Laser RF Double-Resonance Spectroscopy in Molecular Beams

2 Laser-RF Double-Resonance Spectroscopy in Molecular Beams 

The Rabi technique of radio frequency or microwave spectroscopy in atomic or molecular beams [10.14-10.17] has made outstanding contributions to the accurate determination of ground state parameters, such as the hfs splittings in atoms and molecules, small Coriolis splitting in rotating and vibrating molecules, or the narrow rotational structures of weakly bound van der Waals complexes [10.18]. Its basic principle is illustrated in Fig. 10.9. A collimated beam of molecules with a permanent dipole moment is deflected in a static 

The technique is restricted to atoms or molecules with a sufficiently large difference of the dipole moments in the two levels since the change of deflection in the inhomogeneous fields must be detectable. Furthermore, the detection of neutral particles with a universal ionization detector is generally not very sensitive except for those molecules (for example, alkali atoms or dimers) that can be monitored with a Langmuir-Taylor detector. 

The laser version of the Rabi method (Fig. 10.9b) overcomes both limitations. The two magnets A and B are replaced by the two partial beams 1 and 2 of a laser that cross the molecular beam perpendicularly at the positions A and B. If the laser frequency is tuned to the molecular transition in) kn) the lower level / ) is partly depleted due to optical pumping in the first crossing point A. Therefore the absorption of the second beam in the crossing point B is decreased, which can be monitored through the laser- 

These advantages allow the extension of the laser Rabi technique to a large variety of different problems [10.20], which shall be illustrated by three examples  

With a tunable single-mode cw dye laser any wanted transition can be selected and the hfs constants can be obtained for arbitrary rotational lev- 

The small magnetic hyperfine structure (hfs) in the state of a homonu- 

Example 10.4. Hyperfine-Structure of Highly Excited Atomic Levels 

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