ABSTRACT
This chapter discusses radiatively excited regions, which contain mainly ions and electrons and a few atoms. Molecules cannot exist with appreciable abundances within these intensely ultraviolet-radiated regions. The differences introduced by considering the effects of the presence of molecules in the initially non-ionized gas are not sufficient to justify the extra complexities. Many – but not all – of the most important features of radiatively excited regions arise from the presence of hydrogen, the most abundant element. It is, therefore, instructive to consider first the physics of radiatively excited regions of pure hydrogen. Hydrogen atoms in any energy level can undergo photoionization, provided photons of sufficient energy are available. The gas in a radiatively excited region is almost fully ionized. Bremsstrahlung (or free–free) radiation is produced by the acceleration of electrons in the fields of the positive ions. In principle, the simplest method of measuring the electron temperature is to look at the width of an emission line.
