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Multi-particle momentum imaging experiments are now capable of providing detailed information on the properties and the dynamics of quantum systems in Atomic, Molecular and Photon (AMO) physics. Historically, Otto Stern can be considered the pioneer of high-resolution momentum measurements of particles moving in a vacuum and he was the first to obtain sub-atomic unit (a.u.) momentum resolution (Schmidt-Böcking et al. in The precision limits in a single-event quantum measurement of electron momentum and position, these proceedings [1]). A major contribution to modern experimental atomic and molecular physics was his so-called molecular beam method [2], which Stern developed and employed in his experiments. With this method he discovered several fundamental properties of atoms, molecules and nuclei [2, 3]. As corresponding particle detection techniques were lacking during his time, he was only able to observe the averaged footprints of large particle ensembles. Today it is routinely possible to measure the momenta of single particles, because of the tremendous progress in single particle detection and data acquisition electronics. A “state-of-the-art” COLTRIMS reaction microscope [4–11] can measure, for example, the momenta of several particles ejected in the same quantum process in coincidence with sub-a.u. momentum resolution. Such setups can be used to visualize the dynamics of quantum reactions and image the entangled motion of electrons inside atoms and molecules. This review will briefly summarize Stern’s work and then present in longer detail the historic steps of the development of the COLTRIMS reaction microscope. Furthermore, some benchmark results are shown which initially paved the way for a broad acceptance of the COLTRIMS approach. Finally, a small selection of milestone work is presented which has been performed during the last two decades.

Time-of-flight spectrometer configurations combined with position sensitive single particle counting detectors have been simulated with a Monte Carlo code to find an efficient solution for coincident low energy electron detection. Photoemission experiments on polycrystalline silver have been performed at a synchrotron radiation facility with a likewise wide-angle ARPES (angle resolved photoemission spectroscopy) setup. Low-energy electron pairs created from secondary electron emission have been detected in coincidence obtaining their full momentum vectors.

Interatomic Coulombic decay (ICD) in Ar2, ArKr and Kr2 following Ar 2p or Kr 3d Auger decay has been investigated by means of momentum-resolved electron-ion-ion coincidence spectroscopy. This sequen- tial decay leads to Coulombic dissociation into dication and monocation. Simultaneously determining the kinetic energy of the ICD electron and the kinetic energy release between the two atomic ions, we have been able to unambiguously identify the ICD channels. We find that, in general, spin-conserved ICD, in which the singlet (triplet) dicationic state produced via the atomic Auger decay preferentially decays to the singlet (triplet) state, transferring the energy to the other atom, is faster than spin-flip ICD, in which the Auger final singlet (triplet) dicationic state decays to the triplet (singlet) state. However, spin-flip ICD may take place when spin-conserved ICD becomes energetically forbidden. Dipole-forbidden ICDs from Kr2+(4s− 21 S)-B (B = Ar or Kr) to Kr2+(4p− 21 D, 3P)-B + are also observed.

Many-particle dynamics in atomic and molecular physics has been investigated by using the COLTRIMS reaction microscope. The COLTRIMS technique visualizes photon and ion induced many-particle fragmentation processes in the eV and milli-eV regime. It reveals the complete momentum pattern in atomic and molecular many-particle reactions comparable to the bubble chamber in nuclear physics.  2005 Elsevier B.V. All rights reserved.

We present a joint experimental and theoretical study of the gerade and ungerade amplitudes of the photo double ionization of helium at excess energies of 100 eV and 450 eV above the threshold. We describe a method of extracting the amplitudes from a COLTRIMS data set. The experimental results are well reproduced by convergent close-coupling (CCC) calculations. The fully differential cross section data underlying this study can be found in our companion papers immediately preceding this one.

