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Journal of Physics B Atomic Molecular and Optical Physics

Many-electron atoms in high magnetic fields

J P Connerade, G Droungas, Roger Elliott, Xinghong He, Natalia E. Karapanagioti, M A Farooq, Hanqing Ma, J. P. Marangos, Muhammad Zaman Nawaz

Published Jul 14, 1994• 11 citations
Publisher

Abstract

We present a report on experiments performed at Imperial College on diamagnetic effects in many-electron atoms. These include both atomic-beam experiments with DC fields, for which the measurement technique involves detecting Rydberg atoms after laser excitation, and experiments on columns of atomic vapour, in which case we have studied Faraday rotation induced by a pulsed magnetic field. In the atomic-beam experiments, interesting differences between the sigma + and sigma - spectra have been observed. We also describe theoretical work related to the interpretation of the data for both sets of experiments.

DOI
10.1088/0953-4075/27/13/007

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We present a report on experiments performed at Imperial College on diamagnetic effects in many-electron atoms. These include both atomic-beam experiments with DC fields, for which the measurement technique involves detecting Rydberg atoms after laser excitation, and experiments on columns of atomic vapour, in which case we have studied Faraday rotation induced by a pulsed magnetic field. In the atomic-beam experiments, interesting differences between the sigma + and sigma - spectra have been observed. We also describe theoretical work related to the interpretation of the data for both sets of experiments.
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A quasihydrogenic complete diamagnetic manifold is investigated experimentally. The spectra are obtained in a uniform magnetic field by exciting in a one-step process, using polarised laser light, the lithium atoms of an atomic beam. The whole structure of the M=0 odd diamagnetic multiplet has been recorded for the first time. This experiment provides a confirmation for the existence of the two classes of states predicted for the diamagnetic Hamiltonian of atomic hydrogen in the weak-field limit. The fundamental features of diamagnetism still subsist for such high field strengths so that the diamagnetic interaction cannot be regarded as a perturbation with respect to the Coulomb problem. The intermanifold coupling and the influence of effects due to the ionic core in the lithium atom are evaluated quantitatively.
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.
The simultaneous development of single mode c.w. dye laser excitation and thermoionic detection over the last five years has supplied with one of the most powerful experimental technique for atomic physics studies under vapour phase conditions. One of the most interesting application concerns problems of fundamental physics which can be tackled on Rydberg atoms especially the ones dealing with the atomic properties in external fields. Several selected examples are discussed to the light of generalized vectorial models of diamagnetism, linear Stark effect and crossed electric and magnetic fields effects. Although the experimental results are highly illustrative of the pure hydrogenic behaviour in external fields, none of them have been obtained on the hydrogen atom itself.
We report diamagnetic spectra of sodium atoms observed in a strong magnetic field up to 3 T and compare them with theoretical calculations based on the formulation of an exact quantum-defect theory. We focus on one-photon absorption processes from the ground 3s3s3s state to the excited npnpnp Rydberg states whose magnetic quantum numbers are determined by the polarization of the laser field. The calculations employ BBB-spline basis expansion and the complex spatial coordinate rotation technique, and their results agree well with the observed ones when atomic core effects are appropriately taken into account. When graphed against magnetic field, level anticrossings and core-induced interaction of the npnpnp states provide clear signatures for studying the effects of quantum defect on the diamagnetic spectra.