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Physical Review A

Theory of two-color pump-probe determination of ionization dynamics

Richard Taïeb, Valérie Vëniard, A. Maquet

Published Jun 1, 1998• 6 citations
Publisher

Abstract

We show, through numerical simulations, that it is, in principle, possible to follow in time the structure and the population dynamics of an atomic system ``dressed'' by a strong time-varying laser field. The proposed time-resolved spectroscopy scheme exploits the properties of the currently developed sources of coherent extreme ultraviolet radiations, generated via high-order harmonic conversion. With pulse durations within the femtosecond range, it is possible to conceive two-color pump-probe schemes that would help to shed light on issues currently under discussion about the dynamics of strong-field photoionization.

DOI
10.1103/physreva.57.r4098

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We show, through numerical simulations, that it is, in principle, possible to follow in time the structure and the population dynamics of an atomic system ``dressed'' by a strong time-varying laser field. The proposed time-resolved spectroscopy scheme exploits the properties of the currently developed sources of coherent extreme ultraviolet radiations, generated via high-order harmonic conversion. With pulse durations within the femtosecond range, it is possible to conceive two-color pump-probe schemes that would help to shed light on issues currently under discussion about the dynamics of strong-field photoionization.
We present a theoretical study of the main characteristics of two-color photoionization spectra obtained when atoms are subjected to an intense radiation pulse containing the fundamental of an infrared laser and one of its higher harmonics. The frequency of the latter is chosen to be high enough that the atom can be ionized by a single photon. The calculation is performed by numerical integration of the time-dependent Schrödinger equation for a (three-dimensional) hydrogen atom. We present the general properties of the spectra and angular distributions as well as the possible occurrence of interferences between the laser-assisted process and above-threshold ionization. Our results indicate that such interference effects, which can lead to partial coherent control of the photoionization process, can be observed at moderate laser intensities within the reach of currently operated laser and harmonic sources.
Two-photon ionization of atomic helium has been measured by combining femtosecond extreme-ultraviolet pulses from the free-electron laser in Hamburg (FLASH at DESY) with intense light pulses from a synchronized neodymium-doped yttrium lithium fluoride laser. Sidebands appear in the photoelectron spectra when the two laser pulses overlap in both space and time. Their intensity exhibits a characteristic dependence on the relative time delay between the ionizing and the dressing pulses and provides an inherent time marker for time-resolved pump-probe experiments. The measurements of the sidebands are in good agreement with theoretical predictions and allow for a direct analysis of two-photon ionization, free from processes related to interference between multiple quantum paths.
Using high-order harmonic attosecond pulse trains, we investigate the photoionization dynamics and transient electronic structure of a helium atom in the presence of moderately strong (∼10(12) W cm(-2)) femtosecond laser pulses. We observe quantum interferences between photoexcitation paths from the ground state to different laser-dressed Floquet state components. As the intensity ramps on femtosecond time scales, we observe switching between ionization channels mediated by different atomic resonances. Using precision measurements of ion yields and photoelectron distributions, the quantum phase difference between interfering paths is extracted for each ionization channel and compared with simulations. Our results elucidate photoionization mechanisms in strong fields and open the doors for photoabsorption or photoionization control schemes.
Strong-field coherent control deals with the efficient excitation of a quantum system into a preselected final state. In order to understand the underlying control mechanisms, the transient dynamics of the laser-dressed states need to be considered in addition. In this paper, we present a route towards a complete picture of non-perturbative coherent control. To this end, we study near-resonant chirped excitation of potassium atoms as a model system for resonant strong-field control. Combining a two-colour pump–probe scheme with photoelectron spectroscopy, we simultaneously observe final state and transient excitation dynamics in the strongly driven atom. As demonstrated on the prototype scenario, the scheme enables a detailed understanding of the physical mechanisms governing the interaction. Our results highlight the power of two-colour time-resolved photoelectron spectroscopy to shed light onto the different aspects of non-perturbative coherent control.
We study the nonlinear and resonant process of two-photon ionization of atoms (He and H) in a pump-probe scheme. The pump pulse prepares the quantum system in a superposition of the ground state and an excited bound state. By varying the phase difference between the pulses, we show how it is possible to coherently control the dressed-state population during the probe pulse. Our main result is that for certain laser parameters, the control over the dressed-state population leads to strong control of the ionization probability during the probe pulse. The effect arises due to one of the dressed states becoming stabilized against ionization. Contrasting effects from circular and linear polarized pulses demonstrate how such “bound states in the continuum” are sensitive to the degeneracy of the coupled continuum.
Metastable states are important actors in the ionisation of atoms and molecules. Sub-femtosecond extreme ultraviolet pulses can coherently populate several transiently bound states at once, thus starting the attosecond clocks which are required to monitor and control ultrafast electronic evolution above the ionisation threshold. Here we illustrate, from a theoretical point of view, the effects coherent superpositions of 1 P o doubly excited states in the helium atom have on channel-resolved photoelectron spectra as well as on the transient absorption spectrum of the atom in the extreme ultraviolet region, when they are created by a single-attosecond pulse in the presence of a strong few-cycle near-infrared/visible pulse which acts as a probe. Interference fringes varying rapidly with the pump-probe time delay are visible in both photoelectron and transient absorption spectra. From such fringes, the wave packet itself can conceivably be reconstructed. Conversely, all observables are modulated by the characteristic beating periods of the wave packet, so that control of partial ionisation yields, branching ratios, and light absorption or amplification can be achieved.