5 - Electron–ion collisions

Summary

In ionized plasmas spectral formation is due to particle collisions or radiative excitations. In astrophysical situations there is usually a primary energy source, such as nuclear reactions in a stellar core, illumination of a molecular cloud by a hot star or accretion processes around a black hole. The ambient energy is transferred to the kinetic energy of the particles, which may interact in myriad ways, not all of which are related to spectroscopy.

Electron collisions with ions may result in excitation or ionization. The former process is excitation of an electron into discrete levels of an ion, while the latter is excitation into the continuum, or ionization, as shown in Fig. 3.1 and discussed in Chapter 3. A practically complete description of the (e + ion) excitation process requires collisional information on the ions present from an observed astrophysical source, and for all levels participating in spectral transitions. As the excitation energy from the ground state to the higher levels increases, the ionization energy E_I is approached. The negative binding energy of the excited states increases roughly as E ~ –z²/n², where z is the ion charge. As n → ∞, E → 0, i.e., the electron becomes free.

At first sight, therefore, it might seem like a very large number of levels need to be considered for a given atomic system in order to interpret its spectrum completely.