We present a joint experimental and theoretical study for the fully differential cross section of the photo double ionization (PDI) of helium with linearly polarized light at the excess energies Eexc = 100 eV and 450 eV above the threshold. The fully differential cross section is obtained by measuring the three-dimensional momentum vectors of one electron and the He 2+ ion in coincidence using the COLTRIMS method. We give an overview of the momentum distribution of the three-body continuum 100 eV above the threshold. We show angular distributions for both electrons for various energy sharings at Eexc = 100 eV and 450 eV. The experimental results are well reproduced by a set of convergent close-coupling (CCC) calculations.

We present a new time-of-flight spectrometer for energy and angle-resolved measurements of electrons emitted in coincidence from a surface. By using a projection method, we can cover nearly 2π of the solid angle above the sample resulting in a very high coincidence efficiency. The use of this new spectrometer for the double photoemission process from surfaces will be demonstrated.

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.

We simultaneously measured the momentum transferred to a free-floating molecular double slit and the momentum change of the atom scattering from it. Our experimental results are compared to quantum mechanical and semiclassical models. The results reveal that a classical description of the slits, which was used by Einstein in his debate with Bohr, provides a surprisingly good description of the experimental results, even for a microscopic system, if momentum transfer is not ascribed to a specific pathway but shared coherently and simultaneously between both.

We have measured the ionization and fragmentation of Helium, Neon and Argon Dimers induced by ion impact and observed two different pathways, the sequential ionization on each atom and the interatomic Cou-lombic decay.

We discuss a novel micro-channel plate (MCP) photomultiplier with resistive screen (RS-PMT) as a detection device for space- and time-correlated single photon counting, illustrated by several applications. The photomultiplier tube resembles a standard image intensifier device. However, the rear phosphor screen is replaced by a ceramic “window” with resistive coating. The MCP output is transferred through the ceramic plate to the read-out electrode (on the air side) via capacity-coupling of the image charge. This design allows for an easy reconfiguration of the read-out electrode (e.g. pixel, charge-sharing, cross-strip, delay-line) without breaking the vacuum for optimizing the detector performance towards a certain task. It also eases the design and manufacturing process of such a multi-purpose photomultiplier tube. Temporal and spatial resolutions well below 100 ps and 100 microns, respectively, have been reported at event rates as high as 1 MHz, for up to 40 mm effective detection diameter. In this paper we will discuss several applications like wide-field fluorescence microscopy and dual γ/fast-neutron radiography for air cargo screening and conclude with an outlook on large-area detectors for thermal neutrons based on MCPs.

Radiation damage to living tissue stems not only from primary ionizing particles but to a substantial fraction from the dissociative attachment of secondary electrons with energies below the ionization threshold. We show that the emission yield of those low energy electrons increases dramatically in ion–atom collisions depending on whether or not the target atoms are isolated or embedded in an environment. Only when the atom that has been ionized and excited by the primary particle impact is in immediate proximity of another atom is a fragmentation route known as interatomic Coulombic decay (ICD) enabled. This leads to the emission of a low energy electron. Over the past decade ICD was explored in several experiments following photoionization. Most recent results show its observation even in water clusters. Here we show the quantitative role of ICD for the production of low energy electrons by ion impact, thus approaching a scenario closer to that of radiation damage by alpha particles: We choose ion energies on the maximum of the Bragg peak where energy is most efficiently deposited in tissue. We compare the electron production after colliding He + ions on isolated Ne atoms and on Ne dimers (Ne 2 ). In the latter case the Ne atom impacted is surrounded by a most simple environment already opening ICD as a deexcitation channel. As a consequence, we find a dramatically enhanced low energy electron yield. The results suggest that ICD may have a significant influence on cell survival after exposure to ionizing radiation.

In the present paper, we show that the absorption of a single photon can singly ionize both atoms of a helium dimer (He${}_{2}$): ionization with simultaneous excitation of one atom followed by de-excitation via interatomic Coulombic decay leads to the ejection of an electron from each of the the two atoms of the dimer. Using the Cold Target Recoil Ion Momentum Spectroscopy technique (COLTRIMS), we obtained angular distributions of these electrons in the laboratory frame and the molecular frame. We observe a pronounced variation of these distributions for different regions of kinetic-energy releases of the ions.

