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Physical Review Letters

Optical Detection of Level Crossing in a J=12 State

Alan C. Gallagher, Allen Lurio

Published Jan 1, 1963• 21 citations
Publisher

Abstract

It is shown theorptically that a level-crossing signal from the m/sub j/ = plus or minus 1/2 sublevels of a J = 1/2 state populated by optical excitation from a J = 1/2 or 3/2 ground state can be observed by observing the scattering of circularly polarized light from the substance. Both the incidert-light and scattered-light directions are assumed perpendicular to the applied d-c magnetic field. The effect is measured in Tl, and the lifetime of the S/sub 1/2 level of Tl is calculated to be (7.8 plus or minus 0.2) x 10/sup -9/ sec by analysis of the width of this level. (T.F.H.)

DOI
10.1103/physrevlett.10.25

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Soviet Physics Uspekhi•Apr 30, 1975•19 citations
It is shown theorptically that a level-crossing signal from the m/sub j/ = plus or minus 1/2 sublevels of a J = 1/2 state populated by optical excitation from a J = 1/2 or 3/2 ground state can be observed by observing the scattering of circularly polarized light from the substance. Both the incidert-light and scattered-light directions are assumed perpendicular to the applied d-c magnetic field. The effect is measured in Tl, and the lifetime of the S/sub 1/2 level of Tl is calculated to be (7.8 plus or minus 0.2) x 10/sup -9/ sec by analysis of the width of this level. (T.F.H.)
We discuss experimental and theoretical studies of coherent excitation of magnetic sublevels in nD states of cesium that cross in an external electric field. Crossings of mF magnetic sublevels of hyperfine F levels with &Dgr;mF = ±2 lead to resonances in the linearly polarized laser induced fluorescence, while crossings with &Dgr;mF = ±1 lead to resonances in the circularly polarized laser induced fluorescence. These resonances can be exploited to observe alignment to orientation conversion. From the level crossing signals it is possible to measure atomic properties, such as the tensor polarizability &agr;2 and the hyperfine constant A. Alignment to orientation conversion involves the deformation of the spatial distribution of an atom's angular momentum.
Level-crossing effects have been detected non-optically with the use of a Rabi-type atomic-beam method. The sodium beam was excited by intense linearly polarised radiation from a CW narrow-band dye laser. Several superimposed resonances were observed in the atomic-beam signal when scanning the magnetic field around zero. The two narrower resonances are interpreted as high-order non-linear effects. They arise from the coupling of laser-induced Zeeman coherences in the ground state (including a hexa-decapole moment) with the populations of the Zeeman sublevels.
Subnatural-width resonances due to coherent effects in optical pumping at hyperfine (hf) transitions of Rb are investigated using a linearly or circularly polarized single-frequency diode laser beam. The sign and amplitude of the resonances in the fluorescence are measured by scanning the laser frequency over the hf transitions of the Rb D 1 and D 2 lines. The fluorescence perpendicular to the laser beam is detected as a function of magnetic field parallel to the beam propagation and scanned around zero value. In the case of linear polarization of the field, dark resonances are observed for hf transitions with F g → F e = F g -1 and F g → F e = F g , and bright resonances are observed for transitions with F g → F e = F g + 1. This is in agreement with the recently developed theories. For all (except F g = 2→ F e = 3, 85 Rb, D 1 line) hf transitions the sign of the resonances reverses when the polarization of the field changes to circular. The amplitudes of the resonances correspond to the probability of the hf transitions and to the calculated loss rate of the population to the ground hf level not excited by the laser field. The observed narrow resonances (FWHM: few tens of kHz) and their sensitivity to weak magnetic fields are interesting for high-resolution spectroscopy and for development of simple and precise magnetometers.
Summary form only given. The resonance laser interaction with an atomic system with degenerate levels creates coherent properties in the medium. Depending on the laser polarization and the experimental geometry it could be alignment, orientation or optical pumping. This coherence could be destroyed by application of a magnetic field, which removes the level degeneracy. This process has an effect on the optical characteristics of the medium, such as absorption and fluorescence from the levels involved. They show the resonance dependence on the applied magnetic field with widths, depending on /spl gamma//sub a/, /spl gamma//sub b/, and /spl gamma//sub ab/, where /spl gamma//sub I/ is the width of the upper or lower state of the transition, resonant to the laser beam, /spl gamma//sub ab/ is the width of the optical transition a/spl rarr/b. Using different experimental geometry, /spl gamma//sub a/, /spl gamma//sub b/, and /spl gamma//sub ab/ could be measured, as well as some rate constants of these levels. We investigated the coherent properties of the 2/sup 3/ P/spl rarr/3/sup 3/ D transition of He I under resonant laser radiation. Due to the peculiarities of the coherent signals, registered in the lateral and forward direction the cross-section for velocity selective collisions for the 3/sup 3/ D He I level were obtained. The influence of the strong laser field on the widths and shapes of the signals was determined. The cross-section, for depolarizing collisions of the 3/sup 3/ D level was measured, too.
The history of the discovery, study, and explanation of the effect discovered fifty years ago by the German physicist Hanle and its development and generalization by the French school of physicists led by Kastler are described. The effect is observed optically in an increase in the degree of depolarization of the resonance fluorescence. It is studied in the range of magnetic fields in which the Zeeman splitting of the atomic magnetic sublevels does not exceed the level widths determined by thermal relaxation mechanisms and electromagnetic interaction. It is a direct optical manifestation of interference of degenerate atomic states and is intimately connected with the conservation law for the angular momentum of the photon-atom system. In the review, different applications of the effect to the measurement of mean lifetimes, linewidths, and Stark splitting constants of atoms in excited states are given. The resonance scattering of light by atoms in the ground state in a weak magnetic field is considered in detail—in particular, in the presence of radio-frequency magnetic fields oriented in different ways. The different possible types of parametric resonances in one or two radio-frequency fields for arbitrary orientation of the constant magnetic field are interpreted as the effect of crossing of the levels of an atom that is dressed by the radio-frequency field. Applications of the effect in the quantum magnetometry of ultraweak fields (down to 10-10 Oe) are examined.