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Journal of Physics B Atomic and Molecular Physics

Continuum state mixing induced by the weak magnetic field of the Earth in the photoionisation Stark spectra of sodium

J. M. Lecomte, Eliane Luc-Koenig

Published Aug 14, 1985• 1 citation
Publisher

Abstract

The long-range diamagnetic interaction arising from the Earth's magnetic field produces a striking change in the photoionisation Stark spectra. The profiles of the supplementary resonances observed for the excited sodium atoms are typical of continuum-continuum state mixing.
DOI
10.1088/0022-3700/18/15/019

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We report experimental results on the photoionization spectra of excited sodium atoms in a well-defined ml=1{m}_{l}=1ml​=1 substate in the presence of a dc electric field. The various spectra corresponding to a definite polarization of the ionizing light exhibit characteristic features. It appears in addition that they are strikingly perturbed by the Earth's magnetic field.
We report the observation of a structure in the ionization spectrum of sodium, induced by a strong radiation field embedding the 5s 2^{2}2S1/2{\mathit{S}}_{1/2}S1/2​ state into the continuum. The structure, probed by single-photon ionization, shows a marked asymmetry, typical of a laser-induced continuum structure.
The effects of saturation of a bound-bound atomic resonance on bound-continuum decays in investigated using the dressed state concept. Two special cases are discussed: the modification of spontaneous radiative decay by coupling to a third bound atomic state, and the changes in single-photon photoionisation when the initial discrete state is coupled by an external field to another bound state. For strong external fields Rabi oscillations and dynamic Stark splittings profoundly alter the 'decay'; but for weak fields the alterations at line centre are no less dramatic.
The diamagnetic and Stark shifts of strongly perturbed Rydberg spectra such as those of the alkaline earths are expressed in terms of multichannel---quantum-defect parameters. The theory is applicable when the shifts produced by the external fields are smaller than the neighboring Rydberg separations. It is applied to evaluate the diamagnetic shifts in Ba, an atom in which Rydberg and doubly excited channels are strongly mixed. In such complex spectra, the diamagnetic effect can serve a useful purpose as a tool to disentangle the interacting series. The radial sizes and, therefore, the diamagnetic shifts are quite different for singly and doubly excited states even when they are at the same energy. In fact, for the field strengths considered ($\ensuremath{\simeq}40$ kG), the diamagnetic contribution of the doubly excited configuration is quite negligible. As a result, the observed shift is a direct measure of the amount of singly excited configuration in the state of interest. Exactly similar conclusions apply to Stark shifts. The last part of this paper considers states of very high excitation further up in the spectrum which are no longer in a perturbative regime. Spectra of atoms in a magnetic field have shown equally spaced resonances near the ionization threshold with characteristic spacings. By the consideration of the effects of electric fields which are also often simultaneously present, a criterion is established for the nature of these spacings depending on the relative strengths of the crossed electric and magnetic fields. The spacing seen in any particular experiment is shown to depend on the mass and quantum defect of the atom, the strengths of the external fields, and experimental conditions such as the temperature of the atomic vapor whose absorption spectrum is being studied.