Laser-Microwave Spectroscopy

There is also a variety of experimental methods, including, for example, simple optical pumping with subsequent rf transitions, but also photon-echo microwave nuclear double resonance, or the generation of multiphoton Lamb dips with a laser and a microwave field. 

In order to handle efficiently the large number of experiments carried out by laser-microwave spectroscopy and to illustrate the techniques and their differences as compared to work utilizing, e.g., pure laser or pure radiofrequency excitation, the organization of this chapter is by experimental methods rather than by the physical goals pursued in the different experiments. Typical examples will be described in some detail, and reference to other or related work is made. However, the list of references may not be complete. 

---

R. Neumann, F. Trager and G. zu Putlitz. Laser-microwave spectroscopy. In H. J. Beyer and H. Kleinpoppen (eds). Progress in Atomic Spectroscopy, part D, Plenum, New York, 1987. 

This chapter assesses the state of the art in laser microwave spectroscopy, i.e., investigations where both laser radiation and microwave radiation are involved. The many concepts and methods applied to numerous different atomic, molecular, and solid state systems are outlined, not always with emphasis on completeness of all the later references to the same method. This chapter hopefully stimulates further development of laser microwave spectroscopic methods and applications, even crossing the border of different disciplines. 

A classification scheme for laser-microwave spectroscopy based on well-defined three-step processes is displayed in Figure 5. It illustrates the great variety of possible experimental methods. The steps can be carried out in many different ways depending, for example, on the physical properties of the species under study. To be more specific, details of the three steps are as follows ... 

Figure. Classification scheme for laser-microwave spectroscopy.

Besides this technique, numerous combinations of optical and nonop-tical methods in step 1 and step 3 have been used. Examples will be given in the following text in order to illustrate the large variety of ways in which laser-microwave spectroscopy can be performed. 

Table 1. Hfs Splittings of the 2 S State for (a) and Li (b). Measured by Laser-Microwave Spectroscopy with an Ion Beam"...

Combined laser-microwave spectroscopy based on optical pumping was also performed in the solid state. Spectral line broadening caused, e.g., by strain and phonon interaction, can be overcome by extreme cooling and specific site selective procedures. Very narrow lines are attainable particularly in the spectra of rare earth ions doped to crystals in low concentration. Rare earth ions, therefore, play an important role in solid state spectroscopy, as will be illustrated in the course of this section. 

Laser-microwave spectroscopy was also reported of the triplet state of photosynthetic bacteria, namely, of Rhodospirillum rubrum, Rhodo-pseudomonas spheroides, and Chromaticum vinosum, in chemically reduced cellular preparations at 2 K. The authors found similarities of the triplet state frequencies, spectral features, and intersystem crossing rates that suggest that the reaction centers in photosynthetic bacteria possess a common structure. 

Detection of microwave transitions via fluorescence becomes more and more difficult with increasing principal quantum number n of the level under study, since the oscillator strength of an optical transition from a level with n to a lower level decreases as n. The field ionization technique therefore favorably replaces fluorescence detection for large principal quantum numbers. Laser-microwave spectroscopy combinedwith field ionization was first realized by Gallagher et and by Fabre et aL Two-step... 

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). 

Summaries and reviews on infrared laser microwave spectroscopy of molecules can be found in Refs. 225-227, 246-252. 

It is characteristic of the technology of microwave spectroscopy that frequencies are measurable to very high precision. Until the introduction of infrared lasers, microwave spectroscopy far outran vibrational spectroscopy in the precision and accuracy of spectral measurements. The primary piece of information obtained from a microwave spectrum is the rotational constant, and given the precision available with this type of experiment, high-precision values of the rotational constant are obtained. This, in turn, implies that very precise values of the bond length of a diatomic molecule can be deduced from a microwave spectrum. In practice, measurement precision corresponding to a few parts in 10,000 is achieved. 

---