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Applications of High Field and Short Wavelength Sources (HFSW)

NEW TYPE OF AC-STARK EFFECT MANIFESTATION AT MULTIPHOTON IONIZATION OF Sr AND Ba ATCHMS

І. І. Бондар, В. В. Суран, M. I. Dudich

Published Jan 1, 1999• 0 citations
Publisher

Abstract

The experimental investigation of influence of perturbation on multiphoton ionization of Sr and Ba atoms using tunable IR radiation of laser on colour centres (ω = 8200-9100 cm−1 ) was performed. The investigations were performed at field strength e-106÷107 V/cm. The laser pulse duration was τ= 4 • 10−8 s.
DOI
10.1364/hfsw.1999.mc4

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We studied experimentally the two-photon excitation of 5d7s/sup 3/D/sub 2/ state of Ba atom strongly perturbed in result of ac-Stark effect realisation. The perturbation of this state was fulfilled by influence of YAG-laser radiation. For excitation of this state we used the tunable dye-laser radiation. The investigations were performed by the resonant ionization spectroscopy method. We measured the Ba/sup +/ ions yield caused by the ionization of Ba atoms through the two-photon resonance with perturbed 5d7s/sup 3/D/sub 2/ state in cases when the electric vectors of both used radiations were parallel and orthogonal.
The influence of the Stark effect on multiphoton ionization of Ba atoms under conditions when the dynamic polarizability depends strongly on the frequency of the laser radiation is investigated. It is found that for some electric field strengths ε of the laser radiation this effect gives rise to resonance peaks in the Ba + ion yield as a function of the laser radiation frequency at frequencies corresponding to single-photon transitions between the excited states. These frequencies can differ substantially from the frequencies corresponding to the conventional multiphoton excitation of these states from the ground states of the atoms. Peaks in the ion yield as a function of ε behave differently from the conventional Stark effect—their position on the frequency scale does not depend strongly on ε . The conditions under which such an induced resonance structure appears are determined.
A model of multiple resonances between groups of discrete and autoionizing levels is proposed for the description of the process of multiphoton ionization. The phenomena of the extremely rapid broadening and growth of amplitudes of the multiphoton resonances, observed experimentally in the yield of Sr+ and Ba+ ions [ J. Phys. B.14, 2253– 2359 ( 1981);Elementary Processes in Laser Radiation Field ( 1988), pp. 122– 153)] are explained on the basis of this model. The strong-coupling method was used in numerical calculations for Sr and Ba atoms in a strong laser field.
The resonant multiphoton resonances shifts and widths and ionization cross-sections for multielectron atoms in a intense laser radiation field are studied. It is carried out a new consistent approach to atom in a strong realistic laser field, based on the relativistic energy formalism (S-matrix adiabatic formalism), relativistic Dirac equation Green function method and relativistic many-body perturbation theory with the Dirac-Kohn-Sham zeroth approximation. In relativistic theory, the Gell-Mann and Low adiabatic formula for energy shift is connected with electrodynamical scattering matrice, which includes interaction with as a laser field as a photon vacuum field (radiative decay). The optimized basis of the relativistic orbitals is generated with using a minimization procedure for the gauge-non-invariant contribution (the fourth-order of the QED perturbation theory) to the radiation width of atomic state. An approach to treating the multiphoton atomic processes is outlined on example of H, Cs, Kr,Mg etc. Analysis shows that the shift and width of the multi-photon resonance line for the interaction “atom- multimode laser pulse” is greater than the corresponding shift and width for a case of the “atom- single-mode pulse” (the Lorenz pulse model) interaction. From the physical point of view it is naturally provided by action of the photon-correlation effects and influence of the laser pulse multi-modity.