The atomic physics of excitation, ionization, and recombination is the story of frolicking electrons – like space traveling aliens, these little leptons are busy jumping up and down when bound to their “home planet” atom/ion. Launching freely into space, they adventure out and engage in a series of friendly energy exchanges with fellow particles and photons in an expansive plasma. Landing on and being captured by some other random “planet” atom/ion, the cavorting continues. In this chapter, we follow the dynamic lives of electrons, photons, and ions and present an abridged review of the physics of collisional excitation, ionization, and recombination. We describe photoionization, Auger ionization, direct collisional ionization, excitation auto-ionization, radiative and dielectronic recombination, and charge exchange. We show that detailed balancing and reaction cross sections, rates, and rate coefficients are the heart of chemical-ionization modeling of absorbers. We then present the cosmic photoionization rate of HI, HeII, MgII, CIV, and OVI as a function of redshift. We conclude with a comprehensive treatment of the heating and cooling functions of astrophysical gas.

The hot ionized medium (HIM) represents gas at T ∼ 106 K and n ∼ 0.004 cm−3. It constitutes gas that has been shock-heated by supernova explosions, and which has not yet had time to cool by free--free emission. The properties of a spherically expanding shock front are described by the Sedov–Taylor solution; when radiative losses from the post-shock gas are large, the expanding supernova remnant transitions to the snowplow solution. The hot gas inside a supernova-blown bubble is in collisional ionization equilibrium, which permits a calculation of the ionization state of each element as a function of temperature. Emission lines from ionized iron and absorption lines of ionized oxygen (seen in absorption toward hot white dwarfs) provide information about the density and temperature of the hot gas in the Local Bubble within which the Sun lies.