International audienceA strong laser field may tunnel ionize a molecule from several orbitals simultaneously, forming an attosecond electron–hole wavepacket. Both temporal and spatial information on this wavepacket can be obtained through the coherent soft X-ray emission resulting from the laser-driven recollision of the liberated electron with the core. By characterizing the emission from aligned N 2 molecules, we demonstrate the attosecond contributions of the two highest occupied molecular orbitals. We determine conditions where they are disentangled in the real and imaginary parts of the emission dipole moment. This allows us to carry out a tomographic reconstruction of both orbitals with angstrom spatial resolution. Their coherent superposition provides experimental images of the attosecond wavepacket created in the ionization process. Our results open the prospect of imaging ultrafast intramolecular dynamics combining attosecond and angstrom resolutions

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International audienceWe photoionize nitrogen molecules with a train of extreme ultraviolet attosecond pulses together with a weak infrared field. We measure the phase of the two-color two-photon ionization transition ͑molecular phase͒ for different states of the ion. We observe a 0.9␲ shift for the electrons produced in the ionization channels leading to the X 2 ⌺ g + , vЈ = 1, and vЈ = 2 states. We relate this phase shift to the presence of a complex resonance in the continuum. By providing both a high spectral and temporal resolution, this general approach gives access to the evolution of extremely short-lived states, which is often not accessible otherwise. DOI: 10.1103/PhysRevA.80.011404 PACS number͑s͒: 33.80.Eh, 33.60.ϩq, 42.65.Ky, 82.53.Kp Ionization of atoms and molecules by absorption of ul-trashort extreme ultraviolet ͑xuv͒ radiation provides rich structural information on the considered species. The ioniza-tion process releases an electron wave packet, which can be described as a coherent superposition of partial waves. The relative contributions and phases of the partial waves can be extracted from photoelectron angular distributions at a given energy ͓1͔. However, the temporal structure of the ejected wave packet, which is imposed by the phase relation between different energy components, is not accessible with such experiments. To access this phase, one needs to couple two energy components of the electron wave packet and record the resulting interference. This can be achieved by absorption of high-order harmonics of an infrared laser pulse in the presence of the fundamental field. An intense laser pulse propagating in a gas jet produces coherent xuv radiation constituted of odd harmonics ͑2q +1͒␻ 0 of the fundamental frequency ␻ 0. These harmonics are all approximately phase locked with the fundamental and form an attosecond pulse train ͑APT͒ ͓2͔. In photoionization experiments with high harmonics, the photoelectron spectrum exhibits equidistant lines resulting from single-photon ionization ͓Fig. 1͑a͔͒. If an additional laser field with frequency ␻ 0 is added, two-photon ionization can occur: absorption of a harmonic photon accompanied by either absorption or stimulated emission of one photon ␻ 0. New lines ͑sidebands͒ appear in the spectrum, in between the harmonics ͓Fig. 1͑a͔͒. Since two coherent quantum paths lead to the same sideband, interferences occur. They are observed in an oscillation of the sideband amplitude as the delay ␶ between the probe ͑ir͒ and harmonic fields is scanned ͓2,3͔. This is the basis of the reconstruction of attosecond beating by interference of two-photon transitions ͑RABBITT͒ technique. The phase of the oscillation is determined by the phase difference between consecutive harmonics ͑phase locking͒ and by additional phase characteristics of the ionization process. The same process can be described in the time domain. The APT creates a train of attosecond electron wave packets. The additional laser field acts as an optical gate on the electrons , which can be used to re...

This tutorial presents the most important aspects of the molecular self-probing paradigm, which views the process of high harmonic generation as "a molecule being probed by one of its own electrons". Since the properties of the electron wavepacket acting as a probe allow a combination of attosecond andÅngström resolutions in measurements, this idea bears great potential for the observation, and possibly control, of ultrafast quantum dynamics in molecules at the electronic level. Theoretical as well as experimental methods and concepts at the basis of self-probing measurements are introduced. Many of these are discussed on the example of molecular orbital tomography.

The recently developed attosecond light sources make the investigation of ultrafast processes in matter possible with unprecedented time resolution. It has been proposed that the very mechanism underlying the attosecond emission allows the imaging of valence orbitals with Ångström space resolution. This controversial idea together with the possibility of combining attosecond and Ångström resolutions in the same measurements has become a hot topic in strong-field science. Indeed, this could provide a new way to image the evolution of the molecular electron cloud during, e.g. a chemical reaction in 'real time'. Here we review both experimental and theoretical challenges raised by the implementation of these prospects. In particular, we show how the valence orbital structure is encoded in the spectral phase of the recombination dipole moment calculated for Coulomb scattering states, which allows a tomographic reconstruction of the orbital using first-order corrections to the plane-wave approach. The possibility of disentangling multi-channel contributions to the attosecond emission is discussed as well as the necessary compromise between the temporal and spatial resolutions.

This review presents the technological infrastructure that will be available at the Extreme Light Infrastructure Attosecond Light Pulse Source (ELI-ALPS) international facility. ELI-ALPS will offer to the international scientific community ultrashort pulses in the femtosecond and attosecond domain for time-resolved investigations with unprecedented levels of high quality characteristics. The laser sources and the attosecond beamlines available at the facility will make attosecond technology accessible for scientists lacking access to these novel tools. Time-resolved

We report on recent progress on laser-plasma acceleration using a low energy and high-repetition rate laser system. Using only few milliJoule laser energy, in conjunction with extremely short pulses composed of a single optical cycle, we demonstrate that the laser-plasma accelerator (LPA) can be operated close to the resonant blowout regime. This results in the production of high charge electron beams (>10 pC) with peaked energy distributions in the few MeV range and relatively narrow divergence angles. We highlight the importance of the plasma density profile and gas jet design for the performance of the LPA. In this extreme regime of relativistic laser-plasma interaction with near-single-cycle laser pulses, we find that the effect of group velocity dispersion and carrier envelope phase can no longer be neglected. These advances bring LPAs closer to real scientific applications in ultrafast probing.

We have simulated two-color photoionization of N(2) by solving the time-dependent Schrödinger equation with a simple model accounting for the correlated vibronic dynamics of the molecule and of the ion N(2)(+). Our results, in very good agreement with recent experiments [Haessler et al., Phys. Rev. A 80, 011404 (2009)], show how a resonance embedded in the molecular continuum dramatically affects the phases of the two-photon transition amplitudes. In addition, we introduce a formal relation between these measurable phases and the photoelectron release time, opening the way to attosecond time-resolved measurements, equivalent to double-slit experiments in the time domain.