ABSTRACT
Most electronic transitions of interest fall into the visible and near-ultraviolet regions of the spectrum. This range of photon energies commonly corresponds to electrons being moved among valence orbitals. These orbitals are important to an understanding of bonding and structure, so are of particular interest in physical chemistry and chemical physics. For this reason, most of this chapter will concentrate on visible and near-UV spectroscopy, roughly the region between 200 and 700 nm, but there are no definite boundaries to the wavelengths of interest. Some of the valence orbitals will be so close in energy as to give spectra in the near-infrared region. Conversely, some valence transitions will be at high enough energy to lie in the vacuum ultraviolet, below about 200 nm, where air absorbs strongly and instrumentation must be evacuated to allow light to pass. In this region are also transitions of electrons to states of higher principal quantum number, known as Rydberg states. At still higher energies, in the x-ray region, are transitions of inner-shell electrons, and their spectroscopy has become an extremely useful tool, especially for studying solids and their surfaces. However, these other regions will not be covered in detail here.
