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Narrowing of autoionizing resonances in a laser-radiation field

Yu. I. Geller, A. K. Popov

Published Jan 1, 1985• 0 citations
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Abstract

This paper discusses narrowing of autoionizing resonances in photoabsorption in a laser-radiation field, and splitting autoionizing resonances in a strong radiation field. Narrowing of autoionizing resonances in photoabsorption is examined. General expressions are explained. Nonlinear susceptibility in the presence of several close-lying autoionizing levels is discussed. A review of the capabilities of nonlinear spectroscopy of autoionizing states is given.

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Soviet Journal of Quantum Electronics•Aug 31, 1983•4 citations
This paper discusses narrowing of autoionizing resonances in photoabsorption in a laser-radiation field, and splitting autoionizing resonances in a strong radiation field. Narrowing of autoionizing resonances in photoabsorption is examined. General expressions are explained. Nonlinear susceptibility in the presence of several close-lying autoionizing levels is discussed. A review of the capabilities of nonlinear spectroscopy of autoionizing states is given.
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.
We discuss the possibility of narrowing the atomic autoionization states via their resonance mixing in a field. The results show that, in contrast to the interference process [Yu. I. Geller and A. K. Popov, Laser Stimulation of Nonlinear Resonances in Continuous Spectra (Nauka, Novosibirsk, 1981)] or to the mixing of two isolated states [A. I. Andryushin et al., Sov. Phys.---JETP 61, 678 (1985)], with mixing of multiplets one may expect substantial narrowing of autoionization states owing to their intersection with bound-electron states.
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.
We study theoretically the photoabsorption spectra of an attosecond extreme ultraviolet (XUV) pulse by a laser-dressed atomic system. A weak XUV excites an autoionizing state which is strongly coupled to another autoionizing state by a laser. The theory was applied to explain two recent experiments [Loh, Greene, and Leone, Chem. Phys. 350, 7 (2008); Wang, Chini, Chen, Zhang, Cheng, He, Cheng, Wu, Thumm, and Chang, Phys. Rev. Lett. 105, 143002 (2010)] where the absorption spectra of the XUV lights were measured against the time delay between the laser and the XUV. In another example, we study an attosecond pulse exciting the 2s2p(1P)2s2p{(}^{1}\phantom{\rule{-0.16em}{0ex}}P)2s2p(1P) resonance of helium which is resonantly coupled to the 2s2(1S)2{s}^{2}{(}^{1}\phantom{\rule{-0.16em}{0ex}}S)2s2(1S) resonance by a moderately intense 540-nm laser. The relation between the photoabsorption spectra and the photoelectron spectra and the modification of the transmitted lights in such an experiment are analyzed. The role of Rabi flopping between the two autoionizing states within their lifetimes is investigated with respect to the laser intensity and detuning.
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.