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 4π 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.

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Recoil-ion and electron momentum spectroscopy: reaction-microscopes

Angle-resolved photoelectron spectroscopy has been performed for more than 70 years in various guises, but recently its potential to help solve in detail problems in the photoionization dynamics and intramolecular dynamics of gas-phase molecules has been recognized. One key development has been the design of experiments in appropriate geometries to extract information that pertains to the molecular frame, another has been the development of imaging spectrometers, and a third is the use of ultrafast lasers to cause photoionization. In this review, which is aimed at experimentalists, simple expressions for photoelectron angular distributions (PADs) in various experimental geometries are given and their applications explained.

Photoelectron angular distributions.

Femtosecond time-resolved photoelectron imaging (TRPEI) is a variant of time-resolved photoelectron spectroscopy used in the study of gas-phase photoinduced dynamics. A new observable, time-dependent photoionization-differential cross section provides useful information on wave-packet motions, electronic dephasing, and photoionization dynamics. This review describes fundamental issues and the most recent works involving TRPEI.

Femtosecond time-resolved photoelectron imaging.

Electron impact dissociation of oxygen-containing molecules A critical review

Recent experimental and theoretical developments in time-resolved X-ray spectroscopies

Nonlinear processes of rare gas atoms in intense extreme ultraviolet (EUV) free-electron laser (FEL) fields have been investigated by photoelectron spectroscopy Simultaneous recording of fluctuating laser spectra from self-amplified spontaneous emission (SASE) FEL and nonlinear responses of target atoms provided a deeper understanding on the multiphoton processes in (i) double ionization of Ar and (ii) double excitation of He, in intense EUV laser fields Results on two-color time-resolved experiment are also presented

Shot-by-Shot Photoelectron Spectroscopy of Rare Gas Atoms in Ultrashort Intense EUV Free-Electron Laser Fields

The decay processes of core-valence doubly excited states near the N K edge of NO have been studied using electron spectroscopy. Electron yields measured as a function of photon energy and kinetic energy enable the clear identification of atomic Auger lines associated with the dissociation of doubly excited states. The atomic Auger lines exhibit Doppler profiles, allowing the entire reaction scheme of such dissociation processes to be determined.

Dissociation of core-valence doubly excited states in NO followed by atomic Auger decay.

We present a new realization of the time-domain double-slit experiment with photoelectrons, demonstrating that spontaneous radiation from a bunch of relativistic electrons can be used to control the quantum interference of single-particles. The double-slit arrangement is realized by a pair of light wave packets with attosecond-controlled spacing, which is naturally included in the spontaneous radiation from two undulators in series. Photoelectrons emitted from helium atoms are observed in the energy-domain under the condition of detecting them one by one, and the stochastic buildup of the quantum interference pattern on a detector plane is recorded. 

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Time domain double slit interference of electron produced by XUV synchrotron radiation.

Indirect double photoionization processes of Ne have been studied in the photon energy range from 65.5 to 68.3 eV by using a threshold-photoelectron−photoelectron coincidence method, where intermediately formed Ne+ states have been defined by the detection of threshold photoelectrons and fast electrons from their autoionizing decay have been analysed in coincidence. The present investigation revealed that each Ne+ Rydberg series converging to Ne2+(1Se) shows notable selectivity in the autoionizing decay to the final Ne2+ states.

Autoionization selectivity of Ne+ Rydberg states converging to Ne2+(1Se)

Direct measurements of the photoelectrons or Auger electrons associated with inner shell ionization of positively charged ions are extremely difficult and rarely realized. We use here an alternative method to simulate such measurements, based on core-valence double photoionization of the neutral species. As an example, we obtain the spectroscopy, lifetimes and Auger decays of the states arising from 2p inner shell ionization of an Ar+ ion. Observations compare well with theoretical predictions obtained with multiconfigurational Dirac-Fock formalism.

Yasumasa Hikosaka

Papers

Shot-by-Shot Photoelectron Spectroscopy of Rare Gas Atoms in Ultrashort Intense EUV Free-Electron Laser Fields

Dissociation of core-valence doubly excited states in NO followed by atomic Auger decay.

Time domain double slit interference of electron produced by XUV synchrotron radiation.

Autoionization selectivity of Ne+ Rydberg states converging to Ne2+(1Se)

Decay of a 2p inner-shell hole in an Ar + ion