In this chapter, we will examine the various line cooling processes of interstellar gas. Together with the heating processes, which are discussed in the next chapter, this will allow us to solve the energy balance for the gas in a wide variety of environments. This will be applied in HII regions (Section 7.3), HI regions (Section 8.2.2), photodissociation regions (Section 9.3), and molecular clouds (Section 10.3). This chapter starts off with a refresher on the concepts of electronic, vibrational, and rotational spectroscopy (Section 2.1). We will then discuss the cooling rate (Section 2.2) with the emphasis on two-level systems (Section 2.3). A two-level system analysis is a powerful tool for understanding the details of gas emission processes and we will encounter this many times in subsequent chapters. We will then consider in detail the cooling processes in ionized (Section 2.4) and neutral (Section 2.5) gas. The last section (Section 2.6) discusses the cooling law.

Spectroscopy

Table 2.1 summarizes typical properties of transitions. These are, of course, directly related to the binding energies of the species involved. Electronic binding energies for atoms increase from left to right in the periodic system from about 5 eV to some 20 eV. For hydrogen and helium the lowest electronic transitions are fairly high (10.2 and 21.3 eV, respectively); a substantial fraction of the ionization energy. Multi-electron systems have electronic orbitals that are low in energy compared with their ionization potentials.