Most of the observable matter in the Universe is ionized plasma. The two main sources of ionization are collisional ionization due to electron impact as discussed in Chapter 5, and photoionization due to a radiative source. Among the prominent radiation sources we discuss in later chapters are stars and active galactic nuclei. The nature of these sources, and physical conditions in the plasma environments activated by them, vary considerably. The photoionization rate and the degree of ionization achieved depends on (i) the photon distribution of the radiation field and (ii) the cross section as a function of photon energy. In this chapter, we describe the underlying physics of photoionization cross sections, which turns out to be surprisingly full of features revealed through relatively recent experimental and theoretical studies. Theoretically, many of these features arise from channel coupling, which most strongly manifests itself as autoionizing resonances, often not considered in the past in the data used in astronomy. The discussion in this chapter will particularly focus on the nearly ubiquitous presence of resonances in the cross sections, which later would seen to be intimately coupled to (e + ion) recombination (Chapter 7).

The interaction of photons and atoms inducing transitions between bound states has been discussed in Chapter 4. Here we describe the extension to the bound–free transitions. We first revisit a part of the unified picture of atomic processes in Fig. 3.5.