Microwave Spectroscopy


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Microwave spectroscopy

The study of the interaction of matter and electromagnetic radiation in the microwave region of the spectrum. See Spectroscopy

The interaction of microwaves with matter can be detected by observing the attenuation or phase shift of a microwave field as it passes through matter. These are determined by the imaginary or real parts of the microwave susceptibility (the index of refraction). The absorption of microwaves may also trigger a much more easily observed event like the emission of an optical photon in an optical double-resonance experiment or the deflection of a radioactive atom in an atomic beam. See Molecular beams

At room temperature, the relative population difference between the states involved in a microwave transition is a few percent or less. The population difference can be close to 100% at liquid helium temperatures, and microwave spectroscopic experiments are often performed at low temperatures to enhance population differences and to eliminate certain line-broadening mechanisms. The population differences between the states involved in a microwave transition can also be enhanced by artificial means. When the molecules or atoms with inverted populations are placed in an appropriate microwave cavity, the cavity will oscillate spontaneously as a maser (microwave amplification by stimulated emission of radiation). See Maser

The magnetic dipole and electric quadrupole interactions between the nuclei and electrons in atoms and molecules can lead to energy splittings in the microwave region of the spectrum. Thus, microwave spectroscopy has been used extensively for precision determinations of spins and moments of nuclei. See Hyperfine structure, Nuclear moments

The rotational frequencies of molecules often fall within the microwave range, and microwave spectroscopy has contributed a great deal of information about the moments of inertia, the spin-rotation coupling mechanisms, and other physical properties of rotating molecules. See Molecular structure and spectra

The magnetic resonance frequencies of electrons in fields of a few thousand gauss (a few tenths of a tesla) lie in the microwave region. Thus, microwave spectroscopy is used in the study of electron-spin resonance or paramagnetic resonance. See Magnetic resonance

The cyclotron resonance frequencies of electrons in solids at magnetic fields of a few thousand gauss (a few tenths of a tesla) lie within the microwave region of the spectrum. Microwave spectroscopy has been used to map out the dependence of the effective mass on the electron momentum.

For other applications See Atomic clock.

Microwave Spectroscopy

 

a branch of radio spectroscopy in which the spectra of substances are studied in the centimeter and millimeter bands (microwaves or superhigh frequencies). Since most band and band-inversion spectra of molecules, which cannot be observed in solids and liquids, lie in this range, microwave spectroscopy is often identified with the radio spectroscopy of gases.

Microwave spectroscopy is an effective method of physical and chemical research. Measurement of the frequencies of the band spectra of molecules makes possible highly accurate determination of the structure of molecules, and also the study of the nature of the chemical bond. The band absorption spectrum of a molecule depends on its configuration (that is, on whether the molecule is a linear, spherical, symmetric, or asymmetric rotating body). The band spectrum of any molecule may be calculated if its moments of inertia, which depend on the configuration and dimensions of the molecule, are known. Comparison of theoretically computed band spectra of molecules with experimentally observed spectra makes possible determination of the configuration of a molecule, the length of its bonds, and the angles between them.

The concept of the molecule as a rigid formation is approximate. The vibrations of the atoms that make up a molecule lead to splitting of the lines of the band spectrum and to the formation of fine structure. So-called l-doubling of lines is possible in the spectra of linear molecules and molecules of the symmetrically rotating type, and inversion splitting is possible in the spectra of asymmetrically rotating molecules, which have an inversion plane. For example, l-doubling spectra are observed in the HCN molecule, and the transitions between doubling levels fall in the band of wavelengths λ ˜ 3 mm. The ammonia molecule (NH3, ND3, NHD2) is the only molecule for which inversion splitting of energy levels is observed. The inversion spectrum of NH3 lies in the wavelength band λ = 1.3 cm, and the ND3 spectrum lies in the band λ ˜ 15–18 cm. Both of these molecules were used in the first lasers.

The hyperfine structure of molecular band spectra is caused by weak interactions of the electric and magnetic moments of atomic nuclei with one another and with the field generated by the electrons in the molecule. The quadrupole hyperfine structure of spectra is caused by the interaction of the nuclear quadrupole moment with the intramolecular electric field, and the magnetic hyperfine structure is associated with the interaction of the magnetic moments of nuclei with one another and with the magnetic field because of the rotation of the molecule as a whole. Observation of the quadrupole hyperfine structure yields information on the spin and quadrupole and magnetic moments of the nuclei that make up the molecule.

To investigate the band spectra of molecules, waves from an SHF generator are passed through a wave-guide cell filled with the gas to be studied. From there the waves strike a detector, whose signal is fed to a recorder (for example, an oscillograph). The detector signal is proportional to the power absorbed in the waveguide. By smoothly changing the generator frequency, the resonance frequency ν and the degree (intensity) of absorption are determined. Cavity resonators, which have a higher quality factor, are sometimes used instead of a wave-guide cell. The shortcoming of resonator cells as compared to wave-guide cells is their narrow band range; a separate resonator must be designed for virtually every spectral line. To increase the sensitivity of radio spectroscopes, the line intensity is modulated by means of an electric or magnetic field. Modulation takes place as a result of line splitting in the electric field (the Stark effect) or magnetic field (the Zeeman effect).

Sufficiently powerful monochromatic generators (klystrons) exist in the SHF band; therefore, the resolving power of a radio spectroscope is determined by the width of the spectral line, which in a gas depends primarily on the Doppler effect and on collisions of molecules with one another and with the cell walls. The line width Δν, which is due to molecular collisions, can be reduced by lowering the pressure in the cell. It is usually of the order of 0.13 newtons per sq m (N/m2), or 10−3 mm of mercury (mm Hg), and Δν ˜ (1–5) × 104 hertz (Hz).

The method of molecular beams, in which there are virtually no collisions of molecules with molecules, is used to decrease the width of spectral lines. In this case the line width can be reduced to a value of approximately 103 Hz. This makes possible the observation not only of quadrupole but also of magnetic hyperfine structure. The use of molecular beams entails a decrease in line intensity. However, there are special methods that increase line intensity. Their essence is that the absorption coefficient of a wave is proportional to the difference in population of the energy levels between which the transition takes place. If the particles are “removed” from the upper energy level, or if the population of the lower level is decreased severalfold, then the intensity of the spectral line will increase by a factor of kT/hν (where T is the temperature of the gas, k is the Boltzmann constant, and is the energy of the absorbed quantum of the SHF electromagnetic field). In a molecular beam this can be accomplished by using nonuniform electric or magnetic fields, and in an equilibrium gas can be accomplished by means of auxiliary radiation.

REFERENCES

Townes, C, and A. Schawlow. Radiospektroskopiia. Moscow, 1959. (Translated from English.)
Gordy, W., W. Smith, and R. Trambarulo. Radiospektroskopiia. Moscow, 1955. (Translated from English.)

A. N. ORAEVSKII

microwave spectroscopy

[′mī·krə‚wāv spek′träs·kə·pē]
(spectroscopy)
The methods and techniques of observing and the theory for interpreting the selective absorption and emission of microwaves at various frequencies by solids, liquids, and gases.
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