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Physical Review A

Strong-field short-pulse photoionization of Rydberg atoms: Interference stabilization and distribution of the photoelectron density in space and time

Mikhail Vladimirovich Fedorov, Olga V. Tikhonova

Published Aug 1, 1998• 20 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.58.1322

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Laser Physics•Jan 1, 2000•2 citations
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.
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.
Interference stabilization of atoms is investigated numerically. Raman-type transitions to Rydberg states with higher values of the electron orbital momentum are taken into account. These transitions are shown to change qualitatively theoretical predictions concerning the dependence of the time of ionization on the light-field-strength amplitude : if the weak-field time of ionization, as usual, is a falling function of , in a strong-field region the function is predicted to be more or less constant and of the order of the classical Kepler period in a rather large interval of , in contrast with the earlier prediction according to which, in a strong field, was assumed to be a rather fast growing function. The results derived are shown to be in a rather good agreement with the existing experiments. Alternative theoretical models are discussed.
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.
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.
Stimulated by a recent experiment [L. D. Noordam et al., Phys. Rev. Lett. 68, 1496 (1992)], we present a model of Rydberg-atom photoionization whose essential feature is the resonance between the initially populated Rydberg state and an additional state lying below Rydberg quasicontinuum. Despite the infinite number of discrete states involved in our model we have obtained the expressions for photoionization yield and discrete-state populations in completely analytical form. These expressions cover both short- and long-time scales, i.e., when laser-pulse duration is shorter or longer than the classical round-trip time of the Rydberg electron. The short-time scale corresponds to the conditions met in the experiment of Noordam et al. and, on the basis of our expressions, we have gained at least qualitative understanding of their observations. In the case of long pulses, which in our considerations are never longer than 54 ps, near resonance is shown to be the origin of a profound ionization suppression, particularly for some specific frequencies, which we identify as those ensuring the formation of a perfectly dark dressed state of zero width and thus unionizable. This dark state is shown to be populated with a probability increasing when laser gets stronger. We discuss the connection between stabilization and formation of this perfectly dark state as well as of additional dressed states, which are dark in the sense that they have the property of becoming very narrow as laser intensity increases. The undamped perfectly dark state, created at specific laser frequencies, has given us rise to introduce what we call the zero-pole approximation for ionization yield, which, as we have shown, works well if duration and intensity of laser pulse are appropriately combined.
The phenomenon of interference stabilization of Rydberg atoms is studied. The residual probability W res of finding an atom in bound (Rydberg) states and the threshold of stabilization V thr are calculated in their dependence on the pulse duration τ (for pulses with smooth envelopes). At τ longer than the Kepler period t K , both W res (τ) and V thr (τ) are shown to oscillate with approximately the Kepler period. At τ < t K (the case often referred to as transient stabilization), the threshold of stabilization V thr (τ) is shown to be a growing function of a decreasing pulse duration τ. Transient stabilization is shown to be explained by the same physical reasons as interference stabilization, and this implies that the concept of interference stabilization can be used in both limits of long and short pulses.