The COLTRIMS Reaction Microscope C‐REMI can image the momentum vectors of all emitted charged fragments in an atomic or molecular reactions similar to the bubble chamber in high energy particle physics. C‐REMI can detect fragments with “zero” kinetic energy in an ultrahigh vacuum environment by projecting them with weak electromagnetic fields onto position‐sensitive detectors. Geometrically a nearly 4π collection solid angle and a nearly 50% efficiency for a fivefold multi‐coincidence can be achieved. Measuring time‐of‐flight and detector position the momenta of the fragments can be measured with excellent resolution (<0.01 a.u.; see A1 in the Appendix). Thus, multivector correlations in momentum space are measured, which provide insight into the entangled dynamics of atomic and molecular quantum systems. From these vector‐correlations phases and energies can be deduced which allow for relative time measurements even in the zeptosecond range. C‐REMI provides a “spyhole” into the secrets of ultrafast dynamics of atomic and molecular processes. It is applied today around the globe in numerous research projects in physics and chemistry. The purpose for writing this article is to demonstrate the universal application possibilities of C‐REMI, and its high multi‐coincidence efficiency and high momentum resolution. This paper will not give a review on all milestone experiments performed with C‐REMI.

AbstractMulti-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.

The transition between two distinct ionization mechanisms in femtosecond laser fields at 785 nm is observed for C60 molecules. The transition occurs in the investigated intensity range from 3 to 20 TW/cm2 and is visualized in electron kinetic energy spectra below the one-photon energy (1.5 eV) obtained via velocity map imaging. Assignment of several observed broad spectral peaks to ionization from superatom molecular orbitals (SAMOs) and Rydberg states is based on time-dependent density functional theory simulations. We find that ionization from SAMOs dominates the spectra for intensities below 5 TW/cm2. As the intensity increases, Rydberg state ionization exceeds the prominence of SAMOs. Using short laser pulses (20 fs) allowed uncovering of distinct six-lobe photoelectron angular distributions with kinetic energies just above the threshold (below 0.2 eV), which we interpret as over-the-barrier ionization of shallow f-Rydberg states in C60.

Strong laser fields can be used to trigger an ultrafast molecular response that involves electronic excitation and ionization dynamics. Here, we report on the experimental control of the spatial localization of the electronic excitation in the C-60 fullerene exerted by an intense few-cycle (4 fs) pulse at 720 nm. The control is achieved by tailoring the carrier-envelope phase and the polarization of the laser pulse. We find that the maxima and minima of the photoemission-asymmetry parameter along the laser-polarization axis are synchronized with the localization of the coherent electronic wave packet at around the time of ionization.

The fragmentation pathways and dynamics of ethylene molecules after core ionization are explored using coincident measurements of the Auger electron and fragment ions by employing the cold target recoil-ion momentum spectroscopy method. The influence of several factors on the dynamics and kinematics of the dissociation is studied. These include propensity rules, ionization mechanisms, symmetry of the orbitals from which the Auger electrons originate, multiple scattering, conical intersections, interference, and possible core-hole localization for the double ionization of this polyatomic molecule. Energy correlation maps allow probing the multidimensional potential energy surfaces and, in combination with our multiconfiguration self-consistent field calculations, identifying the populated electronic states of the dissociating dication. The measured angular distributions of the Auger electrons in the molecular frame further support and augment these assignments. The deprotonation and molecular hydrogen ion elimination channels show a nearly isotropic Auger electron angular distribution with a small elongation along the direction perpendicular to the molecular axis. For the symmetric breakup the angular distributions show a clear influence of multiple scattering on the outgoing electrons. The lowest kinetic energy release feature of the symmetric breakup channel displays a fingerprint of entangled Auger and photoelectron motion in the angular emission pattern identifying this transition as an excellent candidate to probe core-hole localization at a conical intersection of a polyatomic molecule.

We demonstrate that the electron emission from atoms generated by an attosecond pulse train made of both odd and even harmonics in the presence of a weak IR field can be temporally controlled on the attosecond time scale. In addition, we show that such electron emission carries information about the relative phase between consecutive odd and even harmonics in the attosecond pulse train.

The transition between two distinct mechanisms for the laser-induced field-free orientation of CO molecules is observed via measurements of orientation revival times and subsequent comparison to theoretical calculations. In the first mechanism, which we find responsible for the orientation of CO up to peak intensities of 8 × 10(13) W/cm(2), the molecules are impulsively oriented through the hyperpolarizability interaction. At higher intensities, asymmetric depletion through orientation-selective ionization is the dominant orienting mechanism. In addition to the clear identification of the two regimes of orientation, we propose that careful measurements of the onset of the orientation depletion mechanism as a function of the laser intensity will provide a relatively simple route to calibrating absolute rates of nonperturbative strong-field molecular ionization.

We demonstrate that the electron emission from atoms can be temporally controlled on an attosecond time scale. Electron wave-packets are formed by ionizing an atomic target with an attosecond pulse train composed of both odd and even high-order harmonics in the presence of a relatively weak infrared field. We show that interference between one- and two-photon transitions produces a large asymmetry in the angular distribution of the photoelectrons. The direction of the emission can be controlled on an attosecond time scale by varying the time delay between the two pulses. In addition, we show that such asymmetric emission is also related to the properties of the attosecond pulse train. The temporal analysis of the modulated electron emission, based on an accurate description of the atomic physics of the photoionization process, then provides a way to measure the temporal profile of the attosecond pulse.

