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Journal of the Optical Society of America B

Ac Stark effect and trapping of population on Rydberg levels in a strong ionizing field

Mikhail Vladimirovich Fedorov, A. M. Movsesian

Published Aug 1, 1989• 24 citations
Publisher

Abstract

Ac Stark splitting and ionization broadening of atomic Rydberg levels in a strong electromagnetic field are described. The quasi-energy atomic spectrum is shown to consist of a quasi-continuum plus two sets of narrow quasi-energy levels: weakly split and broadened almost field-free levels and strongly perturbed levels, whose width is a decreasing function of the field strength amplitude. The strong-field asymptotics of the photoelectron spectrum and of the time-dependent total photoionization probability are described. The photoelectron spectrum is shown to have a multipeak structure, and the shape of each peak is shown to be different from the Lorenz shape found earlier. This effect is explained by the contribution of the quasi-continuous quasi-energy spectrum. The same reason explains a partially nonexponential time dependence of the total photoionization probability.

DOI
10.1364/josab.6.001504

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Ac Stark splitting and ionization broadening of atomic Rydberg levels in a strong electromagnetic field are described. The quasi-energy atomic spectrum is shown to consist of a quasi-continuum plus two sets of narrow quasi-energy levels: weakly split and broadened almost field-free levels and strongly perturbed levels, whose width is a decreasing function of the field strength amplitude. The strong-field asymptotics of the photoelectron spectrum and of the time-dependent total photoionization probability are described. The photoelectron spectrum is shown to have a multipeak structure, and the shape of each peak is shown to be different from the Lorenz shape found earlier. This effect is explained by the contribution of the quasi-continuous quasi-energy spectrum. The same reason explains a partially nonexponential time dependence of the total photoionization probability.
Photoionization of highly excited atoms by a strong microwave field is considered. Secondary coherent population of some neighboring Rydberg levels in the process of ionization is shown to be a reason for interference of transitions from these levels to the continuum. Interference is shown to be a reason for some new effects: stabilization of the atom, suppression of its ionization, appearance of a multipeak structure in the photoelectron spectrum, field-induced narrowing of its peaks, narrowing and localization of quasi-energy atomic spectrum in a strong field, etc.
Photoionization of Rydberg atoms is considered in the framework of the quasiclassical (WKB) approach. Distribution of the photoelectron density in space and time is found and analyzed. This distribution is shown to consist of two main parts: the temporary population, which exists only as long as the light field is on; and the population corresponding to irreversible ionization. The irreversible-ionization wave packets are shown to experience the effect referred to as Coulomb narrowing. An origin of this effect and the conditions of its existence are discussed. In a strong field, rather simple analytical expressions are found for the partial probabilities of above-threshold ionization and for the total probability of ionization. In contrast to many earlier works, in the present theory neither the rotating-wave nor ``pole'' approximations are used, and the results are derived for an arbitrary pulse shape. Strong-field interference stabilization of Rydberg atoms is confirmed to occur at pulse durations shorter than the Kepler period. The arising regimes of interference stabilization are analyzed in comparison with the predictions of earlier works.
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.
We have measured the energy spectrum of photoelectrons emitted in multiphoton ionization of atoms by a short-pulse, tunable laser. The spectrum shows prominent resonances, confirming the observations of Freeman et al. [Phys. Rev. Lett. 59, 1092 (1987)]. The study of the position and intensity of such resonances, as a function of laser wavelength and intensity, probes the structure of the atom in the presence of the strong electromagnetic field and allows their spectroscopic assignment. A systematic study of the ac Stark shift of one of the resonances, the 7s[3/2]\ifmmode^\circ\else\textdegree\fi{} level of xenon, is presented.
Excitation and photoionization of Rydberg wave packets are described. Interference of bound-free transitions from many coherently populated Rydberg levels is shown to affect the dependence of the photoionization probability on the duration of interaction. Strong-field interference effects in such a system are described also. The most important of these is the field-induced interference stabilization of Rydberg atoms in a strong ionizing field. The intensity range in which this effect can exist is determined. The results derived are used for a qualitative model describing supercontinuum emission in the process of multiphoton ionization.
The energy levels of atoms and molecules exposed to intense, short, high-frequency laser pulses undergo time-dependent distortion known as dynamic Stark shift (DSS). These level shifts are induced by the coupling with nonresonant (nonessential) states and follow the temporal intensity profile of the laser pulse. Owing to the different DSSs of the individual atomic levels, transient resonance suppressions and enhancements significantly modify the multiphoton ionization pathway, leaving clear fingerprints (asymmetry, splitting, and shifting) in the energy spectrum of emitted photoelectrons. Here we investigate this phenomenon by solving the time-dependent Schr\"odinger equation of the valence electron of Na and Li in increasing levels of complexity: (i) developing minimal models for two-photon transition and (2+12+12+1)-photon ionization, (ii) applying a spectral method without continuum-continuum couplings, and (iii) propagating the electron wave packet on a large spatial grid accounting for all possible couplings. We show that appropriately detuned transform limited pulses provide a high level of control in strong-field excitation, when the atomic levels are dynamically shifted. Furthermore, we demonstrate the role of DSSs in the multipeak structure of the Autler-Townes doublet, found when several Rabi oscillations are induced during the strong-field resonant ionization.