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Physical Review Letters

Observation of Hyperfine Level Crossing in Stimulated Emission

J. S. Levine, Paul A. Bonczyk, Ali Javan

Published Feb 17, 1969• 29 citations
Publisher

Abstract

A level-crossing effect in stimulated emission is used to determine precisely the hfs in an excited electronic state of ${\mathrm{Xe}}^{129}$. The effect arises from a resonant change in nonlinear polarization induced by a monochromatic field as appropriate tunable level pairs are made to cross.

DOI
10.1103/physrevlett.22.267

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A level-crossing effect in stimulated emission is used to determine precisely the hfs in an excited electronic state of Xe129{\mathrm{Xe}}^{129}Xe129. The effect arises from a resonant change in nonlinear polarization induced by a monochromatic field as appropriate tunable level pairs are made to cross.
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.
We have performed an analysis of the effect of hyperfine structure in an experiment in which an aligned excited state is probed using linearly polarized light. The state is born with a given electronic alignment resulting from a collisional process. It is probed after a time which is short in comparison to its radiative lifetime but much longer than ω−1hfs, where ℏωhfs is the hyperfine splitting. This means that hyperfine structure has ample time to ‘‘develop.’’ It is therefore essential to take into account both the hyperfine structure of the level and the manner in which the alignment is affected by recoupling to the new basis. Ratios of fluorescent intensities for different laser polarization configurations contain information on the alignment of the system. We show how these may be calculated for a system with several isotopes, some of which have hyperfine structure, and where many components are probed with appreciable strength. A comparison is made with our experimental results for the Hg(6 3P1) level probed on the 6 3P1–7 3S1 transition. The presence of hyperfine structure has a considerable influence on the measurements. It is important to understand such effects if one is to obtain information on the nascent electronic alignment by observation of the fluorescent intensities.
New studies of Na2{\mathrm{Na}}_{2}Na2​ hyperfine structure (hfs) by the laser-induced-fluorescence molecular-beam magnetic-resonance technique are described which show that state selection and detection by strong optical fields can produce rf-magnetic-resonance spectra which differ drastically from the Rabi shape, even though the optical and rf fields do not act simultaneously on the molecules. We suggest that in the case of optically unresolved ground-state sublevels these effects are due to the creation of ground-state coherence by absorption and stimulated-emission cycles in the laser field, and we present a model calculation which exhibits the essential features of the observed spectra. We also give a proof that no model based only upon populations can explain the negative signals we observe.
Level-crossing experiments yielding Doppler-free signals are normally performed using a CW light source with a broad spectral distribution ("white" excitation), and the signals obtained are described by a well-known formula due to Breit. By using a single-mode dye laser or a pulsed laser for the excitation, strong modifications of level-crossing signals can be obtained. Profound changes in hyperfine structure crossing signals are demonstrated in experiments, performed in a cell, on the 9 2 D 3/2 state of 133 Cs. Practical utilisation of unconventional excitation in level-crossing and optical double resonance spectroscopy is discussed.