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Applications of High Field and Short Wavelength Sources (HFSW)

Laser-Assisted Multicolor Photoionization of Atoms with Higher Harmonics

A. Maquet, Richard Taïeb, Valérie Vëniard

Published Jan 1, 1997• 1 citation
Publisher

Abstract

We discuss, via a numerical calculation, the main properties of atomic photoelectron spectra, as they would be obtained by using a radiation pulse containing higher harmonics together with the laser which has been used to generate them. We address more precisely the physically relevant situation in which the harmonics have much weaker intensities than the one of the laser. In such multicolor photoionization processes, the atom can simultaneously absorb harmonic UV photons and exchange, i.e. absorb and/or emit (via stimulated emission) laser IR photons. An interesting outcome of our study is to show that such new effects should be observable with currently developed harmonic sources.

DOI
10.1364/hfsw.1997.fd3

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We discuss, via a numerical calculation, the main properties of atomic photoelectron spectra, as they would be obtained by using a radiation pulse containing higher harmonics together with the laser which has been used to generate them. We address more precisely the physically relevant situation in which the harmonics have much weaker intensities than the one of the laser. In such multicolor photoionization processes, the atom can simultaneously absorb harmonic UV photons and exchange, i.e. absorb and/or emit (via stimulated emission) laser IR photons. An interesting outcome of our study is to show that such new effects should be observable with currently developed harmonic sources.
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.
We consider harmonics generation and wave mixing by two-color multiphoton resonant excitation of three-level atoms and molecules in strong laser fields. The coherent part of the spectra corresponding to multicolor harmonics generation is investigated. The obtained analytical results on the basis of a generalized rotating wave approximation are in a good agreement with numerical calculations. The results applied to the hydrogen atoms and homonuclear diatomic molecular ions show that one can achieve efficient generation of moderately high multicolor harmonics via multiphoton resonant excitation by appropriate laser pulses.
We theoretically study the process of photoelectron emission from the helium atom using a high-intensity short-wavelength laser at a resonance condition of the residual singly charged ion. Photoionization followed by strong resonance coupling in the ion leads to a change in the photoelectron spectrum due to Rabi oscillations in the residual ion. Similarly to resonance fluorescence at high laser intensity, the photoelectron spectrum at high intensities evolves into a multipeaked structure. The number of peaks in the photoelectron spectrum is related to the number of Rabi cycles following the photoionization process. Moreover, the strong laser-induced coupling to nonresonant states of the residual ion has an imprint on the photoelectron spectrum, leading to additional, isolated peaks at the lower- or higher-energy sides. The effect should be observable at current seeded extreme ultraviolet (XUV) free-electron lasers and persists after volume integration in a realistic experimental geometry.