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This chapter develops the theory of the hyperfine structure of atoms involving nuclear magnetic dipole and electric quadrupole moments. The Zeeman effect in weak, intermediate, and strong magnetic fields is considered. The experimental measurement of the hyperfine structure of ground state atoms by the techniques of optical pumping, atomic beam magnetic resonance, and optical double resonance is explained. The caesium beam atomic clock, the importance of hyperfine structure experiments in hydrogen, and the investigation of hyperfine structure of excited states are discussed.

The earliest magnetic resonance experiments were with non-paramagnetic molecules in ¹Σ states which have no net electronic magnetic moment. They were later applied to paramagnetic atoms, which gave multiple energy levels due to the interactions of different electron and nuclear orientation states from which the hyperfine separations, Δν_hfswere obtained. The paramagnetic atoms also permitted measurements of the electron magnetic moment and the magnetic moment due to the electrons orbital motion. To obtain the correctly calibrated values of nuclear magnetic moments, one needs to know the absolute value of both the frequency and the applied magnetic field as briefly discussed in Chapter 6. There are well known procedures for frequencies, but the problem is much more difficult for magnetic fields. In the first experiments the magnetic fields were calibrated with a conventional ballistic galvanometer. To obtain better precision, the next experiments measured the fields in terms of the magnetic moment of the electron, which was presumed to have the exact value predicted by Dirac. However, atomic beam experiments later showed this assumption was incorrect, but the fields could be correctly calibrated in terms of the orbital magnetic moment of the electron being one Bohr magneton. Nuclear magnetic moments are accurately expressed in nuclear magnetons in the tables of Chapter 6 and values of the hyperfine separations are given Chapter 9. The discovery that the hyperfine structure for atomic hydrogen differed from the predicted value stimulated the development of QED and the discovery of the anomalous electron magnetic moment. Accurate measurements of Δν_hfs, are given in tables for many atoms.

This chapter describes the principles of major nonlinear and ultrahigh resolution laser spectroscopic techniques, including saturation spectroscopy, two-photon spectroscopy, coherent Raman spectroscopy, nonlinear polarization spectroscopy, and laser cooling and trapping spectroscopy. All these spectral techniques are based on the use of one or several laser beams (at least one of which is tunable), and there is no need for any ordinary spectrometers or dispersion elements (such as prisms, gratings, or Fabry–Perot etalon). The most remarkable advantages of these novel spectroscopic techniques are their Doppler-free capability and ultrahigh spectral resolution, which enable researchers to measure hyperfine structures, isotope shifts, Stark and Zeeman splitting, and to establish new optical frequency standards (atomic clocks) as well.

This chapter discusses the physics of the hydrogen atom and the electron-positron bound state positronium. It describes the energy levels of these atoms, including the fine structure and hyperfine structure. It discusses the lifetimes of the two species of positronium.

This chapter provides a broad overview of the spectroscopic principles required in order to perform quantitative spectroscopy of atmospheres. It couples the details of atmospheric spectroscopy with the radiative transfer processes and also with the assessment of rotational, vibrational, and electronic spectroscopic measurements of atmospheres. The principles apply from line-resolved measurements (chiefly microwave through infrared) through ultraviolet and visible measurements employing absorption cross sections developed from individual transitions. The chapter introduces Einstein coefficients before in turn discussing rotational spectroscopy, vibrational spectroscopy, nuclear spin, and electronic spectroscopy.