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Scientific Herald of Uzhhorod University Series Physics

ROTATION OF POLARIZATION PLANE OF PROBE FIELD BY LASER-INDUCED STRUCTURE IN HELIUM ATOM CONTINUUM

Грызлова Елена Владимировна, A N Grum-Grzhimailo, С. И. Страхова

Published Aug 5, 2007• 2 citations
Publisher

Abstract

The ensemble of helium atoms in two electromagnetic fields has been investigated. The laser field coupling results in interference between two helium autoionizing states (AIS). The probe field scans the region of AIS and leads to photoionization. Rotating wave approximation was used to obtain the density matrix equation de­scribing the process. Nonlinear susceptibility of the atomic ensemble at the probe field frequency was found. Nonlinear effects such as polarization plane rotation and acquired ellipticity are calculated.

DOI
10.24144/2415-8038.2007.21.109-113

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The ensemble of helium atoms in two electromagnetic fields has been investigated. The laser field coupling results in interference between two helium autoionizing states (AIS). The probe field scans the region of AIS and leads to photoionization. Rotating wave approximation was used to obtain the density matrix equation de­scribing the process. Nonlinear susceptibility of the atomic ensemble at the probe field frequency was found. Nonlinear effects such as polarization plane rotation and acquired ellipticity are calculated.
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We derive a parametrized formula for the photoionization cross section in the vicinity of an autoionizing state (AIS) coupled with another AIS by a strong laser field. The rotating-wave approximation is used and adiabaticity is assumed. In the probe field spectrum two Fano resonances contribute to the cross section, while a single resonance occurs in the strong-field spectrum. The correlation between the resonance shape and the complex energies of the laser-driven two-level system is described. Calculations are performed for the and AIS pair in He and for the and pair in Mg. The results agree with those obtained using time-dependent approaches. The formula for the cross section describes the spectral features of the ion yield measured in the two-photon ionization of the Mg atom via the AIS coupled with the state. The appearance of laser-induced degenerate states in the cross section shape is studied.
Process of photon scattering on atoms is considered at energy above the ionization threshold. Influence of the resonant structure in the continuum, induced by a laser field due to coupling with the level 41 S in helium, on the cross sections of elastic and inelastic scattering of a probe radiation on atom in the meta-stable state 21 S is theoretically investigated. In the rotating wave approximation approach dependences of resonant scattering cross sections on the probe and laser radiation characteristics are obtained. The frequency and polarization dependences of the scattering cross sections to the final states of atom 11 S, 21 S and 41 S are calculated.
We present the experimental and theoretical investigation of the effects of the strong electromagnetic coupling of two autoionizing states (AIS's) on the photoionization properties of an atomic system. These effects are observed by probing the ladder system from the ground state of the atom through a weak two-photon transition to the region of the lower AIS. The experimental observation of the modification of the total ionization and partial photoelectron spectra at different wavelengths of the coupling and probe lasers are accurately reproduced by a theoretical analysis which uses a density-matrix approach. Both theory and experiment indicate a strong inhibition of the ionization of the system in the case when both lasers are resonant with the two atomic transitions, while displaying a rapidly varying asymmetry induced as the detuning between the two states and the fields increases. In the extreme case of a large detuning, two separate structures appear as distinct total ionization and photoelectron peaks. The above findings combined with the full theoretical treatment indicate that the phenomena observed are an experimental demonstration of ac Stark splitting between two AIS's. \textcopyright{} 1996 The American Physical Society.
The effect of the resonant structure in the continuous spectrum induced by an external laser field due to the coupling of continuum states with the 1s4s 1 S level in helium on cross sections of elastic and inelastic scattering of probe radiation on an atom in the metastable 1s2s 1 S state at an energy above the ionization threshold is theoretically studied. Explicit expressions for the angular distribution and the degree of polarization of scattered photons as functions of probe and laser radiation characteristics are derived in the rotating wave approximation. Frequency, angular, and polarization dependences of the cross sections of scattering into final 1s 2 1 S, 1 s2s 1 S, and 1s4s 1 S atomic states are calculated.
The effect of a laser field on the cross section of photoionization of an atom by probe radiation in the region of closely lying autoionizing resonances is studied theoretically. The cross sections in the region of the overlapping autoionizing Rydberg states 5p 5(2 P 1/2)6d′ J = 1 and 5p 5(2 P 1/2)8s′ J = 1 resonantly coupled with the discrete state 5p 5(2 P 3/2)7p[1/2]1 in the xenon atom are calculated. The behavior of the shape and position of resonances in relation to the frequency and intensity of laser radiation is studied.