We show that a single photon can ionize the two helium atoms of the helium dimer in a distance up to 10 {\deg}A. The energy sharing among the electrons, the angular distributions of the ions and electrons as well as comparison with electron impact data for helium atoms suggest a knock-off type double ionization process. The Coulomb explosion imaging of He_2 provides a direct view of the nuclear wave function of this by far most extended and most diffuse of all naturally existing molecules., Comment: 10 pages, 5 figures

4 páginas 4 figuras.-PACS:36.40.Mr,33.20.Tp.-.-- et al., Using synchrotron radiation we simultaneously ionize and excite one helium atom of a helium dimer (He2) in a shakeup process. The populated states of the dimer ion [i.e., He*+(n=2,3)-He] are found to deexcite via interatomic Coulombic decay. This leads to the emission of a second electron from the neutral site and a subsequent Coulomb explosion. In this Letter we present a measurement of the momenta of fragments that are created during this reaction. The electron energy distribution and the kinetic energy release of the two He+ ions show pronounced oscillations which we attribute to the structure of the vibrational wave function of the dimer ion., This work was supported by the Koselleck Programm of DFG, R. E. G. acknowledges support by the Helmholtz Society, Grant No. VH-NG-331.

We report a kinematically complete experiment on the production of CO{sup +} autoionizing states following photoionization of carbon monoxide below its vertical double ionization threshold. Momentum imaging spectroscopy is used to measure the energies and body-frame angular distributions of both photo- and autoionization electrons, as well as the kinetic energy release (KER) of the atomic ions. This data, in combination with ab initio theoretical calculations, provides insight into the nature of the cation states produced and their subsequent dissociation into autoionizing atomic (O*) fragments.

At photon energies near the Ne K edge it is shown that for I s ionization the Auger electron, and for 2s ionization the fast photoelectron, launch vibrational wave packets in a Ne dimer. These wave packets then decay by emission of a slow electron via interatomic Coulombic decay (ICD). The measured and computed ICD electron spectra are shown to be significantly modified by the recoil induced nuclear motion.

Journal of physics / B 42, 141002 (2009). doi:10.1088/0953-4075/42/14/141002, Published by IOP Publ., Bristol

Few-photon multiple ionization of N2 was studied differentially in a reaction microscope using 44 eV, approximately 25 fs, intense ( approximately 10(13) W/cm(2)) photon pulses from FLASH. Sequential ionization is observed to dominate. For various intermediate charge states N(2)(n+0 we find a considerable excess of photons absorbed compared to the minimum number that would energetically be required. Photoionization of aligned N(2)(n+) ions, produced by photon absorption in sequential steps, is explored and few-photon absorption pathways are traced by inspecting kinetic energy releases and fragment-ion angular distributions.

We used cold target recoil ion momentum spectroscopy for a comprehensive study of the decay of a neon dimer $({\mathrm{Ne}}_{2})$ after removal of a $1s$ electron from one of the atoms of the dimer. Multiple decay pathways are found and identified, mostly being connected to different types of interatomic Coulombic decay (ICD) such as the ``direct'' ICD which happens via the transfer of a virtual photon, the ``exchange'' ICD via an electron transfer and the electron transfer mediated decay. A quantitative theoretical analysis of these decay processes can be found in the preceding paper by Demekhin et al. [Phys. Rev. A 78, 043421 (2008)], as well as in Stoychev et al. [J. Chem. Phys. 129, 074307 (2008)].

Recoil-ion momentum distributions for two-photon double ionization of He and Ne (variant Planck's over omega=44 eV) have been recorded with a reaction microscope at FLASH (the free-electron laser at Hamburg) at an intensity of approximately 1 x 10(14) W/cm2 exploring the dynamics of the two fundamental two-photon-two-electron reaction pathways, namely, sequential and direct (or nonsequential) absorption of the photons. We find strong differences in the recoil-ion momentum patterns for the two mechanisms pointing to the significantly different two-electron emission dynamics and thus provide serious constraints for theoretical models.

