The rise time of intense radiation determines the maximum field strength atoms can be exposed to before their polarizability dramatically drops due to the detachment of an outer electron. Recent progress in ultrafast optics has allowed the generation of ultraintense light pulses comprising merely a few field oscillation cycles. The arising intensity gradient allows electrons to survive in their bound atomic state up to external field strengths many times higher than the binding Coulomb field and gives rise to ionization rates comparable to the light frequency, resulting in a significant extension of the frontiers of nonlinear optics and (nonrelativistic) high-field physics. Implications include the generation of coherent harmonic radiation up to kiloelectronvolt photon energies and control of the atomic dipole moment on a subfemtosecond $(1{\mathrm{f}\mathrm{s}=10}^{\mathrm{\ensuremath{-}}15}\mathrm{}\mathrm{s})$ time scale. This review presents the landmarks of the 30-odd-year evolution of ultrashort-pulse laser physics and technology culminating in the generation of intense few-cycle light pulses and discusses the impact of these pulses on high-field physics. Particular emphasis is placed on high-order harmonic emission and single subfemtosecond extreme ultraviolet/x-ray pulse generation. These as well as other strong-field processes are governed directly by the electric-field evolution, and hence their full control requires access to the (absolute) phase of the light carrier. We shall discuss routes to its determination and control, which will, for the first time, allow access to the electromagnetic fields in light waves and control of high-field interactions with never-before-achieved precision.

Intense few-cycle laser fields: Frontiers of nonlinear optics

Summary form only given. Fundamental processes in atoms, molecules, as well as condensed matter are triggered or mediated by the motion of electrons inside or between atoms. Electronic dynamics on atomic length scales tends to unfold within tens to thousands of attoseconds (1 attosecond [as] = 10-18 s). Recent breakthroughs in laser science are now opening the door to watching and controlling these hitherto inaccessible microscopic dynamics. The key to accessing the attosecond time domain is the control of the electric field of (visible) light, which varies its strength and direction within less than a femtosecond (1 femtosecond = 1000 attoseconds). Atoms exposed to a few oscillations cycles of intense laser light are able to emit a single extreme ultraviolet (XUV) burst lasting less than one femtosecond. Full control of the evolution of the electromagnetic field in laser pulses comprising a few wave cycles have recently allowed the reproducible generation and measurement of isolated sub-femtosecond XUV pulses, demonstrating the control of microscopic processes (electron motion and photon emission) on an attosecond time scale. These tools have enabled us to visualize the oscillating electric field of visible light with an attosecond "oscilloscope", to control single-electron and probe multi-electron dynamics in atoms, molecules and solids. Recent experiments hold promise for the development of an attosecond X-ray source, which may pave the way towards 4D electron imaging with sub-atomic resolution in space and time.

Attosecond Physics

THE subject of quantum electrodynamics is extremely difficult, even for the case of a single electron. The usual method of solving the corresponding wave equation leads to divergent integrals. To avoid these, Prof. P. A. M. Dirac* uses the method of redundant variables. This does not abolish the difficulty, but presents it in a new form, which may be dealt with in two ways. The first of these needs only comparatively simple mathematics and is directly connected with an elegant general scheme, but unfortunately its wave functions apply only to a hypothetical world and so its physical interpretation is indirect. The second way has the advantage of a direct physical interpretation, but the mathematics is so complicated that it has not yet been solved even for what appears to be the simplest possible case. Both methods seem worth further study, failing the discovery of a third which would combine the advantages of both.

http://ieeexplore.ieee.org/iel5/3/22993/01069961.pdf

Quantum electrodynamics

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.

