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Physical Review A

General theory of laser-induced quantum beats. II. Sequential laser excitation; effects of external static fields

Mark P. Silverman, S. Haroche, M. Groß

Published Oct 1, 1978• 18 citations
Publisher

Abstract

A general theory of laser-induced quantum beats introduced in paper I is extended to include the effects of sequential laser excitation by several lasers for atomic systems either free or subjected to external static fields. A detailed study is made of the dependence of the quantum beat signal on the polarizations of the exciting laser pulses and detected spontaneous emission. The theory is applied to the echelon excitation of sodium $\mathrm{nD}$ Rydberg states. An analysis of the Zeeman beats produced by excitation in an external magnetic field suggests new ways to measure the fine-structure interaction constant of the Rydberg levels.

DOI
10.1103/physreva.18.1517

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Pulsed dye lasers are used to excite sodium atoms into a coherent superposition of high Rydberg $\mathrm{nd} (^{2}\mathrm{D}_{\frac{3}{2}},^{2}D_{\frac{5}{2}})$ states. Analysis of the field ionization behavior of these atoms reveals the time evolution of the superposition state. Rapid application of a weak electric field projects different components of the superposition state onto states having identifiably different field ionization behavior. Analysis of the signal from subsequent field ionization as a function of the time of weak-field application reveals fine-structure quantum beats. These beats result from the time development of the original superposition state. Measurements of the fine-structure quantum beat frequency are presented for the principal quantum numbers $n=32, 34, 35, 36, 38, \mathrm{and} 40$.

Physical Review A•Jun 1, 1981•31 citations
A general theory of laser-induced quantum beats introduced in paper I is extended to include the effects of sequential laser excitation by several lasers for atomic systems either free or subjected to external static fields. A detailed study is made of the dependence of the quantum beat signal on the polarizations of the exciting laser pulses and detected spontaneous emission. The theory is applied to the echelon excitation of sodium nD\mathrm{nD}nD Rydberg states. An analysis of the Zeeman beats produced by excitation in an external magnetic field suggests new ways to measure the fine-structure interaction constant of the Rydberg levels.
A theory of stepwise pulsed-laser excitation of cascade Zeeman quantum beats has been developed for a four-level system. The laser pulses are treated in the broad-line approximation and the full optical-pumping equations for the atomic-density matrix of each level have been obtained. These can be used to describe saturation and light shift effects. The weak-pumping theory solutions to the optical pumping equations have been used to obtain expressions for the cascade quantum-beat intensity versus time curves. Larmor precession, spontaneous-emission decay and the populating of the final cascade level during the excitation pulse are all allowed for. A Wiener-Levy phase diffusion model with slowly varying amplitudes is used for the laser pulses, and the final theoretical expressions for the quantum beats depend on the time-dependent intensities of the laser pulses as well as on their polarisations. Hyperfine effects have been ignored. The theory applies to both coincident pulses and to pulses with a time delay between them. An illustrative application to the case of two very short but coincident pulses (during which Larmor precession, spontaneous emission etc. can be ignored) is given. In an accompanying paper the theoretical expressions are further evaluated and successfully applied to the interpretation of cascade Zeeman quantum beat experiments in Fe I.
A theory of optical free-induction decay (FID) induced by two exciting light pulses in three-level atomic systems subjected to a constant magnetic field is presented. The theory takes into account the hyperfine interaction, the level degeneracy, and the dependence of the FID signal on the polarizations of the exciting pulses. An analysis of the FID signal in a magnetic field suggests new ways to measure the g factors of the levels, the hyperfine structure constants, and the relaxation rates. The results obtained are extended to include the echelon excitation of the multilevel system by a definite set of light pulses with specially chosen frequencies and polarizations.
For pt.I see ibid., vol.20, p.251 (1987). Theoretical expressions for cascade light intensity as a function of time have been evaluated for the case of cascade Zeeman beats produced by two-step excitation with broad-line laser pulses, and the results are compared with experimentally observed cascade Zeeman beats in Fe I. The beats were detected in time-resolved fluorescence from intermediate levels populated by cascades from highly excited levels of Fe I, using three different stepwise-excitation-cascade sequences. In the case of coincident laser pulses, good agreement between theory and experiment is obtained over a range of magnetic fields when allowance is made for Larmor precessions during the excitation period, for the width and shape of the exciting laser pulses and for the contribution of impurity processes. The effects of precession during the excitation period are especially important for the excitation sequence a 5 D 4 to z 5 F degrees 5 to e 5 F 5 , for which short-pulse theories predict no quantum beats for the experimental geometry used (parallel linear polarisations in the two excitation steps). For non-coincident pulses a predicted oscillation in the amplitude of the cascade beats with magnetic field has also been observed.
The theory of double-resonance phenomena, or the simultaneous action of optical transitions and rf perturbations between the Zeeman sublevels of an atom, is extended to the induced emission and absorption processes effected by the monochromatic radiation and the discrete resonances involved in the gas laser. Expressions for the probability amplitudes of the Zeeman sublevels under such conditions are derived from time-dependent perturbation theory, the lifetimes of the states being introduced in a phenomenological way. Zeeman splittings of both upper and lower states involved in the laser transition are considered, and an effective term diagram used to discuss the results. Small signal approximations are applied to consider some specific cases, which show that the light beats seen in spontaneous emission when the rf frequency ${\ensuremath{\omega}}_{0}$ equals the Larmor frequency, and in other instances, do not occur in induced emission unless such frequencies coincide with axial resonances of the laser cavity. Variations in the intensity and resonance effects are indicated at Zeeman splittings corresponding to ${\ensuremath{\omega}}_{0}$, with similar variations in the amplitude of any beat frequencies due to multiple axial resonances. Additional coherence effects occur where levels cross, which will give rise to changes in the intensity and polarization of the induced emission at the corresponding frequency.
Pulsed dye lasers are used to excite sodium atoms into a coherent superposition of high Rydberg nd(2D32,2D52)\mathrm{nd} (^{2}\mathrm{D}_{\frac{3}{2}},^{2}D_{\frac{5}{2}})nd(2D23​​,2D25​​) states. Analysis of the field ionization behavior of these atoms reveals the time evolution of the superposition state. Rapid application of a weak electric field projects different components of the superposition state onto states having identifiably different field ionization behavior. Analysis of the signal from subsequent field ionization as a function of the time of weak-field application reveals fine-structure quantum beats. These beats result from the time development of the original superposition state. Measurements of the fine-structure quantum beat frequency are presented for the principal quantum numbers n=32,34,35,36,38,and40n=32, 34, 35, 36, 38, \mathrm{and} 40n=32,34,35,36,38,and40.