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arXiv:1709.02978

Probability of Spontaneous Emission in Processes of the Above Threshold Ionization of Atoms

K. V. Ivanyan

Published Sep 9, 2017• 0 citations
arXivHTMLPDF

Abstract

Expression for the probability of the spontaneous emission of high-order harmonics when the emitter phases are synchronized and are obtained in the region where the multiphoton approximation is applicable to the description of the ionization of an atom (where the adiabaticity parameter satisfies gamma > 1). The dependence of this probability on the main parameters of the pump wave and the atomic medium is established. Criteria for observing emission are formulated with the consideration of phase locking.

DOI
10.48550/arxiv.1709.02978

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Expression for the probability of the spontaneous emission of high-order harmonics when the emitter phases are synchronized and are obtained in the region where the multiphoton approximation is applicable to the description of the ionization of an atom (where the adiabaticity parameter satisfies gamma > 1). The dependence of this probability on the main parameters of the pump wave and the atomic medium is established. Criteria for observing emission are formulated with the consideration of phase locking.
Expression for the probability of induced emission of high-order harmonics is obtained in the region where the multiphoton approximation is applicable to the description of the ionization of an atom. The dependence of this probability on the main parameters of the pump wave and the atomic medium is established. Criteria for observing emission are formulated with the consideration of phase locking. The possibility of amplifying a UV probe wave aimed through the region where the atoms interact with the pump wave is considered.
For phase-locked emitters, provided a certain relation exists between the pump wave and atomic beam parameters, a saturation effect is shown to be possible for which the intensity of the high-order harmonics ceases to depend on the atomic density. By means of a simple model that includes variations in the intensity of the pump wave in the plane transverse to the focal axis, an expression is obtained for the optimum atomic density of the medium corresponding to intensity saturation. The dependence of the optimum atomic density on the laser power and harmonic number obtained is found to be in qualitative agreement with recently published experimental data.
The review is devoted to new effects discovered recently in the course of studying the process of nonlinear ionization of atoms in the field of laser radiation—above-threshold ionization, manifestation of multielectron structure of complex atoms, formation of multiply charged ions, tunneling ionization, and emission of short-wavelength radiation. The main experimental data and their theoretical interpretation are presented. A discussion is given of the criteria for the applicability of the lowest non-vanishing order of non-steady-state perturbation theory to the description of the interaction of an atom with the electromagnetic field.
Continuum–continuum, bound–bound, bound–free, and free–bound atomic field-induced transitions are investigated. The initial atomic state is supposed to be given by a wave packet of coherently populated levels of continuous or discrete spectra. Interference effects were found to increase the probabilities of these types of transition and to change their dependence on the pulse duration τ. Interference is shown to occur also in the two-photon ionization probability with highly excited Rydberg levels as intermediate states. Criteria of applicability of the energy diffusion model are formulated. The different diffusion criteria are shown to be incompatible with each other, owing particularly to described interference effects. An alternative picture of multiphoton excitation is suggested. This picture is similar to the above-threshold ionization and predicts the appearance of some maxima in the spectrum of excited electrons below the ionization threshold.
A quantum-mechanical formulation for the generation of third-harmonic radiation in an atomic medium undergoing three-photon resonant multiphoton ionization is given. The generation of the third harmonic is treated as a cooperative emission and its pressure dependence is shown to be in agreement with experiments. The formulation is general enough to include both transient and steady-state phenomena and includes both traveling-wave and standing-wave cases. A comparison of the results of the present theory with those of Jackson and Wynne is given.