ABSTRACT
Why should a book about molecular spectroscopy include a chapter on atomic spectroscopy? There are a number of reasons why we want to consider atomic spectra before moving on to the chemically more interesting subject of molecular spectra. Atoms present the simplest possible examples for applying some of the principles laid out in the previous chapters. In this chapter, we consider aspects of selection rules which also apply to more complicated systems. We are concerned with the electronic spectra of atoms, and we can concentrate on this topic without worrying about the rotational and vibrational structure that accompanies an electronic transition in a molecule. The absorption wavelengths of an atom coincide with those for emission, which is not necessarily the case in molecular electronic spectra, as we shall see. We also discuss topics such as the effect of internal fields (spin-orbit coupling and hyperfine effects due to nuclear spin) and external fields (the Stark and Zeeman effects) on atomic spectra. These concepts are relevant to molecular spectra as well, but are easier to discuss quantitatively in the atomic case. This chapter also provides an opening into the very interesting topic of laser emission, and we consider the operating principles at work in some common atomic lasers. Lasers are important not only because they exemplify stimulated emission, but also because they are frequently employed as sources in spectroscopic experiments. The field of atomic spectroscopy is an old one, but still of vital interest in fields ranging from analytical chemistry to quantum optics.
