histogram mode. Both modes allow 2D position and time-of-flight (TOF) spectroscopy with approximately 1 nanosec TOF resolution. Furthermore, we currently test a delay-line anode on printed circuit that operates with image charge pick-up from a high-resistive collecting anode. With an image charge detection method this 3D-imaging technique can be applied to commercial sealed MCP single-photon detectors. While a simple high-resistive collection anode is placed inside the tube, a position sensitive pick-up electrode can be mounted next to it outside the vacuum wall.

Sequential and nonsequential contributions to double ionization in strong laser fields

We demonstrate experimentally the difference between a sequential interaction of a femtosecond laser field with two electrons and a nonsequential process of double ionization mediated by electron-electron correlation. This is possible by observing the momentum distribution of doubly charged argon ions created in the laser field. In the regime of laser intensities where the nonsequential process dominates, an increase in laser power leads to an increase in the observed ion momenta. At the onset of the sequential process, however, a higher laser power leads to colder ions. The momentum distributions of the ions from the sequential process can be modelled by convolving the single-ionization distribution with itself.

Absolute triple differential cross sections for photo-double ionization of helium - experiment and theory

We have measured absolute triple differential cross sections for photo-double ionization of helium at 20 eV excess. The measurement covers the full ranges of energy sharing and emission angles of the two photoelectrons. We compare our data for selected geometries with the convergent close-coupling (CCC) calculations as well as 2SC calculations by Pont and Shakeshaft and 3C calculations by Maulbetsch and Briggs. In terms of the absolute magnitude and the trend in the shapes of the triple differential cross section for different geometries we find good agreement of the CCC and published 2SC calculations with our measurement, though differences with respect to the observed shape of individual patterns still exist.

Kinematically complete experiments using cold target recoil ion momentum spectroscopy

Abstract Cold Target Recoil Ion Momentum Spectroscopy allows the detection of the three-dimensional momentum vector of the recoiling product ion from ion, electron or photon atom collisions with 4π solid angle and high resolution. It can be combined with large area position-sensitive detectors for electron detection or measurement of the projectile charge-state and scattering angle. Such ‘reaction microscopes’ cover the full correlated momentum space of all fragments of an atomic reaction yielding kinematically complete information for each reaction event. For the first time in atomic collision physics fully differential data became available in the sense that not only the momenta of all fragments, but also the complete momentum space is observed in one experiment. Recent results achieved with this new technique for slow p-He collisions and threshold photo ionization of He will be discussed.

A design study for an internal gas-jet target for the heavy-ion storage ring CRYRING

This paper presents the design of the internal gas-jet target, CRYJET, which is being constructed for investigations of, e.g., fast ion--atom collisions in the heavy-ion storage and cooler ring CRYRING at the Manne Siegbahn Laboratory, Stockholm University. The goal for the design work was to create an ultra-cold He target (< = 10 mK in the longitudinal direction and 0.5 mK transverse temperature) with a density of ∼ 1012 atoms/cm3. Care was taken in order to minimize the influence from the jet on the very low background pressure in the storage ring (∼ 10-11 mbar). The low temperature is essential for the resolution in the experiments. The high density will enable us to get sufficient luminosities for investigations of processes with cross sections down to the 10-27 cm2 range. The gas-jet target will be equipped with two recoil-ion-momentum spectrometers in order to extract detailed information about the collision dynamics.

Photo double ionization of He by circular and linear polarized single-photon absorption

We have measured absolute fully differential cross sections for photo double ionization of helium by circularly and linearly polarized light 20 eV above threshold. The data have been obtained by measuring in coincidence the momentum vector of the He 2+ ion and one of the electrons, covering 4π solid angle for all particles. We give an overview over the momentum distribution in the three-body continuum and show fivefold differential cross sections. We find a swirl in the electron momentum space for double ionization by circularly polarized light. The present data supersede earlier data from our group (V mergel et al 1998 Phys. Rev. Lett. 80 5301).

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.