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Physical Review A

Numerical versus analytical models of laser-induced population redistribution in Rydberg atoms

Antoni Wójcik, R. Parzyński

Published Jan 1, 1999• 3 citations
Publisher

Abstract

Our recent approximate analytical results on the evolution of the initial state and the excitation from this state of a band of high Rydberg states by an optical pulse [Phys. Rev. A 55, 2144 (1997)] are compared with more exact numerical results. The difference between the two approaches is that the approximations of representative atom-field couplings and equidistant spacing of high Rydberg states, applied in the analytical model, are removed in the numerical model. We find that the approximate analytical model is not able to reproduce all the results of the more exact numerical model, if the laser pulse is very short (i.e., much shorter that the Kepler period of the resonantly excited high Rydberg state) or very intense (i.e., when the Rabi period for the resonant transition is substantially shorter than the above Kepler period). This mainly concerns the redistribution of the population over different Rydberg states. The analytical model predicts a symmetric distribution centered around the resonantly excited Rydberg state, irrespective of the pulse duration and intensity, while the numerical model allows an asymmetric distribution with its maximum shifted towards lower Rydberg states. Despite the above-stated difference, the analytical model is able to give preliminary results for the total population of all the excited Rydberg states and the evolution of the initial state, imitating at least qualitatively the results of the numerical model in certain time and intensity scales.

DOI
10.1103/physreva.59.597

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Our recent approximate analytical results on the evolution of the initial state and the excitation from this state of a band of high Rydberg states by an optical pulse [Phys. Rev. A 55, 2144 (1997)] are compared with more exact numerical results. The difference between the two approaches is that the approximations of representative atom-field couplings and equidistant spacing of high Rydberg states, applied in the analytical model, are removed in the numerical model. We find that the approximate analytical model is not able to reproduce all the results of the more exact numerical model, if the laser pulse is very short (i.e., much shorter that the Kepler period of the resonantly excited high Rydberg state) or very intense (i.e., when the Rabi period for the resonant transition is substantially shorter than the above Kepler period). This mainly concerns the redistribution of the population over different Rydberg states. The analytical model predicts a symmetric distribution centered around the resonantly excited Rydberg state, irrespective of the pulse duration and intensity, while the numerical model allows an asymmetric distribution with its maximum shifted towards lower Rydberg states. Despite the above-stated difference, the analytical model is able to give preliminary results for the total population of all the excited Rydberg states and the evolution of the initial state, imitating at least qualitatively the results of the numerical model in certain time and intensity scales.
Following the experiment of Jones et al. [Phys. Rev. Lett. 71, 2575 (1993)], we present a theory of coherent redistribution of population in a model Rydberg atom exposed to a sequence of two identical laser pulses separated in time and shorter in duration than the Rydberg-electron Kepler period. For different initial-population conditions, we solve the problem of redistribution in a fully analytical and rigorously nonperturbative way, thus obtaining discrete-state population amplitudes, dependent on laser intensity, and a time delay between the pulses. Using this solution, we study the population in a given state versus pulse delay (Ramsey fringes), as well as the corresponding Fourier spectrum, for both low and high laser intensities. From the number of frequency components and their positions in the Fourier spectrum, we deduce the redistribution of the initial-state population over a number of states by the first pulse. Within our model calculations we achieve qualitative agreement with the experimental observation of Jones et al. that an increase in laser intensity results in a general increase in the number of frequency components in the Fourier spectrum. Our theoretical results are also consistent with the experimental finding of Noordam et al. [Phys. Rev. Lett. 68, 1496 (1992)] that Raman transitions via the continuum between different Rydberg states are essential for population redistribution. We suggest new experiments to measure the effect of the initial-state depletion on the Fourier spectrum. As follows from our model calculations, some frequency components (both low and optical) present in the low-intensity Fourier spectrum should vanish in the high-intensity spectrum due to complete depletion of the initial state by the first pulse.
An analytical study is presented of two models of intense-laser ionization from an isolated initial state via a band of high-Rydberg states in a system which bears many features of the hydrogen atom. The models differ in the way the population from the directly excited Rydberg states can migrate to the states of higher angular momenta. In one of them it migrates via nonresonant and in the other via resonant degenerate Raman coupling. Within these models we draw conclusions about the efficiency of migration, the initial-state lifetime, the redistribution of population, the laser intensities stabilizing the atom against ionization, the effect of the initial state on the threshold stabilizing intensity, as well as about stabilizing mechanisms versus the initial-state choice.
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.
Assuming the initial population in the hydrogenic 50s state, we simulate Rydberg-state redistribution and photoionization by a nonresonant optical-frequency pulse versus the number of different n manifolds included. The inclusion of additional n manifolds, around the n = 50 manifold distinguished by the initial-state choice, is shown to invalidate the photoionization predictions based on the single-n manifold approximation. The calculations are performed assuming a linearly polarized, 620 nm pulse, turned on and off suddenly, of ten-optical-period duration and peak intensity 109 W cm-2 ensuring effective transient mixing of both-parity Rydberg states around n = 50.
We introduce a model of laser pulse ionization of a high Rydberg state whose core is a chain of coupled V-type transitions with exact resonance in each arm of the V. Through this chain the population is allowed to migrate resonantly from the initial state of low angular momentum to other bound states of higher angular momenta, both of low and high energy. Along with the redistribution of the initial population over different bound states, we discuss the effect of the redistribution on atomic stabilization, exploiting an analytical solution of the model that holds in a limiting case. The resonant migration mentioned is shown to be able to enhance atomic stabilization against ionization by an optical-frequency pulse.
In a wide basis of bound hydrogenic states (20< n< 80,l< 29),(20<~n<~80,l<~29),(20< n< 80,l< 29), we simulate redistribution of the initial 50s50s50s state population by a nonresonant, optical-frequency pulse of the strength at which Rydberg-to-Rydberg Rabi frequencies exceed the pulse frequency. A small transient flow of the population out of the 50l50l50l manifold (l=0,1,2,3,…)(l=0,1,2,3,\dots{})(l=0,1,2,3,…) is observed and the effect of this $n\ensuremath{\ne}50$ redistribution on the formation of the photoionization signal from Rydberg states is calculated in the limit of weak bound-free coupling. This $n\ensuremath{\ne}50$ redistribution is found to totally cancel the predictions of the single-n manifold approximation (n=50(n=50(n=50 in this case) in photoionization.