Ionization is the dominant response of atoms and molecules to intense laser fields and is at the basis of several important techniques, such as the generation of attosecond pulses that allow the measurement of electron motion in real time. We present experiments in which metastable xenon atoms were ionized with intense 7-micrometer laser pulses from a free-electron laser. Holographic structures were observed that record underlying electron dynamics on a sublaser-cycle time scale, enabling photoelectron spectroscopy with a time resolution of almost two orders of magnitude higher than the duration of the ionizing pulse.

We present angle- and energy-resolved measurements of photoelectrons produced in strong-field ionization of Xe using a tunable femtosecond laser. An occurrence of highly oscillatory patterns in the angular distribution at low photoelectron kinetic energy is observed that correlates with channel closing/opening over a wide range of laser parameters. The correlation is investigated both experimentally and by means of systematic analysis of numerical solutions of the time-dependent Schrödinger equation. Our experimental and numerical results are in quantitative agreement with the semi-classical model introduced by Arbó et al (2008 Phys. Rev. A 78 013406), which relates the oscillatory patterns to interference between photoelectrons produced during different cycles of the laser pulse in the course of non-resonant ionization of the atom. We observe that an increase of the laser intensity eventually leads to qualitative invariance of the pattern, defining a limit on the applicability of the semi-classical model.

We use attosecond angular streaking to place an intensity-averaged upper limit of 12 attoseconds on the tunneling delay time in strong field ionization of a helium atom. This is much shorter than the Keldysh time.

We present angle- and energy-resolved measurements of photoelectrons produced in strong-field ionization of Xe and Ar using a tunable femtosecond laser in the wavelength range between 600 and 800 nm. Systematic analysis of the experimental data that are quantitatively reproduced by numerical solutions of the time-dependent Schrodinger equation with integration over the laser focal volume demonstrates the dominance of resonance-enhanced ionization. Continuous variation of the laser wavelength allows the identification of a number of consecutive channel-switching effects with a reliable assignment of the intermediate Rydberg states involved. The appearance of the resonant sub-structure in the electron energy spectra is influenced by the presence of a non-resonant contribution. At relatively low laser intensity, a coherent addition of resonant and non-resonant ionization processes is observed. Due to the absence of an intensity dependence in the resonance-enhanced ionization, we observe the persistence of Freeman resonances at the transition to the tunnelling regime.

We present a study of the photoelectrons energy distribution from ionization of Argon by a linearly polarized, intense, mid-infrared laser field with special attention to the recently discovered Low Energy Structure (LES) [1] whose origin is not yet fully understood. In this paper we will go deeper in the analysis of the LES by studying its angular distribution and examine its behavior in circularly polarized light.

We used attosecond angular streaking to measure attosecond ionization dynamics in the non-adiabatic tunneling regime of helium using slightly elliptically polarized 5.9 fs pulses with a peak intensity ranging from 2.3 to 3.5 x 1014 W/cm2 (corresponding to a Keldysh parameter variation of 1.45 to 1.17). With our technique we could demonstrate intensityindependent “instantaneous” ionization with an accuracy of 50 as. Numerical simulations based on the time-dependent Schrödinger equation confirm such ionization behavior with no distinct electron wave packets. This implies that we would not expect a tunneling time or multi-photon ionization delay in the ionization dynamics

The discovery of an overlooked but apparently ubiquitous spike in the mid-infrared photoelectron spectra of molecular and atomic gases suggests that we don’t know as much as we thought we did about the ionization of matter in strong fields. Over the past thirty years, extensive studies of strong-field photoionization of atoms have revealed both quantum and classical aspects including above-threshold ionization1, electron wave-packet drift, quiver and rescattering motions. Increasingly sophisticated spectroscopic techniques2 and sculpted laser pulses3 coupled with theoretical advances have led to a seemingly complete picture of this fundamental laser–atom interaction. Here, we describe an effect that seems to have escaped observation: the photoelectron energy distribution manifests an unexpected characteristic spike-like structure at low energy, which becomes prominent using mid-infrared laser wavelengths (λ>1.0 μm). The low-energy structure is observed in all atoms and molecules investigated and thus seems to be universal. The structure is qualitatively reproduced by numerical solutions of the time-dependent Schrödinger equation but its physical origin is not yet identified.

We use attosecond angular streaking to place an intensity-averaged upper limit of 12 attoseconds on the tunneling delay time in strong field ionization of helium. This is far shorter than most tunneling times discussed before.

A systematic experimental study of the ionization of argon by mid-infrared light confirms half-a-century-old predictions and paves the way to the development of brighter, shorter attosecond pulse sources. In 1964 Keldysh1 helped lay the foundations of strong-field physics by introducing a theoretical framework that characterized atomic ionization as a process that evolves with the intensity and wavelength of the fundamental field. Within this context, experiments2 have examined the intensity-dependent ionization but, except for a few cases, technological limitations have confined the majority to wavelengths below 1 μm. The development of intense, ultrafast laser sources in the mid-infrared (1 μm<λ<5 μm) region enables exploration of the wavelength scaling of the Keldysh picture while enabling new opportunities in strong-field physics, control of electronic motion and attosecond science. Here we report a systematic experimental investigation of the wavelength scaling in this region by concurrently analysing the production of energetic electrons and photons emitted by argon atoms interacting with few-cycle, mid-infrared fields. The results support the implicit predictions contained in Keldysh’s work, and pave the way to the realization of brighter and shorter attosecond pulsed light sources using longer-wavelength driving fields.

