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Production of narrow nonlinear autoionizing-like resonances in a continuum by strong laser radiation

Yu. I. Geller, A. K. Popov

Published Jan 1, 1985• 0 citations
OpenAlex

Abstract

This paper investigates nonlinear processes in transitions to the continuum, and their manifestations in the photoabsorption, polarization, and angular distribution of the electrons, as well as in the nonlinear susceptibility in the case of frequency gases. Change of photoabsorption line shape by strong laser radiation upon transition to the continuum is examined. Conditions for the appearance of an induced autoionizing-like resonance in absorption are described, and allowance for the energy dependence of the matrix elements of the transitions to the continuum are examined. A discussion of narrow nonlinear resonances in ionization and recombination is presented. Induction of nonlinear resonances in a continuum when strong/radiation interacts with several close-lying discrete states is described. A presentation is given on the use of laser to induce nonlinear autoionizinglike resonances in a continuum to generate coherent radiation on the basis of frequency mixing in gases. Doppler broadening of nonlinear resonance is discussed. Phase modulation, frequency modulation and amplitude-phase modulation are outlined.

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This paper investigates nonlinear processes in transitions to the continuum, and their manifestations in the photoabsorption, polarization, and angular distribution of the electrons, as well as in the nonlinear susceptibility in the case of frequency gases. Change of photoabsorption line shape by strong laser radiation upon transition to the continuum is examined. Conditions for the appearance of an induced autoionizing-like resonance in absorption are described, and allowance for the energy dependence of the matrix elements of the transitions to the continuum are examined. A discussion of narrow nonlinear resonances in ionization and recombination is presented. Induction of nonlinear resonances in a continuum when strong/radiation interacts with several close-lying discrete states is described. A presentation is given on the use of laser to induce nonlinear autoionizinglike resonances in a continuum to generate coherent radiation on the basis of frequency mixing in gases. Doppler broadening of nonlinear resonance is discussed. Phase modulation, frequency modulation and amplitude-phase modulation are outlined.
Efficient harmonic generation at laser-induced autoionizing state in continuum, were observed for the first time by our laboratory [1,2,3]. In this paper, nonlinear frequency mixing in alkali metal vapors at induced continuum structure in Na, are studied in detail. Besides, measurement of nonlinear optical susceptibility at induced autoionizing resonance, is performed. Tunable ultraviolet radiations by four-photon processes near induced resonances are obtained. Estimations of the real and imaginary parts of the nonlinearities, are estimated. We will specially note, that the resonant photoabsorption of ions is experimentally investigated since it gives information about the autoionizing states, which is impossible to obtain on the base only of the emission spectra. We use also laser inducing for obtaining of powerful radiation source in VUV. Thus, many actual problems as the selective impact on matter, laser photochemistry and laser isotope separation are connected directly with the induced autoionizing nonlinear resonances. The main interest is to control the spectral characteristics of continuum. The discrete level from the continuum structure leads to appearance of asymmetrical autoionizing resonance onto the background of the broad-band absorption line.
Absorption properties of a system consisting of two autoionizing states coupled strongly either by internal interactions or by external fields are investigated. The autoionizing states may themselves correspond to those induced by laser fields as in resonant multiphoton ionization. The analysis is based on the density matrices and incoherent processes like spontaneous emission and radiative recombination are included. Optical susceptibilities exist in a number of special cases for autoionizing transitions. Exact results for such optical susceptibilities are obtained. Explicit numerical results for susceptibilities, corresponding traditionally to strong-field inverse Raman effect, are given with a view to recent work on dc-field-coupled autoionizing states and on laser-induced autoionization. The behavior of both absorptive and dispersive parts is discussed for a range of parameters.
A study was made of the influence of autoionization-like resonances in the continuum, induced by strong optical radiation, on the nonlinear susceptibilities governing third harmonic generation and the optical activity of sodium vapor. It was demonstrated experimentally for the first time that it should be possible to study high-lying states of atoms and molecules by the method of nonlinear polarization spectroscopy of the fifth-order optical nonlinearity. The experimental results were compared with the theory. An analysis was made of the feasibility of using this effect for spectroscopy of the states in the continuum and for generation of short-wavelength radiation in gaseous media.
Experimental investigations of the laser-induced continuum structure in the vicinity of autoionizing states (AIS's) of Ca are reported for both ionization and third-harmonic generation. The shape of the induced structures is shown to be strongly affected by the presence of the AIS's, as well as exhibiting a detuning- and laser-intensity-dependent transition from a window resonance dip to a peak. The observed modification of the continuum is discussed in terms of interfering excitation channels.
The review systematizes the experimental material and reports the fundamental theoretical results on the study of nonlinear resonance effects in spectra of atomic systems situated in the radiation field of lasers. The theoretical part explains the topics of greatest importance and significance for understanding of the experimental studies: the Stark effect in an optical field; the nonequilibrium velocity distribution of the atoms; the shape of the spectral line for a transition adjacent to a transition combining with a strong field; the simultaneous manifestation of the Stark effect and of Doppler broadening in the form of a very sharp structure on the line contour; a brief mention of the influence of collisions, recoil, and other factors on the sharp structure in spectra. The experimental part reports the results of experiments on the observation of the Stark effect in the optical region of the spectrum, the fine structure of the profile of a Doppler-broadened line and its properties, namely the dependence on the observation direction, the anisotropy in the field of a standing wave, the polarization properties, etc. in spectra of spontaneous or stimulated emission, absorption, and generation.