Radio-Frequency Spectroscopy

the aggregate of methods that are based on the resonance absorption of radio waves and are used to investigate the structure of a substance and the physical and chemical processes in it. Radio-frequency spectroscopy studies matter in the solid, gaseous, and liquid states. A number of investigations of the structure of atoms and molecules have been made by means of molecular and atomic beams, where interaction between particles is essentially absent. Radio-frequency spectroscopy differs from optical spectroscopy, infrared spectroscopy, and Mössbauer gamma-ray spectroscopy (seeMÖSSBAUER EFFECT) in the small energies of the absorbed quanta. Consequently, it is able to study fine interactions in a substance that cause the splitting of energy levels into very closely spaced levels. Another advantage of radio-frequency spectroscopy is that by simultaneously irradiating a substance with radio waves of several different resonance frequencies, the relative population of energy levels can be changed and transitions masked in the optical band by secondary interactions can be observed.

A number of separate areas of research exist in radio-frequency spectroscopy. Microwave spectroscopy, for example, investigates transitions between energy levels that are due to the rotational motions of molecules possessing a permanent electric dipole moment; to the fine structure of the vibrational levels that results from inversion in molecules such as ammonia (seeMOLECULAR GENERATOR); or to the fine structure of rotational levels that is connected with the interaction of nuclear quadrupole moments with inhomogeneous molecular electric fields. Since the free rotation of molecules is inhibited in liquids and solids, gases are investigated in microwave spectroscopy. Resonance absorption is usually observed in the frequency range between 10¹⁰ and 10¹¹ hertz (Hz).

Another area of research is nuclear magnetic resonance (NMR). NMR is the resonance absorption of radio waves that is due to the transitions induced between energy levels by the interaction of the magnetic moments of the nuclei with an external magnetic field H. The frequency of these transitions is ω = _γH, where γ is the ratio of the magnetic moment of the nucleus to its spin. In a field H = 10⁴ gauss (G), NMR is observed between 1 and 50 MHz. In the observed NMR spectrum, the lines are broadened and split by the interaction of the nuclei with each other and with the electron shells. In solids, the NMR spectrum is due primarily to the direct interaction between the magnetic dipole moments of the nuclei; for nuclei with spin I > ½, the spectrum is also due to the interaction of electric quadrupole moments of the nuclei with inhomogeneous molecular and crystal electric fields. In nuclear quadrupole resonance, these magnetic transitions are observed in the absence of an external magnetic field. The width of an NMR spectral line for a solid is about 10⁴ Hz, and in this case we speak of low-resolution NMR. High resolution is possible with liquids and gases, where the thermal motion of the particles averages the interactions mentioned above. As a result, the NMR line is sharply narrowed—for example, to 10^–2 Hz in pure organic liquids. In this case, the spectrum is determined by the magnetic fields of the electron shells and by indirect interaction between the nuclear spins through the electron shells.

A third area of research is electron paramagnetic resonance (EPR). EPR is the resonance absorption of radio waves that is due to transitions induced between levels by the interaction of an external magnetic field H with the magnetic moments of unpaired electrons of atoms, ions, and free radicals or with the magnetic moments of charge carriers in metals and semiconductors. The EPR frequency is proportional to the external field. For example, when H = 10⁴ G, ω ~ 10^l0–10¹¹ Hz. In the observed EPR spectrum the lines are broadened and split by the interaction of the electrons with the internal fields in crystals, with the electronic environment in free radicals, and with conduction electrons in metals and semiconductors. Additional splitting of an EPR line may result from the interaction of the electrons with nuclei having magnetic moments.

Cyclotron resonance is the subject of a fourth area of research. It is observed in metals and semiconductors placed in a magnetic field H when the frequency of the wave coincides with the cyclotron frequency of the charge carriers. Cyclotron resonance is due to transitions between orbital levels of the conduction electrons formed by the levels’ interaction with the field H. The cyclotron resonance spectrum in metals is determined by the energy spectrum of the conduction electrons; in semiconductors it is determined by the band structure and by the concentration, mobility, and effective mass of the electrons and holes.

Radio-frequency spectroscopy also makes use of ferromagnetic resonance, ferrimagnetic resonance, and antiferromag-netic resonance. In magnetically ordered media there is observed a resonance absorption of radio waves that is associated with the collective motion of the magnetic moments of the electrons. The range of resonance frequencies is usually from 10¹⁰ to 10¹³ Hz. The spectrum is determined by the interaction of the electrons with the external magnetic field, by the effects of crystalline anisotropy, and by the demagnetizing factors; in antifer-romagnetics the exchange interaction is also important here.

The methods of radio-frequency spectroscopy are used to study molecular structure; the character of molecular motion in liquids and solids; chemical kinetics; the mechanism of chemical reactions; the dependence of reactivity on molecular and stereochemical structure (NMR and EPR); the energy spectrum and properties of semiconductors and metals (NMR, EPR, and cyclotron resonance), of magnetic substances (ferromagnetic resonance), and of antiferromagnetic substances (an-tiferromagnetic resonance); biological processes; and physiologically active substances (NMR and EPR). NMR and EPR are used to monitor and control industrial chemical processes. Radio-frequency spectroscopes and spectrometers are used to investigate, for example, EPR and NMR spectra.