The dissociative electron transfer from He into 10 keV H2+ was measured in a kinematically complete experiment by using the cold target recoil ion momentum spectroscopy imaging technique in combination with a highly resolving molecular fragment imaging technique. The electron transfer into the dissociative b(3)Sigma+_(u) state of H2 could be selected by kinematic conditions. We find a striking double slit interference pattern in the transverse momentum transfer which we can modify by selecting different internuclear distances. Compared to an optical double slit, interference minima and maxima are interchanged. The latter is the result of a phase shift in the electronic part of the wave function.

By using a new imaging technique for pairs of electrons emitted in coincidence from a surface, we investigated the two-electron photoemission from a Cu111 surface in the energy range from = 40 to 60 eV. The full solid angle coverage provides a comprehensive view into the details of the emission process and allows for a quantitative study of the exchange-correlation XC hole in the two-electron momentum final state. Evaluating the data in a center-of-mass frame the XC hole yields a value of 0.76 a.u. as smallest distance in momentum space. Additionally, we can demonstrate a significant dependency of the emission dynamics on the light polarization.

We used Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) to investigate the decay of Ne$_2$ after K-shell photoionization. The breakup into Ne$^{1+}$ / Ne$^{2+}$ shows interatomic Coulombic decay (ICD) occurring after a preceding atomic Auger decay. The molecular frame angular distributions of the photoelectron and the ICD electron show distinct, asymmetric features, which imply localization of the K-vacancy created at one of the two atomic sites of the Ne$_2$ and an emission of the ICD electron from a localized site. The experimental results are supported by calculations in frozen core Hartree-Fock approach., Comment: 4 pages, 4 figures

We investigate the interatomic Coulombic decay (ICD) of neon dimers following photoionization with simultaneous excitation of the ionized atom (shakeup) in a multiparticle coincidence experiment. We find that, depending on the parity of the excited state, which determines whether ICD takes place via virtual dipole photon emission or overlap of the wave functions, the decay happens at different internuclear distances, illustrating that nuclear dynamics heavily influence the electronic decay in the neon dimer.

We have investigated the correlated momentum distribution of both electrons from nonsequential double ionization of helium in a 800 nm, 4.5×1014W/cm2 laser field. Using very high resolution coincidence techniques, we find a so-far unobserved fingerlike structure in the correlated electron momentum distribution. The structure can be interpreted as a signature of the microscopic dynamics in the recollision process. We identify features corresponding to the binary and recoil lobe in field-free (e,2e) collisions. This interpretation is supported by analyzing ab initio solutions of a fully correlated three-dimensional helium model. © 2007 The American Physical Society.

Using H2+ and D2+, we observe two-surface population dynamics by measuring the kinetic energy of the correlated ions that are created when H2+ (D2+) ionize in short (40-140 fs) and intense (10(14) W/cm2) infrared laser pulses. Experimentally, we find a modulation of the kinetic energy spectrum of the correlated fragments. The spectral progression arises from a hitherto unexpected spatial modulation on the excited state population, revealed by Coulomb explosion. By solving the two-level time-dependent Schrödinger equation, we show that an interference between the net-two-photon and the one-photon transition creates localized electrons which subsequently ionize.

Partial photoionization cross sections sigmaN(Egamma) and photoelectron angular distributions betaN(Egamma) were measured for the final ionic states He+ (N4) in the region between the N = 8 and N = 13 thresholds (Egamma78.155 eV) using the cold target recoil ion momentum spectroscopy technique (COLTRIMS). Comparison of the experimental data with two independent sets of theoretical predictions reveals disagreement for the branching ratios to the various HeN(+) states. The angular distributions just below the double ionization threshold suggest an excitation process for highly excited N states similar to the Wannier mechanism for double ionization.

