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Journal of Physics Conference Series

Spectroscopy of laser-induced autoionizing nonlinear resonances in atomic systems

Lubomir I. Pavlov, V V Kovachev

Published Mar 18, 2015• 0 citations

Abstract

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.

DOI: 10.1088/1742-6596/594/1/012035

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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.

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.

The generation of tunable, nanosecond-pulsed extreme-ultraviolet (EUV) light

(84.92\mathrm{nm}

to

84.97\mathrm{nm}

$\ensuremath{\sim}14.6\mathrm{eV})$ using four-wave mixing in optically deep krypton gas was investigated experimentally and theoretically. The sum-frequency mixing scheme generated light from two overlapping autoionizing resonances, ${17s}^{\ensuremath{'}}$ and ${15d}^{\ensuremath{'}}$. We investigated whether the laser-induced atomic coherence effect of electromagnetically induced transparency (EIT) could enhance the intensity of the EUV light generated. The maximum EUV output was seen from the region where the ${15d}^{\ensuremath{'}}$ autoionizing resonance has a transparency window due to Fano-type quantum interference. An analytical model using appropriate atomic parameters explains how background absorption can prevent EIT enhancement of EUV generation; and shows how a background Raman-like coupling term in the nonlinear susceptibility can well describe the measured EUV generation spectrum.

In recent experiments VUV radiation of high spectral brightness has been generated at around 155 nm by two-photon resonant four-wave mixing (difference-frequency generation) in xenon using narrow-bandwidth lasers in the visible and ultraviolet.1 Efficiencies of 0.25% have been achieved, but it was found that the process was heavily saturated. A major saturating process is believed to be single-photon ionization from the two-photon resonant state, 6p J = 0 (Fig. WKK4-1a). Further experiments to investigate the three-photon ionization are described. These experiments involved a detailed study of the ionization process.

The ability to introduce additional resonances in the nonlinear optical susceptibility of a medium by the injection of an additional radiation field is an attractive possibility from the standpoint of efficient tunable frequency mixing processes in the VUV. The possibility of such processes suggest themselves in previous observations in atomic mercury where an enhancement of several orders of magnitude in the third harmonic generation efficiency has been reported when the input frequency is four-photon resonant with the 6S → 6D transition(1).