High-lying Rydberg states of Xe have been ionized using intense $108\text{ }\ensuremath{\mu}\text{m}$ radiation from a free-electron laser. Measured two-dimensional photoelectron images reveal significant above-threshold ionization and contain an indirect contribution resulting from the combined action of the atomic Coulomb field, laser field, and dc electric field of the spectrometer on the electron. The observation of indirect ionization contains information about the electron localization directly after the laser excitation and indicates that the experiments are performed in the multiphoton regime of strong-field ionization. The experiments are compared to and interpreted by means of both classical and quantum-mechanical simulations.

It is well established that electrons can escape from atoms through tunneling under the influence of strong laser fields, but the timing of the process has been controversial and far too rapid to probe in detail. We used attosecond angular streaking to place an upper limit of 34 attoseconds and an intensity-averaged upper limit of 12 attoseconds on the tunneling delay time in strong field ionization of a helium atom. The ionization field derives from 5.5-femtosecond-long near-infrared laser pulses with peak intensities ranging from 2.3 × 1014 to 3.5 × 1014 watts per square centimeter (corresponding to a Keldysh parameter variation from 1.45 to 1.17, associated with the onset of efficient tunneling). The technique relies on establishing an absolute reference point in the laboratory frame by elliptical polarization of the laser pulse, from which field-induced momentum shifts of the emergent electron can be assigned to a temporal delay on the basis of the known oscillation of the field vector.

Sub-femtosecond control of the electron emission in above-threshold ionization of the rare gases Ar, Xe and Kr in intense few-cycle laser fields is reported with full angular resolution. Experimental data that were obtained with the velocity-map imaging technique are compared to simulations using the strong-field approximation (SFA) and full time-dependent Schrödinger equation (TDSE) calculations. We find a pronounced asymmetry in both the energy and angular distributions of the electron emission that critically depends on the carrier-envelope phase (CEP) of the laser field. The potential use of imaging techniques as a tool for single-shot detection of the CEP is discussed.

We present an analytical expression for the observed signal in time- and phase-resolved pump-probe studies, with particular emphasis on terahertz time-domain spectroscopy. Maxwell's equations are solved for the response of damped, harmonic oscillators to a driving probe field in the perturbative regime. Our analytical expressions agree with the one previously reported in the literature [Nemec et al., J. Chem. Phys. 122, 104503 (2005)] in the Drude limit; however, they differ in the case of a vibrational resonance.

This document reports recent theoretical and experimental investigations of strong field ionization and high harmonic generation from mid-infrared lasers at 2 and 4 microns. Numerical solution of the time-dependent Schrodinger equation as well as Strong Field approximation calculations are reported. Photoelectron and high Zn harmonic spectra are discussed. Preliminary experimental results are compared to the theoretical predictions.

Atoms exposed to intense light lose one or more electrons and become ions. In strong fields, the process is predicted to occur via tunnelling through the binding potential that is suppressed by the light field near the peaks of its oscillations. Here we report the real-time observation of this most elementary step in strong-field interactions: light-induced electron tunnelling. The process is found to deplete atomic bound states in sharp steps lasting several hundred attoseconds. This suggests a new technique, attosecond tunnelling, for probing short-lived, transient states of atoms or molecules with high temporal resolution. The utility of attosecond tunnelling is demonstrated by capturing multi-electron excitation (shake-up) and relaxation (cascaded Auger decay) processes with subfemtosecond resolution.

A theoretical investigation is presented that examines the wavelength scaling from near-visible (0.8 mu m) to midinfrared (2 mu m) of the photoelectron distribution and high harmonics generated by a "single" atom in an intense electromagnetic field. The calculations use a numerical solution of the time-dependent Schrodinger equation (TDSE) in argon and the strong-field approximation in helium. The scaling of electron energies (lambda(2)), harmonic cutoff (lambda(2)), and attochirp (lambda(-1)) agree with classical mechanics, but it is found that, surprisingly, the harmonic yield follows a lambda(-(5-6)) scaling at constant intensity. In addition, the TDSE results reveal an unexpected contribution from higher-order returns of the rescattering electron wave packet.

We consider high-harmonic generation in hydrogen atoms exposed to a superposition of two linearly polarized laser fields with different colors. To this end, the full three-dimensional Schrodinger equation for the hydrogen atom is solved numerically to obtain the single-atom response. Phase-matching effects are subsequently taken into account in the limit of small atomic density. Our findings clearly indicate the possibility of creating isolated attosecond pulses from many-cycle (Delta tau(FWHM) = 19 fs) driver pulses by an appropriate tuning of the two laser frequencies, which results in control of the ionization and the recollision dynamics of the electronic wavepacket.

Plateau high harmonics are shown to be chirped. The experimental values of the chirp, in good agreement with purely classical kinematics or strong field approximation quantum theory are positive and inversely proportional to intensity. The various steps for the optimization of the attosecond pulse train are discussed.

The exposure of molecules to short, ultrastrong electric fields leads to multiple ionization and a subsequent Coulomb explosion. We present a comparative study where uracil molecules are exposed to fields generated by high-power laser pulses (tau approximately 75 fs, I > 10(16) W/cm2) or swift highly charged ions (0.5 MeV Xe25+) representing a half-cycle pulse of less than 10 fs duration. Molecular dynamics and structural information contained in the fragmentation pathways can be assessed separately. Despite the similar field strengths large differences in fragment kinetic energies are found which are related to field shape and duration with the aid of molecular dynamics simulations.