Radio-Frequency Spectroscopy in Molecular Beams

Comparison between conventional Rabi method (a) and its laser equivalent (b) 

Optical pumping with lasers offers a new and very convenient variation of the conventional Rabi method, which is illustrated in Figs.10.15b and 10.16. The two inhomogeneous magnetic fields A and B are replaced by two parallel laser beams which cross the molecular beam perpendicularly. The first pump beam depletes the population N(i) of the lower level i. This is monitored by the resultant decrease of the fluorescence intensity Ip induced by the probe laser. When the rf field induces transitions k i between other levels k and the depleted level i, the population N(i) is increased, resulting in a corresponding increase of Ip.  

The laser version of the Rabi method does not rely on the mechanical deflection of molecules but uses the change of the population density in specified levels due to optical pumping. The conventional technique is restricted to atoms or molecules with magnetic or electric dipole moments and the sensitivity depends on the difference between the dipole moments in the two levels connected by the rf transition. The laser version can be applied to all atoms and molecules which can be optically pumped by existing lasers. The background noise is small because only those molecules which are in the specified level contribute to the probe-induced fluorescence. 

Schematic diagram of atomic beam magnetic resonance method using optical pumping and probing with lasers [10.27e] 

Some examples illustrate the achievable sensitivity and accuracy ERTMER and HOFER [10.27c] measured the hyperfine structure of metastable states of scandium which were populated by electron impact. The population achieved in these states was only about 1% of the ground state population. Optical pumping to a higher electronic state with a single-mode dye laser allowed selective depletion of the single hfs sublevels of the metastable F state. The hfs constants could be determined with an accuracy of a few kHz. 

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