Optical pumping microwave induced

Miller, et al., (1974) used MOMRIE (Microwave Optical Magnetic Resonance Induced by Electrons) complementarily with anticrossing spectroscopy in experiments on He, H2, and other molecules. Small differences in electron bombardment excitation cross-sections combined with different radiative lifetimes lead to steady-state population differences between initial and final levels of the microwave transition. By monitoring fluorescence from one of the two involved electronic states, a change in fluorescence intensity signals a microwave resonance. Electron bombardment excitation tends to produce smaller population differences than the chemical pumping scheme exploited in the CN experiments, but the MOMRIE technique is more generally applicable. 

There are a number of methods to overcome this loss of atoms by optical pumping. For example, there could be two laser beams tuned for excitation out of each hfs state, the laser spectrum could be sufficiently broad to excite both hfs states, there could be an rf or microwave field that would induce hfs transitions to return atoms to the appropriate hfs state, or optical pumping could be inhibited by careful choice of experimental conditions. For a variety of carefully considered technical reasons, we employ the last of these alternatives. We use circularly polarized light and the axis provided by the magnetic tuning field to allow only excitations to a particular sublevel of the atomic excited state. The only strongly allowed decay process returns the atom to the original state. 

Figure 10. An example of a pumping scheme for microwave-optical polarization spectroscopy. A rotational transition in the ground state is detected via an anisotropy in the optical absorption. With the i axis chosen parallel to , the microwaves only pump AM = 0 transitions. When the linear polarization of the laser light (Ei.) is rotated by 45 and its frequency tuned to the appropriate transition, the z and x components (pumping IT and

Si hfs sublevels and thus affect the population. Similar to the work described above, this microwave-induced optical pumping can be monitored in a second laser-beam-ion-beam crossing region via a change of the fluorescence intensity. 

Laser-microwave spectroscopy based on nonlinear phenomena developed from the type of experiments on molecules already discussed in Section 3.2 which make use of optical pumping or double resonance. Occasionally, the laser and the rf power were high enough to create the nonlinear phenomena mentioned above, i.e., to saturate the transitions involved and/or to induce multiphoton transitions. The intermediate level in, e.g., two-photon transitions did not have to be a real state but could be virtual as well. Therefore, a drawback often encountered in earlier infared laser-microwave experiments could be avoided if the laser transition frequency did not exactly coincide with the molecular absorption line the Stark or Zeeman effect had to be used for tuning. This results in an undesired line splitting. With laser-microwave multiphoton processes, however, the laser can be operated at its inherent transition frequency. Exact resonance with molecular lines is then achieved by using a nonlinear effect, i.e., a radiofrequency quantum is added to or subtracted from the laser frequency (see Figure 28). 

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