OH in the X 2 n ground state

We have already discussed the high-resolution spectroscopy of the OH radical at some length. It occupies a special place in the history of the subject, being the first short-lived free radical to be detected and studied in the laboratory 1 microwave spectroscopy. The details of the experiment by Dousmanis, Sanders and Townes [4] were described in section 10.1. It was also the first interstellar molecule to be detected by radio-astronomy. In chapter 8 we described the molecular beam electric resonance studies of -doubling transitions in the lowest rotational levels, and in chapter 9 we gave a comprehensive discussion of the microwave and far-infrared magnetic resonance spectra of OH. Our quantitative analysis of the magnetic resonance spectra made use of the results of pure field-free microwave studies of the rotational transitions, which we now describe. 

J = 3/2, 5/2 and 7/2 levels of both fine-structure states. Also shown are the A-doublet transitions observed, first by Dousmanis, Sanders and Townes [4], and subsequently by ter Meulen and Dymanus [165] andMeertsandDymanus [166]. The later studies [166] used molecular beam electric resonance methods which were described in chapter 8, and the most accurate laboratory measurements of transitions within the lowest rotational level were those of ter Meulen and Dymanus [165] using a beam maser spectrometer, also described in chapter 8. In the years following these field-free experiments, attention 

It should not be thought that OH always exhibits the unusual behaviour described above the recent developments in tunable far-infrared sources have had an important impact in astronomy, so that interstellar rotational transitions can now be observed. We described an airborne far-infrared telescope in the first part of this chapter, and figure 10.60 shows two examples of interstellar OH rotational transitions, observed by Watson, Genzel, Townes and Storey [170]. 

All of the high quality data for the OH radical has been combined by Brown, Zink, Jeimings, Evenson, Hinz and Nolt [68], building on an earher analysis of the laser magnetic resonance spectrum [171] and more recent work by Varberg and Evenson [172], to produce a current best set of field-free molecular constants for the OH radical. These are presented in table 10.16. The constants are defined by the following effective Hamiltonian which has been described extensively elsewhere in this book  

The CH radical is the simplest hydrocarbon and its rotational or X-doublet spectrum has been sought by many. The first detection of rotational transitions was a triumph for far-infi ared laser magnetic resonance the experiments carried outby Evenson, Radford and Moran [173] were described in detail in chapter 9. The ri-doublet transition in the lowest rotational level was first observed through radioastronomy by Rydbeck, Ellder, Irvine, Sume and Hjalmarson [ 174]. It was almost a further ten years before laboratory observations of the field-free spectrum were reported. 

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