Microwave electric resonance spectra

The structure of the water dimer found by Dyke et al. [1977] from microwave electric resonance spectra of a supersonic molecular beam is... 

The pure microwave rotational spectrum of LiO was measured seventeen years later by Yamada, Fujitake and Hirota [118], and will be discussed in chapter 10. Their conclusions were generally in agreement with those based on the earlier electric resonance spectrum, except for a reassignment of some of the transitions. 

Nitric oxide, NO, is a chemically stable molecule and not surprisingly has been studied extensively by a range of techniques. Its microwave and far-infrared laser magnetic resonance spectra are discussed in chapter 9. These involve an understanding of both the zero-field levels and also the interactions with an external magnetic field. The pure microwave and millimetre wave spectra are described in chapter 10, but they provide information, which we will use, relevant to the radiofrequency electric resonance spectrum described in this section. 

The a3 n state of CO was first identified through its ultraviolet emission spectrum to the ground state, producing what are now known as the Cameron bands [160, 161, 162], Its radioffequency spectrum was then described by Klemperer and his colleagues in a classic series of molecular beam electric resonance experiments. Its microwave rotational spectrum was measured by Saykally, Dixon, Anderson, Szanto and Woods [163], and the far-infrared laser magnetic resonance spectrum was recorded by Saykally, Evenson, Comben and Brown [164], In the infrared region both electronic... 

Figure 9.5. Observed microwave magnetic resonance spectra of A 02 (top) and1A SO (bottom). The microwave frequency was close to 10 GHz in both cases. The top spectrum is obtained by magnetic field modulation, the bottom by electric field modulation.

Several molecules with 3 A. ground states have been studied by both microwave and far-infrared laser magnetic resonance they include O2, SO and SeO. In O2 the observed transitions are necessarily magnetic dipole, and they are frequently used to calibrate the sensitivity of a FIR laser magnetic resonance spectrometer. The other species have electric dipole transitions, and we shall illustrate the situation by describing the studies of SO carried out by Carrington, Levy and Miller [56], SO was also one of the first free radicals to be studied by pure microwave methods, which we will describe in chapter 10. The analysis of the magnetic resonance spectrum actually made use of the parameters determined earlier by pure microwave studies. SO is an easy radical to study experimentally since it is relatively unreactive and has a lifetime of several... 

There can be no question that the most important species with a 3 E ground state is molecular oxygen and, not surprisingly, it was one of the first molecules to be studied in detail when microwave and millimetre-wave techniques were first developed. It was also one of the first molecules to be studied by microwave magnetic resonance, notably by Beringer and Castle [118]. In this section we concentrate on the field-free rotational spectrum, but note at the outset that this is an atypical system O2 is a homonuclear diatomic molecule in its predominant isotopomer, 160160, and as such does not possess an electric dipole moment. Spectroscopic transitions must necessarily be magnetic dipole only. 

The radiofrequency spectrum of phosphine has been measured in a molecular beam electric resonance spectrometer. The suspected inversion doubling was not observed its dipole moment (ju.) was 0.574 D. The calculated rotational barrier between the staggered and eclipsed conformers of methylphosphine is 1.83 and 1.71 kcal mol, in agreement with the experimental value of 1.96 from microwave measurements. An orbital-by-orbital analysis of the changes which occur upon rotation suggests a hydrogen-bond contribution when the phosphorus lone pair of electrons and a CH bond are appropriately orientated.The existence of a 1—2° tilt of a methyl group towards the phosphorus lone pair of electrons in methylphosphines (138) was a conclusion drawn from a microwave study... 

Inversion splitting of the vibrational spectrum of ammonia has been used to create the first molecular microwave amplifier (maser) [86, 87]. The inversion population in the ammonia maser is achieved by transmission of the molecular beam through a non-homogeneous electric field. Ammonia molecules in symmetric and antisymmetric states interact with the electric field in different ways and they are therefore separated in this field. They are then directed to the resonator. 

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