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Physical Review A

Target dressing effects in laser-assisted x-ray photoionization

Aurelia Cionga, Viorica Florescu, A. Maquet, Richard Taïeb

Published Mar 1, 1993• 55 citations
Publisher

Abstract

X-ray photoionization of hydrogen in the presence of a low-frequency laser field is studied using an approach which partially takes into account the radiative stimulated corrections to both bound and continuum states. Special attention is paid to the cases in which one low-frequency photon is exchanged (absorbed or emitted) between the atomic system and the laser field. It is demonstrated that atomic ``dressing'' effects are important close to the ionization threshold.

DOI
10.1103/physreva.47.1830

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X-ray photoionization of hydrogen in the presence of a low-frequency laser field is studied using an approach which partially takes into account the radiative stimulated corrections to both bound and continuum states. Special attention is paid to the cases in which one low-frequency photon is exchanged (absorbed or emitted) between the atomic system and the laser field. It is demonstrated that atomic ``dressing'' effects are important close to the ionization threshold.
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We calculate the two-color ionization profile of atomic hydrogen in the presence of both a weak tunable high-frequency laser field and a low-frequency laser field of moderate intensity. The high-frequency spectroscopic light is used to probe the structure of Rydberg states dressed by the low-frequency field. We construct the photoionization spectrum from the sum of the resonance contributions by calculating the transition dipole moments between the initial state and the dressed Rydberg states, analogous to the study of photoionization in the presence of a dc field carried out by Alvarez and Silverstone [Phys. Rev. Lett. 63, 1364 (1989)].
Photoelectric effect in atomic hydrogen in the presence of bichromatic laser fields composed of a fundamental frequency and its second or third harmonics is investigated. The expression of the S-matrix for the laser-assisted photoelectric effect is derived in the length gauge, which can simplify the elements of the matrix. A general dressed wavefunction of the atom is derived in perturbation theory. The continuum state of the ejected electron is described by a Coulomb–Volkov wavefunction. Numerical results of photoionization cross section for the distinct second component are obtained and compared with the monochromatic field dressing case. It is shown that the photoionization cross section is modified notably by introducing the second component of field, especially when the colliding system absorbs or emits a considerable number of laser photons.
The strong-field multiphoton ionization of atoms is considered and a theoretical approach dealing nonperturbatively with the radiation field formulated. The general computational scheme is the conventional perturbation theory, but the intermediate states are dressed by the field. We present in detail a method to dress the continuum states and to study the dipole transitions within the continuum. In the high-frequency domain, the proposed procedure rapidly converges over a wide range of field intensity and offers an interesting framework for calculating ionization rates for arbitrary numbers of absorbed (above-threshold) photons and field polarization.
An approximate non-perturbative theory of multiphoton ionization of a hydrogen atom by two strong radiation fields is outlined. The special case is then considered for when one of the fields is of low intensity and high frequency and the other field is of high intensity and low frequency. A set of representative calculations has been performed for this case; that is, (a) angular distributions for given numbers of absorbed and emitted low-frequency photons and different geometries; (b) total cross-sections versus the strong-field intensity; (c) angular distributions summed over all the low-frequency photon exchanges, showing a fourfold symmetry in a geometry in which each ionization channel exhibits, instead, only a twofold symmetry; (d) the average energy gained by the ejected electrons. Calculations are performed for a strong low-frequency field of arbitrary polarization. In many respects, the present calculations are expected to complement those of single-field multiphoton ionization. It is hoped that the results of the calculations reported here may prompt systematic measurements also in this new class of processes, as the predictions are well within the state of the art of present multiphoton-ionization experiments.
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.