Microwave and millimetre wave

In the microwave region tunable monochromatic radiation is produced by klystrons, each one being tunable over a relatively small frequency range, or a backward wave oscillator, tunable over a much larger range. Both are electronic devices. Absorption experiments are usually carried out in the gas phase, and mica windows, which transmit in this region, are placed on either end of the absorption cell, which may be several metres in length. Stark 

Region Source Absorption cell window Dispersing element Detector 

Microwave Klystron backward wave oscillator Mica None Crystal diode 

Millimetre wave Klyston (frequency multiplied) backward wave oscillator Mica polymer None Crystal diode Golay cell thermocouple bolometer pyroelectric 

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The dispersing element to be described in Section 3.3 splits up the radiation into its component wavelengths and is likely to be a prism, diffraction grating or interferometer, but microwave and millimetre wave spectroscopy do not require such an element. 

Both microwave and millimetre wave radiation can be channelled in any direction by a waveguide made from metal tubing of rectangular cross-section, the dimensions depending on the frequency range. The absorption cell is also made from waveguide tubing. 

Because of point 2, rotational microwave and millimetre wave spectroscopy are powerllil techniques for determining dipole moments. However, the direction of the dipole moment cannot be determined. In the case of 0=C=S, for which /r = 0.715 21 0.000 20 D [(2.3857 0.0007) x 10 ° C m], a simple electronegativity argument leads to the correct conclusion - that the oxygen end of the molecule is the negative end of the dipole. However, in CO, the value of 0.112 D (3.74 x 10 C m) is so small that only accurate electronic stmcture calculations can be relied upon to conclude correctly that the carbon end is the negative one. 

Since 1963 spectra of many molecules have been detected, mainly in emission but some in absorption. Telescopes have been constructed with more accurately engineered paraboloids in order to extend observations into the microwave and millimetre wave regions. 

A further feature of the spectmm in Figure 9.24 is the sharp spike at the centre of each P-shaped transition. The reason for this is that saturation of the transition has occurred. This was discussed in Section 2.3.5.2 in the context of Lamb dips in microwave and millimetre wave spectroscopy and referred to the situation in which the two energy levels involved, m(lower) and n(upper), are close together. Under these circumstances saturation occurs when... 

Microwave and millimetre wave 1.3 - 0.35 mm contains information on rotational transitions. 

Spectroscopy covers a very wide area which has been widened further since the mid-1960s by the development of lasers and such techniques as photoelectron spectroscopy and other closely related spectroscopies. The importance of spectroscopy in the physical and chemical processes going on in planets, stars, comets and the interstellar medium has continued to grow as a result of the use of satellites and the building of radiotelescopes for the microwave and millimetre wave regions. 

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. 

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. 

Microwave-microwave double resonance experiments similar to those described in this section have been carried out on Hj [96], Dj [97] and HeJ [98]. In these cases, however, the transitions studied are actually electronic transitions, despite being observed at microwave and millimetre-wave frequencies. We conclude, with regret, that they are beyond the scope of this book. 

The molecule exhibits complex rotational and rotation-vibrational spectra in the microwave and millimetre wave regions due to the internal rotation of the N2 and CO subunits. Since the behaviour of the complex considerably deviates from that of a semi-rigid rotor, the rotational energy levels are treated for different K stacks separately according to... 

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