Optical-RF Double-Resonance Technique

The coupling of the molecular angular momentum J with a possible nuclear spin I leads to a precession of J around the total angular momentum F = / + /, which further reduces the molecular orientation [10.9]. A careful analysis of experiments on optical pumping of molecules gives detailed information on the various coupling mechanisms between the different angular momenta in selected molecular levels [10.10]. 

Another aspect of optical pumping is related to the coherent excitation of two or more molecular levels. This means that the optical excitation produces definite phase relations between the wave functions of these levels. This leads to interference effects, which influence the spatial distribution and the time dependence of the laser-induced fluorescence. This subject of coherent spectroscopy is covered in Chap. 12. 

A thorough theoretical treatment of optical pumping can be found in the review of Happer [10.11]. Specific aspects of optical pumping by lasers with particular attention to problems arising from the spectral intensity distribution of the pump laser and from saturation effects were treated in [10.12,10.13]. Applications of optical pumping methods to the investigation of small molecules were discussed in [10.4]. 

The combination of laser-spectroscopic techniques with molecular beams and RF spectroscopy has considerably enlarged the application range of optical-RF double-resonance schemes. This optical-RF double-resonance method has now become a very powerful technique for high-precision measurements of electric or magnetic dipole moments, of Lande factors, and of fine or hyper-fine splitting in atoms and molecules. It is therefore used in many laboratories. 

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The fundamental advantage of the rf double-resonance technique is the high spectral resolution which is not limited by the optical Doppler width, see (3.30d)... 

Up to now the heterodyne technique is the most accurate method to determine such line splittings. Its accuracy is comparable with the optical-rf double-resonance method but its application range is more general. Two independent lasers are stabilized onto the line centers of two different molecular transitions (Fig.10.48). The output of the two lasers is superimposed on a nonlinear detector, such as a photomultiplier in the visible range or a semiconductor diode in the infrared. 

In the past, optical methods have proven to be a powerful tool in the study of resonances among closely spaced sub-levels. A first example is the radiofrequency-optical double resonance technique Here a Hertzian resonance is induced by means of a rf field the light field is used to create the nonequilibrium population of the sublevels ("optical pumping") that is required for rf excitation, and to detect the rf-induced changes in the optical properties of the san le. 

In this contribution we present two laser spectroscopic methods that use coherent resonance Raman scattering to detect rf-or laser -induced Hertzian coherence phenomena in the gas phase these novel coherent double resonance techniques for optical heterodyne detection of sublevel coherence clearly extend the above mentioned previous methods using incoherent light sources. In the case of Doppler broadened optical transitions new signal features appear as a result of velocity-selective optical excitation caused by the narrow-bandwidth laser. We especially analyze the potential and the limitations of the new detection schemes for the study of collision effects in double resonance spectroscopy. In particular, the effect of collisional velocity changes on the Hertzian resonances will be investigated. 

With this procedure, as with the double-resonance methods in atomic physics, Zeeman and Stark splittings, hyperfine structures and A doublings in molecules can be measured with high precision, even if the observed level splittings are far less than the optical dopp-ler width. From the width of the rf resonance and from the time response of the pumped systems, orientation relaxation rates can be evaluated for individual v J") levels. Other possible applications of this promising technique have been outlined by Zare 30) Experiments to test some of these proposals are currently under investigation and their results will be reported elsewhere. 

Recently, a novel rf-laser double resonance method for optical heterodyne detection of sublevel coherence phenomena was introduced. This so-called Raman heterodyne technique relies on a coherent Raman process being stimulated by a resonant rf field and a laser field (see Fig.l(a)). The method has been applied to impurity ion solids for studying nuclear magnetic resonances at low temperature3 5 and to rf resonances in an atomic vapor /, jn this section we briefly review our results on Raman heterodyne detection of rf-induced resonances in the gas phase. As a specific example, we report studies on Zeeman resonances in a J=1 - J =0 transition in atomic samarium vapor in the presence of foreign gas perturbers. 

Our studies of the effect of velocity-changing collisions in an rf-laser double resonance experiment contribute to a new vista into the role of collisictis in laser spectroscopy of sub-level structures the limitation of the observation time of the active atoms due to narrow-bandwidth optical excitation and simultaneous velocity diffusion can be of importance for a variety of spectroscopic techniques that use a velocity-selective excitation and detection of either sublevel populations or sublevel coherence. On the other hand, the collisional velocity diffusion of sublevel coherence within an optical Doppler distribution can also give rise to new and surprising phenomena as will discussed in the next section. 

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