Recently Cederbaum et al. [Phys. Rev. Lett. 79, 4778 (1997)] predicted a new decay channel of excited atoms and molecules termed interatomic Coulombic decay (ICD). In ICD the deexcitation energy is transferred via virtual photon exchange to a neighboring atom, which releases it by electron emission. We report on an experimental observation of ICD in $2s$ ionized neon dimers. The process is unambiguously identified by detecting the energy of two ${\mathrm{N}\mathrm{e}}^{1+}$ fragments and the ICD electron in coincidence, yielding a clean, background free experimental spectral distribution of the ICD electrons.

We have measured fully differential cross sections for photo double ionization of helium 450 eV above the threshold using the COLTRIMS technique. We have found an extremely asymmetric energy sharing between the two electrons and an angular asymmetry parameter β ≃ 2 for the fast electrons and β ≃ 0 for the slow electrons. The asymmetric energy sharing together with the asymmetry parameter β ≃ 2 for the fast electron indicate that the fast electron absorbs not only most of the photon energy but also its angular momentum. The electron angular distributions show that the very slow electrons (about 2 eV) are emitted isotropically with a slight backward emission to the momentum of the fast electron which is expected from the shake-off mechanism. At the same time, the slow electrons at higher energies are mostly emitted at 90 degree to the fast electron indicating an (e,2e) like collision between the two electrons. All our data are in good agreement to CCC calculations.

High-pressure discharges are generated using Micro-Structured Electrode (MSE) arrays with electrode distances between 50 and 250 µm. These arrays consist either of a planar electrode system bonded to a dielectric substrate or of a matrix of holes perforated in a composite sheet made out of two metallic foils separated by an insulator. Stable discharges were produced for pressures ranging from 10 to 1000 mbar using DC and radio frequency excitation. The optical appearance is that of a homogeneous glow discharge. Spectral investigations have revealed the presence of a large number density of metastable atoms and highly energetic electrons, while the discharge has a non-thermal character. The non-equilibrium high -pressure plasma provided by the MSE arrays is suitable for surface activation, thin film deposition and reduction of pollutants.

We have measured the angular distributions of 1s photoelectrons excited by circularly and linearly polarized light from fixed-in-space CO and N2 molecules, in the vicinity of their shape resonances. A strong circular dichroism, i.e., a strong dependence on the sense of rotation of the polarization vector of the photons, is found for both molecules. State-of-the-art one-electron multiple scattering and partially correlated random phase approximation calculations are in good agreement with many, but not all, aspects of the experimental data.

A high-pressure dc glow discharge based on micro-structured-electrode (MSE) arrays was investigated by diode laser atomic absorption spectroscopy. The microdischarge was studied at constant current in pure Ar for pressures ranging from 50 to 400 mbar. The measurements of the absolute population density of the excited 1s5 ,1 s 4 ,1 s 3, and 1s2 levels of Ar in the discharge revealed a population density of excited atoms in metastable and resonance states in the range 10 12 –10 15 cm −3 . The gas temperature and the electron number density were evaluated from the analysis of the absorption line profiles, taking into account significant broadening mechanisms. The gas temperature, derived from the Doppler broadening, was found to increase with pressure from 380 K at 50 mbar to 1100 K at 400 mbar. The electron number density was calculated from the Stark broadening and shift, and it ranges from 9 × 10 14 to 5 × 10 15 cm −3 . The MSE-sustained discharges are combining the non-equilibrium character with the advantage of high-pressure, which recommends them for non-thermal plasma processing e.g. surface treatment, plasma chemistry and generation of UV and VUV radiation.

