Alfred Maquet

Probing Single-Photon Ionization on the Attosecond Time Scale

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.

Theory of attosecond delays in laser-assisted photoionization

We study the temporal aspects of laser-assisted extreme ultraviolet (XUV) photoionization using attosecond pulses of harmonic radiation. The aim of this paper is to establish the general form of th ...

Attosecond dynamics through a Fano resonance: Monitoring the birth of a photoelectron

Watching as helium goes topsy-turvy Theorists have long pondered the underpinnings of the Fano resonance, a spectral feature that resembles adjacent rightside-up and upside-down peaks. An especially well-studied instance of this feature appears in the electronic spectrum of helium as a transient state undergoes delayed ionization. Two studies have now traced the dynamics of this state in real time. Gruson et al. used photoelectron spectroscopy to extract the amplitude and phase of the electron wave packet after inducing its interference with reference wave packets tuned into resonance at variable delays. Kaldun et al. used extreme ultraviolet absorption spectroscopy to probe the transient state while variably forcing ionization with a strong near-infrared field. Science, this issue pp. 734 and 738 Ultrafast spectroscopy traces the dynamics of a transient excited state in helium underlying appearance of a Fano resonance. The dynamics of quantum systems are encoded in the amplitude and phase of wave packets. However, the rapidity of electron dynamics on the attosecond scale has precluded the complete characterization of electron wave packets in the time domain. Using spectrally resolved electron interferometry, we were able to measure the amplitude and phase of a photoelectron wave packet created through a Fano autoionizing resonance in helium. In our setup, replicas obtained by two-photon transitions interfere with reference wave packets that are formed through smooth continua, allowing the full temporal reconstruction, purely from experimental data, of the resonant wave packet released in the continuum. In turn, this resolves the buildup of the autoionizing resonance on an attosecond time scale. Our results, in excellent agreement with ab initio time-dependent calculations, raise prospects for detailed investigations of ultrafast photoemission dynamics governed by electron correlation, as well as coherent control over structured electron wave packets.

Two-colour IR+XUV spectroscopies: the “soft-photon approximation”

We address several questions related to the nonlinear ionization processes observed when atoms are in the simultaneous presence of intense and coherent infrared (IR) and extreme ultraviolet (XUV) laser pulses. This topic is of much interest in the context of the current development of new XUV and soft-X-ray coherent sources, either from high-order harmonics or from X-ray free-electron laser (XFEL) devices that can be synchronized with IR lasers. The theoretical description of this class of two- (more generally multi-) colour ionization processes is challenging for theory. Here, we discuss the advantages and limitations of the so-called “soft-photon approximation”, which, we believe, provides most useful insights in the analysis of these processes.

Spectral phase measurement of a Fano resonance using tunable attosecond pulses.

Electron dynamics induced by resonant absorption of light is of fundamental importance in nature and has been the subject of countless studies in many scientific areas. Above the ionization threshold of atomic or molecular systems, the presence of discrete states leads to autoionization, which is an interference between two quantum paths: direct ionization and excitation of the discrete state coupled to the continuum. Traditionally studied with synchrotron radiation, the probability for autoionization exhibits a universal Fano intensity profile as a function of excitation energy. However, without additional phase information, the full temporal dynamics cannot be recovered. Here we use tunable attosecond pulses combined with weak infrared radiation in an interferometric setup to measure not only the intensity but also the phase variation of the photoionization amplitude across an autoionization resonance in argon. The phase variation can be used as a fingerprint of the interactions between the discrete state and the ionization continua, indicating a new route towards monitoring electron correlations in time.

Spectrally resolved multi-channel contributions to the harmonic emission in N 2

When generated in molecules, high-order harmonics can be emitted through different ionization channels. The coherent and ultrafast electron dynamics occurring in the ion during the generation process is directly imprinted in the harmonic signal, i.e. in its amplitude and spectral phase. In aligned N2 molecules, we find evidence for a fast variation of this phase as a function of the harmonic order when varying the driving laser intensity. Basing our analysis on a three-step model, we find that this phase variation is a signature of transitions from a single- to a multi-channel regime. In particular, we show that significant nuclear dynamics may occur in the ionization channels on the attosecond timescale, affecting both the amplitude and the phase of the harmonic signal.

Attosecond delays in photoionization: time and quantum mechanics

This article addresses topics regarding time measurements performed on quantum systems. The motivation is linked to the advent of ?attophysics? which makes feasible to follow the motion of electrons in atoms and molecules, with time resolution at the attosecond (1 as = 10?18 s) level, i.e. at the natural scale for electronic processes in these systems. In this context, attosecond ?time-delays? have been recently measured in experiments on photoionization and the question arises if such advances could cast a new light on the still active discussion on the status of the time variable in quantum mechanics. One issue still debatable is how to decide whether one can define a quantum time operator with eigenvalues associated to measurable ?time-delays?, or time is a parameter, as it is implicit in the Newtonian classical mechanics. One objective of this paper is to investigate if the recent attophysics-based measurements could shed light on this parameter?operator conundrum. To this end, we present here the main features of the theory background, followed by an analysis of the experimental schemes that have been used to evidence attosecond ?time-delays? in photoionization. Our conclusion is that these results reinforce the view that time is a parameter which cannot be defined without reference to classical mechanics.

Atoms in strong laser fields: challenges in relativistic quantum mechanics

We address the challenges raised by the question of how to produce quantitative data in order to reproduce and to interpret the results of recent and future experiments performed with ultra-intense laser pulses. The main challenges lie in the problem of implementing reliable numerical codes for describing quantum processes experienced by electrons brought in the relativistic regime in the presence of the field.

Hyper-Raman lines produced during high harmonic generation

Abstract The possibility of producing and detecting hyper-Raman lines in high-harmonic generation with short and strong laser pulses is discussed. It is shown that the strength and positions of the hyper-Raman lines depend very sensitively on the parameters of the laser pulse and the conditions which are required for an effective production are indicated. It is pointed out that the phenomenon of atomic stabilization can contribute to obtaining these conditions.

Elliptic dichroism and angular distribution of electrons in two-photon ionization of atoms

The three-dimensional angular distribution of electrons in two-photon ionization of atoms by an elliptically polarized laser beam, from an initial state with arbitrary angular momentum Ji, is expressed in terms of six frequency-dependent invariant atomic parameters. The specific effect of the field polarization on the angular distribution (the elliptic dichroism, ED) is discussed for the general case of elliptic polarization. The necessary conditions for a non-zero ED are established and its physical origin is explained. The general theory is illustrated by a simple analytical calculation for the -potential model. Numerical calculations of the angular distribution parameters for H and Cs atoms are also performed and the dependence of ED magnitude on the atomic quantum numbers and the photon frequency is discussed for different frequencies.