Optical pumping resonance cells

Millimeter wave spectroscopy with a free space cell such as a Broida oven is more sensitive than lower frequency microwave spectroscopy. However, the higher J transitions monitored by millimeter wave spectroscopy often do not show the effects of hyperfine structure. In the case of CaOH and SrOH, the proton hyperfine structure was measured in beautiful pump-probe microwave optical double resonance experiments in the Steimle group [24,68], They adapted the classic atomic beam magnetic resonance experiments to work with a pulsed laser vaporization source and replaced the microwave fields in the A and C regions by optical fields (Fig. 15). These sensitive, high-precision measurements yielded a very small value for the proton Fermi contact parameter (bF), consistent with ionic bonding and a... 

Finally, optical pumping and double resonance have opened the way to experiments where the species under study can be confined in closed resonance cells, whereas Rabi s technique relied on the spatial separation of atoms or molecules in a beam arrangement. Also, the new methods allowed the investigation of ions that cannot be studied with a Rabi-type apparatus. 

This optical-optical double-resonance technique has already been used for other Doppler-free techniques [10.25], such as polarization spectroscopy (see Sect.10.3). Its applications to molecular beams has, however, the following advantages compared to spectroscopy in gas cells. When the chopped pump laser periodically depletes the level E. and populates level Ej, there are two relaxation mechanisms in gas cells which may transfer the population modulation to other levels. These are collision processes and laser-induced fluorescence (see Fig.8.39). The neighboring levels therefore also show a modulation and the modulated excitation spectrum induced by the probe laser includes all lines which are excited from those levels. If several absorption lines overlap within their Doppler width, the pump laser simultaneously excites several upper states and also partly depletes several lower levels. 

Since the width of the magnetic resonance line is determined by the inverse of the relaxation time for the spins in the system, it is in turn dependent on the method by which the optical pumping cell has been prepared. In evacuated and uncoated cells the relaxation time is of the order of lO s and linewidths of approximately 2 kHz are obtained. In cells which have been coated or which are filled with a buffer gas at the optimum pressure, the... 

Consequently when an optical pumping cell filled with Rb is illuminated with circularly-polarized radiation from a rubidium resonance lamp, all the transitions shown in Fig. 18.4(b) will be excited for those atoms in the F= l level of the ground state. The transitions for atoms in the F=2 hyperfine ground-state level are even more complex and are not shown for this reason. We see that there are many more states involved than in the case shown in Fig.17.1, where the effect of nuclear spin was ignored, and that the transition probabilities of the different decay routes are also more nearly equal. The net result is that hyperfine structure makes the Zeeman pumping process much less efficient. This point is discussed in more detail in early papers by Hawkins (1955) and Franzen and Emslie (1957). 

In either case the resulting population difference enables the hyperfine magnetic resonance transitions AF = 1 to be detected when microwave power at 6835 MHz is fed into the optical pumping cell. Analagous techniques can be used to... 

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