We have developed a photon counting detector system for combined neutron and gamma radiography which can determine position, time and intensity of a secondary photon flash created by a high-energy particle or photon within a scintillator screen. The system is based on a micro-channel plate photomultiplier concept utilizing image charge coupling to a position- and time-sensitive read-out anode placed outside the vacuum tube in air, aided by a standard photomultiplier and very fast pulse-height analyzing electronics. Due to the low dead time of all system components it can cope with the high throughput demands of a proposed combined fast neutron and dual discrete energy gamma radiography method (FNDDER). We show tests with different types of delay-line read-out anodes and present a novel pulse-height-to-time converter circuit with its potential to discriminate gamma energies for the projected FNDDER devices for an automated cargo container inspection system (ACCIS)., Proceedings of FNDA 2011

By using the COLd Target Recoil Ion Momentum Spectroscopy (COLTRIMS) we have investigated the 1s photoionization of neon dimers. Interatomic Coulombic Decay (ICD) takes place in the asymmetric charge breakup Ne2+/Ne1+. This breakup is used to determine whether the inner shell vacancies, resulting out of the photoionization of the dimer, and the valence shell vacancies, resulting out of the ICD, are localized at one of the atoms or delocalized over the two equivalent sites of the neon dimer.

With photon energies 10 eV above the K-edge the 1s photoionization of neon dimers was measured by using COLd Target Recoil Ion Momentum Spectroscopy. All decay channels resulting in the breakup Ne+ + Ne+ and Ne2+ + Ne+were identified. The latter was used to investigate the localization or delocalization of vacancies in the homonuclear diatomic system.

The dissociative electron transfer from He into 10 keV H+2 was measured in a kinematically complete experiment. For electron transfer into the dissociative b3?+u state of H2 we find a striking double slit interference pattern in the transverse momentum transfer which we can modify by selecting different internuclear distances of the hydrogen molecule. Compared to an optical double slit, interference minima and maxima are interchanged.

Multiple ionization (MI), induced by a few EUV photons at energies of 28.2 eV, 38 eV and 44 eV from FLASH (the free-electron laser at Hamburg), has been studied for atoms (He, Ne, Ar) and N2 molecules utilizing our multi-hit coincident technology?the reaction microscope. At comparably low intensities of I 1011?1013 W cm?2 we find the non-sequential (NS) MI mechanism dominating Ar3+ and Ar4+ production. Inspecting recoil ion and electron momentum distributions evidence is provided (i) for preferential back-to-back emission of electrons for NS double ionization of He and (ii) for angular entanglement between two outgoing electrons in sequential ionization (SI). In contrast to atoms, SI is observed to be most effective for MI of N2 molecules at an intensity of ~1013 W cm?2 leading, among others, to N2+2 ? N+ + N+, N3+2 ? N2+ + N+, N4+2 ? N2+ + N2+ Coulomb explosion channels. Fragment ion momentum distributions are investigated and are demonstrated to allow tracing SI pathways.

Coincident measurement of the Auger electron and fragment ion momenta emitted after carbon core-level photoionization of acetylene has yielded new understanding of how the dication fragments. Ab initio calculations and experimental data, including body-frame Auger angular distributions, are used to identify the parent electronic states and together yield a comprehensive map of the dissociation pathways which include surface crossings and barriers to direct dissociation. The Auger angular distributions for certain breakup channels show evidence of core–hole localization. (Some figures in this article are in colour only in the electronic version)

We have identified interatomic Coulombic decay (ICD) processes in the ArKr dimer following Ar 2p Auger decay, using momentum-resolved electron–ion–ion coincidence spectroscopy and simultaneously determining the kinetic energy of the ICD electron and the KER between Ar 2+ and Kr + . We find that the spin-conserved ICD processes in which Ar 2+ (3p −3 3d) 1 Pa nd 3 P decay to Ar 2+ (3p −2 ) 1 Da nd 3 P, respectively, ionizing the Kr atom, are significantly stronger than the spin-flip ICD processes in which Ar 2+ (3p −3 3d) 1 Pa nd 3 P decay to Ar 2+ (3p −2 ) 3 Pa nd 1 D, respectively. (Some figures in this article are in colour only in the electronic version)