/pdf/recoil-ion-and-electron-momentum-spectroscopy-reaction-c7ai542e3l.pdf

Recoil-ion and electron momentum spectroscopy: reaction-microscopes

Organic light-emitting diodes (OLEDs) show promise for applications as high-quality self-emissive displays for portable devices such as cellular phones and personal organizers. Although monochrome operation is sufficient for some applications, the extension to multi-colour devices--such as RGB (red, green, blue) matrix displays--could greatly enhance their technological impact. Multi-colour OLEDs have been successfully fabricated by vacuum deposition of small electroluminescent molecules, but solution processing of larger molecules (electroluminescent polymers) would result in a cheaper and simpler manufacturing process. However, it has proved difficult to combine the solution processing approach with the high-resolution patterning techniques required to produce a pixelated display. Recent attempts have focused on the modification of standard printing techniques, such as screen printing and ink jetting, but those still have technical drawbacks. Here we report a class of electroluminescent polymers that can be patterned in a way similar to standard photoresist materials--soluble polymers with oxetane sidegroups that can be crosslinked photochemically to produce insoluble polymer networks in desired areas. The resolution of the process is sufficient to fabricate pixelated matrix displays. Consecutive deposition of polymers that are luminescent in each of the three RGB colours yielded a device with efficiencies comparable to state-of-the-art OLEDs and even slightly reduced onset voltages.

Multi-colour organic light-emitting displays by solution processing.

In the present chapter, we discuss direct photo double ionization by singlephoton absorption (Sect. 14.2) and Compton scattering (Sect. 14.3). We do not discuss the closely related phenomenon of multiple ionization by two-step processes such as photoionization followed by single- or multiple-Auger decay. We concentrate on the two most fundamental two-electron target systems: the helium atom (Sects. 14.2 and 14.3) and molecular hydrogen (deuterium) (Sect. 14.4). The subject of photo double ionization of helium is now a mature field in which an impressive experimental and theoretical breakthrough has been achieved in the previous 10 years. The theoretical progress is described in Part III of this book, we therefore restrict ourselves here to a phenomenological description and intuitive interpretation of the physical phenomena. For the problem of two-electron processes in molecules, in contrast, the major challenges for experimentalist and theoretician still lie ahead.

From Atoms to Molecules

One of the most fundamental tasks of nowadays atomic physics is the investigation of the dynamics in many-body systems. Its understanding is fundamental for a detailed treatment of practically all complex systems ranging from the simplest three-body problem, a charged projectile colliding with a hydrogen atom, via large molecules up to macroscopic structures of daily live. These many-body effects in systems governed by the Coulomb force (often called correlations) play a dominant role in atomic multiple ionizing reactions. Thus, already a study of double ionization can serve as a tool to investigate the specific many-body properties of bound and continuum wavefunctions as well as of the collision process itself.

/pdf/electron-impact-ionization-of-helium-e-2e-e-3e-investigated-35yadnkeha.pdf

Electron Impact Ionization of Helium [(e,2e) & (e,3e)] Investigated with Cold Target Recoil-Ion Momentum Spectroscopy

The technique of Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) has been developed to study the reaction dynamics in atomic collisions1,2. Recently, this method has been applied to collisions of electrons and photons with helium3,4,5,6 Combining this technique with position sensitive large solid angle electron time-of-flight detectors it became possible to perform experiments of the type (γ,2e)7 and (e,2e) 8, with the recoil-ion momentum measurement replacing the spectroscopy of one of the electrons.

From (γ,2e) to (γ,eR): Kinematically Complete Experiments with Coltrims

Using Recoil-Ion Momentum Spectroscopy (RIMS) we have measured the momenta of recoiling tar- get ions in He(e,2e)He +- and He(e,3e)He++-reactions at impact energies between 270 ev and 3200 eV. The recoil- ion momentum reflecting the sum momentum of all out- going electrons was determined for the first time in all three spatial dimensions separately for single and double ionization by electron impact. The data are compared to results of a nCTMC-calculation.

Lutz Spielberger

Papers

From Atoms to Molecules

Electron Impact Ionization of Helium [(e,2e) & (e,3e)] Investigated with Cold Target Recoil-Ion Momentum Spectroscopy

From (γ,2e) to (γ,eR): Kinematically Complete Experiments with Coltrims

Recoil-ion momentum distribution for and He(e,3e)He + +-reactions He(e,2e)He +

Double Ionization in Strong Fields: Ion Momenta and Correlated Electron Momenta