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Physical Review A

Magnetic resonances verified in agreement with an assumed time-asymmetric evolution

H.G. Weber, G. Miksch

Published Mar 1, 1985• 14 citations
Publisher

Abstract

With use of a narrow-band laser, the free ${\mathrm{NO}}_{2}$ molecule is prepared in a well-defined excited state under conditions that correspond to an optical-frequency--radiofrequency double-resonance experiment. We observe a resonant increase or decrease (dependent on the divergence of the light beam) of the total fluorescence intensity when the radiofrequency field is in resonance with the Zeeman splitting in the ground or excited state of the laser-induced transition. These resonances differ significantly from well-known optical-rf double-resonance signals and laser saturation effects. The resonances are, however, well described with use of a model which assumes an irreversible evolution in the free ${\mathrm{NO}}_{2}$ molecule.

DOI
10.1103/physreva.31.1477

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With use of a narrow-band laser, the free NO2{\mathrm{NO}}_{2}NO2​ molecule is prepared in a well-defined excited state under conditions that correspond to an optical-frequency--radiofrequency double-resonance experiment. We observe a resonant increase or decrease (dependent on the divergence of the light beam) of the total fluorescence intensity when the radiofrequency field is in resonance with the Zeeman splitting in the ground or excited state of the laser-induced transition. These resonances differ significantly from well-known optical-rf double-resonance signals and laser saturation effects. The resonances are, however, well described with use of a model which assumes an irreversible evolution in the free NO2{\mathrm{NO}}_{2}NO2​ molecule.
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.
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.
In an optical radio-frequency double-resonance experiment on NO2{\mathrm{O}}_{2}O2​ optical saturation resonances are observed. These resonances appear superimposed on the normal magnetic resonance spectrum. With increasing laser light intensity they change the resonance spectrum considerably. We present a model to describe the observed saturation phenomena: The resonances are attributed to the interaction of two optical photons and at least one radio-frequency photon with the molecule. The theory shows that saturation resonances appear in the population and in the coherence of the dressed-molecule levels in agreement with the experimental results.
We have made measurements of the spectrum of resonance fluorescence emitted by sodium atoms in an intense, uniform, monochromatic field. The transition used in the experiment was between the 32S123^{2}S_{\frac{1}{2}}32S21​​ (F=2F=2F=2, mF=2{m}_{F}=2mF​=2) ground state and the 32P323^{2}P_{\frac{3}{2}}32P23​​ (${F}^{\ensuremath{'}}=3$, ${m}_{F}^{\ensuremath{'}}=3$) excited state. A separate light beam, circularly polarized and resonant with this transition, was used to prepare the sodium atoms in the F=2F=2F=2, mF=2{m}_{F}=2mF​=2 magnetic sublevel. This pumping scheme was used to obtain the spectrum for both on- and off-resonance excitations. The measurements are in good agreement with Mollow's calculation. In addition, we describe conditions under which we observed asymmetric spectra similar to those of earlier experiments.
We report on a new effect which is observed when the small polyatomic molecule NO2{\mathrm{NO}}_{2}NO2​ is prepared under collision-free conditions into an electronically excited state. As the intensity of the exciting laser light is varied, the degree of polarization of the fluorescence light may undergo a change in sign. The analysis of the experimental results confronts us with an interesting problem concerning the time evolution of an isolated polyatomic system. In this paper we present the experimental results and a simple model which successfully describes these results. According to this model, the inversion of the polarization of the fluorescence light is a consequence of light-induced stability of the optically prepared state.
The optical analog of the nuclear magnetic resonance rotary saturation effect is demonstrated and analyzed. A 9.6-\ensuremath{\mu}m CO2{\mathrm{O}}_{2}O2​ laser resonance excitation of vibrational transitions in gaseous CH3{\mathrm{H}}_{3}H3​F creates an effective splitting in the interaction representation of the optical two-level system. A radio-frequency standing-wave Stark electric field is tuned in rotary saturation resonance with the splitting. The change in transmitted laser beam intensity, because of rotary resonance, is measured in terms of transient optical signals obtained by the Stark switching method of Brewer and Shoemaker. The measured single and multiphoton rotary saturation resonance signals are compared with perturbation and computer solutions of nonlinear Bloch-Maxwell macroscopic equations. Doppler-free rotary saturation spectra are obtained in the presence of inhomogeneous Doppler broadening by a special scheme of Fourier analysis. A discussion is presented of the coupling of energy between two traveling waves that are in double resonance with the optical two-level system. The concept of spin temperature is discussed in relation to a coherently excited optical level system with a long fluorescence lifetime. The possibility of a cross-relaxation double resonance experiment between an optical system and a spin system is outlined.