We present kinematically complete measurements of the photo-double-ionization of ethylene (double CC bond) and acetylene (triple CC bond) hydrocarbons just above the double-ionization threshold. We discuss the results in terms of the coincident kinetic energy of the photoelectrons and the nuclear kinetic-energy release of the recoiling ions. We have incorporated quantum chemistry calculations to interpret which of the electronic states of the dication have been populated and trace the various subsequent fragmentation channels. We suggest pathways that involve the electronic ground and excited states of the precursor ethylene dication and explore the strong influence of the conical intersections between the different electronic states. The nondissociative ionization yield is small in ethylene and high in acetylene when compared with the dissociative ionization channels. The reason for such a striking difference is explained in part on the basis of a propensity rule that influences the population of states in the photo-double-ionization of a centrosymmetric closed-shell molecule by favoring singlet ungerade and triplet gerade final states. This propensity rule and the calculated potential-energy surfaces clarify a picture of the dynamics leading to the observed dication dissociation products.

A novel design for a velocity-map imaging (VMI) spectrometer with high resolution over a wide energy range surpassing a standard VMI design is reported. The main difference to a standard three-electrode VMI is the spatial extension of the applied field using 11 electrodes forming a thick-lens. This permits measurements of charged particles with higher energies while achieving excellent resolving power over a wide range of energies. Using SIMION simulations, the thick-lens VMI is compared to a standard design for up to 360 eV electrons. The simulations also show that the new spectrometer design is suited for charged-particle detection with up to 1 keV using a repeller-electrode voltage of -30 kV. The experimental performance is tested by laser-induced ionization of rare gases producing electrons up to about 70 eV. The thick-lens VMI is useful for a wide variety of studies on atoms, molecules and nanoparticles in intense laser fields and high-photon-energy fields from high-harmonic-generation or free-electron lasers.

In a kinematically complete experiment we have measured the photo double ionization of Helium at a photon energy of 800 eV and observed He2+ ions with 0 momentum, corresponding to a back-to-back-emission of the two electrons. The results are in good agreement with theoretical calculations.

In this work we propose a novel procedure for the characterization of attosecond pulses. The method relies on the conversion of the attosecond pulse into electron wave-packets through photoionization of atoms in the presence of a weak IR field. It allows for the unique determination of the spectral phase making up the pulses by accurately taking into account the atomic physics of the photoionization process. The phases are evaluated by optimizing the fit of a perturbation theory calculation to the experimental result. The method has been called iPROOF (improved Phase Retrieval by Omega Oscillation Filtering) as it bears a similarity to the PROOF technique [Chini et al. Opt. Express 18, 13006 (2010)]. The procedure has been demonstrated for the characterization of an attosecond pulse train composed of odd and even harmonics. We observe a large phase shift between consecutive odd and even harmonics. The resulting attosecond pulse train has a complex structure not resembling a single attosecond pulse once per IR period, which is the case for zero phase. Finally, the retrieval procedure can be applied to the characterization of single attosecond pulses as well.

We investigate the single-photon double ionization of helium at photon energies of 440 and 800 eV. We observe doubly charged ions with close to zero momentum corresponding to electrons emitted back to back with equal energy. These slow ions are the unique fingerprint of an elusive quasifree photon double ionization mechanism predicted by Amusia et al. nearly four decades ago [J. Phys. B 8, 1248 (1975)]. It results from the nondipole part of the electromagnetic interaction. Our experimental data are supported by calculations performed using the convergent close-coupling and time-dependent close-coupling methods.

In this work, we focused on the angular photoelectron emission from helium generated by an Attosecond Pulse Train (APT) in the presence of a weak Infrared (IR) field. The APT creates both an excited and a continuum electron wave packets. By ionizing the excited state with the IR pulse, a delayed new continuum electron wave packet is created. Consequently, a mix of energy-degenerate even and odd parity states is fed in the continuum by one- and two-photon transitions. These interfere, leading to an asymmetric electron emission along the polarization vector. The interference can be controlled by varying the time delay between the APT and IR pulses.

We investigate the dissociation dynamics of electronically and vibrationally excited O-2(+) molecular ions in intense ultrashort infra-red (IR) probe pulses by analyzing fragment-kinetic-energy release (KER) spectra as a function of the pump-probe delay and quantum-beat frequency.

We have investigated the spectral splitting of high harmonics generated in a semi-infinite gas cell. By performing an EUV–IR cross-correlation experiment, we are able to use the phase behaviour of the different sub-peaks of each harmonic to identify them with different electronic trajectories. Both microscopic and macroscopic analyses of the spectra effects are made. The identification of a particular trajectory with a particular component of the splitting on the basis of a single-atom model is found to be incorrect, while the full macroscopic treatment is in agreement with the experiment.

We have kinematically complete measured photo double ionization of Helium at E-gamma=800 eV and observed recoiling ions with nearly 0 momentum that agree well with theoretical predictions of a new photo double ionization mechanism.

M. S. Schöffler1,∗ C. Stuck, M. Waitz, F. Trinter, T. Jahnke, U. Lenz, M. Jones, A. Belkacem, A. Landers, C. L. Cocke, J. Colgan, A. Kheifets, Igor Bray, H. Schmidt-Böcking, R. Dörner, and Th. Weber 1 Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA 2 Institut für Kernphysik, University Frankfurt, Max-von-Laue-Str. 1, 60438 Frankfurt, Germany 3 Department of Physics, Auburn University, Auburn, AL 36849, USA 4 Department of Physics, Kansas State University, Manhattan, KS 66506, USA 5 Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA 6 Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia and 7 ARC Centre for Matter-Antimatter Studies, Curtin University, WA 6845 Perth, Australia (Dated: July 2, 2012)