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Physical Review A

Suppression of multiphoton intrashell resonances in Li Rydberg atoms

A. Waheed, Daniel E. Fregenal, Øyvind Frette, Morten Førre, B.T. Hjertaker, Erik Horsdal, Ingjald Pilskog, Jana Preclíková

Published Jun 28, 2011• 2 citations
Publisher

Abstract

Multiphoton intrashell transitions in strongly driven Li ($n=25$) Rydberg atoms are studied experimentally. Orthogonal dc electric and magnetic fields lift the degeneracy of the $n$ shell and define the eccentricity $e$ 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. $N$-photon resonances with $N=1\ensuremath{-}9$ are studied as a function of $e$. All resonances with $N\ensuremath{\geqslant}3$ are suppressed at certain $e$ values in between $0$ and $1$. 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 $N$ is increased by two, an additional suppression is observed.

DOI
10.1103/physreva.83.063421

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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−9N=1\ensuremath{-}9N=1\ensuremath−9 are studied as a function of eee. All resonances with N\ensuremath⩾3N\ensuremath{\geqslant}3N\ensuremath⩾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.
One-photon intrashell transitions in strongly driven Li (n=25n=25n=25) atoms are studied experimentally. The degeneracy of the nnn shell is lifted by orthogonal dc electric and magnetic fields, which also define the eccentricity of the initial coherent elliptic state. The transitions are driven by a radio frequency pulse linearly polarized parallel to the major axis of the ellipse. A small dc electric field component parallel to the magnetic field splits the one-photon resonance into two, and transitions in between are studied by state-selective field ionization. Unexpected lines in the ionization spectra relating to unknown transitions are found and discussed.
Rydberg atoms of principal quantum number n in a superposition of a harmonic and a slowly varying field pass through several resonances with the harmonic field of frequency ? as the splitting ? of the shell by the slow field varies. These transient resonances which are met when ? N?, where N is an integer, have been studied for the n = 25 shell of Li. Coherent elliptic states were prepared and used as initial states, and the dynamics was probed by the probability Pa for the atom to remain in the initial state. The harmonic field was circularly polarized and had constant amplitude, and the slow field varied such that ? at first decreased, then went through a minimum and finally increased to bring the atoms into resonance at two different times. This led to interference patterns in Pa(?0), where ?0 is the minimum splitting. These were quite regular for coherent elliptic states of low eccentricity e and for strong fields , but less regular for large e and weak . A few states of Li(n = 25) are not hydrogenic due to quantum defects from the (1s)2 core. Without quantum defects the dynamics can be reduced to that of two spin- particles and this reproduces the regular patterns quite well. A full quantal treatment, which is required if quantum defects are important, shows that the more irregular patterns are the result of quite complex dynamics involving non-hydrogenic quasi-eigenstates.
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.
The evolution of Rydberg states of Li in time-dependent fields was studied with special emphasis on the influence of the Li(1s2)+ core. The strengths and rates-of-change of the fields were such that the dynamics involves states of only a single shell (intrashell dynamics). For sufficiently slow variations, the Rydberg states are transformed adiabatically but for faster variations, intrashell transitions do occur. We present experimental approximate probabilities for adiabatic transformation of the Rydberg states and compare with theoretical predictions with and without core effects included. The core can perturb the dynamics significantly. Circumstances under which this is likely to be the case are discussed.
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.