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

We study photoionization of argon atoms excited by attosecond pulses using an interferometric measurement technique. We measure the difference in time delays between electrons emitted from the 3s(2) and from the 3p(6) shell, at different excitation energies ranging from 32 to 42 eV. The determination of photoemission time delays requires taking into account the measurement process, involving the interaction with a probing infrared field. This contribution can be estimated using a universal formula and is found to account for a substantial fraction of the measured delay.

The multiconfiguration time-dependent Hartree-Fock approach for the description of correlated few-electron dynamics in the presence of strong laser fields is introduced and a comprehensive description of the method is given. Total ionization and electron spectra for the ground and first excited ionic channels are calculated for one-dimensional model systems with up to six active electrons. Strong correlation effects are found in the shape of photoelectron peaks and the dependence of ionization on molecule size.

Esta es la versión de autor del artículo publicado en: This is an author produced version of a paper published in:Science 354(6313) (2016): 734-738 DOI: http://dx.doi.org/10.1126/science.aah5188 Copyright: © 2016 American Association for the Advancement of ScienceEl acceso a la versión del editor puede requerir la suscripción del recurso Access to the published version may require subscription However, the rapidity of electron dynamics on the attosecond timescale has precluded their complete measurement in the time domain. Here, we demonstrate that spectrally-resolved electron interferometry reveals the amplitude and phase of a photoelectron wavepacket created through a Fano autoionizing resonance in helium. Replicas obtained by two-photon transitions interfere with reference wavepackets formed through smooth continua, allowing the full temporal reconstruction, purely from experimental data, of the resonant wavepacket released in the continuum. This in turn resolves the buildup of the autoionizing resonance on attosecond timescale. Our results, in excellent agreement with ab initio time-dependent calculations, raise prospects for both detailed investigations of ultrafast photoemission dynamics governed by electron correlation, as well as coherent control over structured electron wave-packets.One Sentence Summary: By monitoring the decay of an excited atom in real time, we reconstruct how photoelectron wavepackets are born and morph into asymmetric Fano profiles. Main Text:Tracking electronic dynamics on the attosecond (as) timescale and Ångström (Å) lengthscale is a key to understanding and controlling the quantum mechanical underpinnings of physical and chemical transformations (1). One of the most fundamental electronic processes in this context is photoelectron emission, the dynamics of which are fully encoded in the released electron wavepacket (EWP) and the final ionic state. The development of broadband coherent sources of attosecond pulses has opened the possibility of investigating these dynamics with attosecond resolution. On such a short timescale, few techniques (2-5) are able to provide access to both spectral amplitude and phase. The spectral derivative of the phase, the group delay, is a practical quantity for describing general wavepacket properties reflecting the ionization dynamics.

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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.