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Physical Review A

Multiphoton intrashell resonances in Rydberg atoms: Bloch-Siegert shifts and widths

Daniel E. Fregenal, Erik Horsdal-Pedersen, Lars Bojer Madsen, Morten Førre, J. P. Hansen, Valentin N. Ostrovsky

Published Mar 15, 2004• 19 citations
Publisher

Abstract

We measure for the first time, to our knowledge, $N$-photon intrashell resonances, widths, and Bloch-Siegert shifts up to $N=23$. Rydberg atoms of Li $(n=25)$ in a rotating electric field and a static, in-plane electric or magnetic field are used. The measurements, in conjunction with theoretical estimates, precisely determine the characteristics of the resonances (shifts and widths) and their dependence on the strength of the rotating field.

DOI
10.1103/physreva.69.031401

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We measure for the first time, to our knowledge, NNN-photon intrashell resonances, widths, and Bloch-Siegert shifts up to N=23N=23N=23. Rydberg atoms of Li (n=25)(n=25)(n=25) in a rotating electric field and a static, in-plane electric or magnetic field are used. The measurements, in conjunction with theoretical estimates, precisely determine the characteristics of the resonances (shifts and widths) and their dependence on the strength of the rotating field.
Bloch-Siegert shifts and widths of resonances in two-level systems are evaluated within an adiabatic approximation applicable in the multiphoton regime. The level splitting is controlled by a static field. Most attention is paid to the case of a circular driving field with the static field in the plane of rotation. This nonconventional setup was used in a recent experiment on intrashell resonances in Rydberg atoms where shifts and widths for NNN-photon resonance with NNN as large as 23 were observed [D. Fregenal et al., Phys. Rev. A 69, 031401(R) (2004)]. For this in-plane configuration the resonance widths are drastically enhanced compared to their values in the conventional configuration (linearly polarized driving field perpendicular to static field), whereas the shifts are only weakly affected.
Multiphoton intrashell transitions in strongly driven Li (n=25n=25n=25) Rydberg atoms are studied experimentally. Orthogonal dc electric and magnetic fields lift the degeneracy of the nnn shell and define the eccentricity eee of the initial coherent elliptic states, which are formed by laser excitation and subsequent adiabatic transformation. The intrashell transitions are driven by a time-harmonic electric field linearly polarized parallel to the major axis of the ellipse. NNN-photon resonances with $N=1\ensuremath{-}9$ are studied as a function of eee. All resonances with $N\ensuremath{\geqslant}3$ are suppressed at certain eee values in between 000 and 111. A similar system was analyzed by Yabuzaki et al. [Phys. Rev. A 10, 1955 (1974)] who found a simple pattern of suppressions that applies also for the present experiments. The results of these experimentally confirm that each time NNN is increased by two, an additional suppression is observed.
The quasienergy spectrum of hydrogenlike Rydberg atoms in a precessing electric field of constant amplitude is found in simple form. Resonant intrashell transitions occur for specific field values when a (small) additional static electric field perpendicular to the precession axis is present. Sequences of such transitions may enhance the microwave ionization of a Rydberg atom. C. H. Cheng and T. F. Gallagher [Phys. Rev. A 61, 063411 (2000)] recently observed a one-photon resonance of this type. We evaluate the widths of one-photon intrashell resonances, and develop a general approach to the N-photon case within the framework of a quasienergy representation.