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Proceedings of the Royal Society of London A Mathematical and Physical Sciences

Configuration interaction effect on diamagnetic phenomena in atoms: strong mixing and Landau regions

K. T. Lu, Frank S. Tomkins, W. R. S. Garton

Published Aug 22, 1978• 51 citations
Publisher

Abstract

Abstract We present a systematic densitometer mapping of diamagnetic effects on the principal series of Ba and Sr in absorption, with magnetic fields up to 4.7 T. The n- and l-mixing spectra are resolved in both σ- and π-polarization while the quasi-Landau resonances are observed near and above the ionization limit only in σ-polarization. Configuration interaction produces striking differences in the intensities and spacings of the n- and l-mixed spectra while only the shapes of the quasi-Landau resonances depend on the atomic systems.

DOI
10.1098/rspa.1978.0142

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Abstract We present a systematic densitometer mapping of diamagnetic effects on the principal series of Ba and Sr in absorption, with magnetic fields up to 4.7 T. The n- and l-mixing spectra are resolved in both σ- and π-polarization while the quasi-Landau resonances are observed near and above the ionization limit only in σ-polarization. Configuration interaction produces striking differences in the intensities and spacings of the n- and l-mixed spectra while only the shapes of the quasi-Landau resonances depend on the atomic systems.
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.
Quasi-Landau resonances have been recorded for lithium atoms in magnetic fields of up to 47 kG, with a spacing of only half the cyclotron frequency rather than 32\frac{3}{2}23​ as expected from previous theory. The intensity distribution over satellites in the lll-mixing region (including the appearance of parity-forbidden transitions) is discussed in terms of magnetic mixing among nearly degenerate levels. An intensity sum rule is demonstrated.
A perturbation method is used to evaluate the energy levels and oscillator strengths of sodium atoms (m=-2,π2=+)in a strong magnetic field. The main features of the calculated spectra which lie in the strong-field-mixing regime agree well with the corresponding experimental results. Besides the conventional quasi-Landau resonance with spacing equal to 1.5?ωc (ωc is the electron cyclotron frequency) near the zero field ionization limit, a series of resonance spacings are found and analysed using the zero order energy formula and the Fourier transform technique. The values of these spacings are in consistent with other theoretical results. Based on the perturbation method, a quantum mechanical explanation for the spacings is given. These resonance spacings are independent of the applied exernal magnetic field.
We report new experiments on the spectrum of Sr atoms in crossed electric and magnetic fields. We apply an experimental geometry which enables the two circular polarizations to be detected and recorded separately, thereby considerably simplifying the observed structure. This experimental procedure allows Stark maps for the diamagnetic structure to be constructed. We compare the data for Sr with analogous data reported recently for Ba. We also present theoretical calculations of the crossed-field spectrum, which agree well with our observations.
We study the spectra of diamagnetic rubidium Rydberg atoms in strong magnetic and weak electric parallel fields, in the nnn-mixing regime. Our emphasis is on isolated pairs of near-degenerate, opposite-parity, diamagnetic states that become mixed by the weak electric field. Such level pairs allow for the generation of nondegenerate states with large, tunable permanent electric dipole moments and large optical excitation cross sections from the atomic ground state. We investigate how the dipole moments and the zero-electric-field energy defects of these level pairs can be tuned using small variations of the electric and magnetic fields. Using calculations, we explore the abundance of such level pairs over wide spectral regions for several magnetic quantum numbers. Applications of polar, diamagnetic Rydberg states in Rydberg-atom interaction experiments are briefly discussed.