Microwave/optical double resonance principles

Figure 11.1. (a) Principles of the microwave/optical double resonance method, (b) Change of polarisation of fluorescent light resulting from AM= 1 radiofrequency transitions. 

Figure 11.10. Principles of microwave/optical double resonance, permitting the observation of rotational transitions in either the ground or excited electronic state [16]. The ground state levels are in thermal equilibrium with the heat bath, and it is assumed that when the molecule in the excited state spontaneously emits a photon, it enters the heat bath rather than returning to the optically-depleted ground state level.

An important development in microwave/optical double resonance, called microwave/optical polarisation spectroscopy, was described by Ernst and Torring [42], The principles of this technique are illustrated in figure 11.22. A linearly polarised probe beam from a tunable laser is sent through the gas sample and a nearly crossed linear polariser, before its final detection. Polarised microwave radiation resonant with a rotational transition in the gas sample is introduced via a microwave horn as shown, and resonant absorption results in a partial change in polarisation of... 

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].

Two of the three constants eqQaa, eqQbb id eqC c were determined from microwave optical double resonance (MODR) spectra of NH2 [1 to 4], microwave absorption spectra of ND2 [5], and by ab initio (UHF) calculations [6]. The third constant follows from eqQaa+ eqQbb + QQcc = 0 [3]- The nuclear quadrupole coupling constants listed in the following table are given in terms of the principle axis system (b C2, c 1 molecular plane). 

These latter discussions in 1. and 2. show that effects of spin-lattice relaxation can in principle be studied by an investigation of the temperature dependence of decay properties, even when the spectral resolution is one order of magnitude smaller than the zero-field spHtting of the triplet. (Compare also Sect. 3.1.) However, since more adequate methods, such as the techniques of microwave double resonance are available, one should use these latter methods with preference. (See the reviews [32,90,130]). But when the optical resolution is sufficient due to a larger zero-field spHtting, as is found for Pt(2-thpy)2, optical investigations of effects of spin-lattice relaxation become highly successful [24,65] and thus will represent the methods of preference as will be shown in detail in Sects. 4.2.7 to 4.2.9. 

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