OPTICAL DOUBLE RESONANCE EXPERIMENTS

Introduction. In our discussion of the Hanle effect in the previous chapter, we assumed that the g-factors of the excited atomic levels were already well known or could be calculated to sufficient accuracy from tlie Lande formula 

However, in the spectra of mercury and many other elements the the existence of intercombination lines shows that L-S coupling does not hold rigorously and that small deviations of the actual g-factors from the L-S values may be observed. 

These deviations contain useful information about the atomic wavefunctions and a technique which would permit the accurate measurement of the Zeeman splittings of excited levels is clearly very desirable. It would also be interesting to make accurate measurements of the hyperfine structures of the excited levels of odd isotopes since these give information about the magnetic dipole and electric quadrupole moments of the atomic nucleus. 

therefore, some means could be devised to measure the splittings between the excited levels directly, then the Doppler width associated with the signal wo-uld be reduced to a negligible value. 

For several years it seemed unlikely that the magnetic resonance method could be applied to excited atoms because of the low population of atoms that can be created in excited levels and the high temperatures required to attain appreciable vapour densities. However, it was pointed out by Brosscl and Kastler (1949) that the excitation of atoms using polarized resonance radiation allowed very large population differences to be established between excited sub- 

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Figure 11.25. Principles of the microwave/optical double resonance experiment for studying rotational transitions in the ground state of FeO, and the experimental arrangement [49].

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... 

Electron-impact Spectra.—A review of electron-scattering spectroscopy has appeared.265 Electron-impact spectra have been reported of helium,263 H and He,267 Li,258 Ba,259 Hg,280 H2,261 CO,282 Rbl and KI,283 NO and N20,284 water,286 ammonia and methane,2866 N02,288 and C02.287 The study on NOa yields the interesting result that the 2B2 state is asymmetric, possessing different equilibrium bond lengths between the N atom and each atom this can explain the results of recent microwave-optical double-resonance experiments.288... 

Double-resonance Spectroscopy.—A review has been given of double-resonance methods in spectroscopy.378 Attention will be focused here on optically (usually phosphorescence) detected magnetic resonance experiments (ODMR). Microwave-optical double-resonance experiments have been carried out on the spectrum of gaseous N02,379 permitting assignment of the rotational = 0—4 side-bands of the 493 nm band. 

P.R. Hemmer, M.K. Kim, M.S. Shahriar, Observation of sub-kilohertz resonances in RF-optical double resonance experiment in rare earth ions in solids. J. Mod. Opt. 47, 1713 (2000)... 

A similar amplification scheme has recently been used in microwave -optical double resonance experiment ... 

Very recently optical-optical double resonance experiments involving two lasers were performed for the study of highly excited electronic states. 

When Brossel and Kastler proposed the intermarriage of optical spectroscopy and radiofrequency resonance nearly four decades ago, they triggered off the development of numerous novel spectroscopic methods— a development that continues even today. The first generation of optical pumping and optical double resonance experiments not only yielded a wealth of very precise data on atoms, molecules, condensed matter, and nuclei alike, but they also initiated the detailed study of the interaction of atoms with radiation, particularly under the aspects of coherence, and of the modification of atomic states through radiation fields. 

Figure 23. Photoluminescence region for the study of BaO in a microwave-optical double resonance experiment. BaO is produced when barium vapor entrained in argon reacts with oxygen. The microwave radiation is introduced with a horn radiator. Two photomultipliers view the photoluminescence region through a combination of filters and through a double monochromator. After Ref. 143.

For optical double resonance experiments two lasers are required which are tuned to different molecular transitions sharing a common level. In Fig. 5 a-c three possible double resonance schemes are illustrated which can be applied to various problems in molecular spectroscopy. 

Brossel and Bitter (1952) for optical double-resonance experiments on the 6 level of... 

Comparison with the Zeeman effect in optical spectroscopy. In optical double-resonance experiments the Doppler shift due to the motion of an atom through the r.f. magnetic field is negligible in comparison with the natural width of the excited levels. Substituting into equation (16.21) a typical atomic lifetime of 10" s leads to a line-width for the magnetic resonance signal, at low r.f. power of... 

Sensitivity of optical double-resonance experiments. In conventional solid state magnetic resonance experiments the necessary population difference between the states is created by the Boltzmann factor exp(jj.B/kT) and is enhanced by working at low temperatures and high field strengths. 

By contrast optical double-resonance experiments can be performed with vapour densities as low as 10° atoms cm". This great increase in sensitivity is due to the high atomic polarization achieved by optical excitation combined with the fact that in these experiments the absorption of an r.f. quantum triggers the detection of a visible or ultraviolet quantum whose energy is some 10° - 10 times greater. Optical double-resonance experiments can therefore be performed on samples containing only a few milligrams of mass-... 

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