There are several excellent books on physical processes in low density nebulae. The most recent one is by Osterbrook (1989), where high density processes in AGNs are also described. A complete description of atomic processes is given in Mihalas (1978). Comprehensive discussions of radiative transfer methods are given in a book edited by Kalkofen (1984, see Canfield et el. there for application to AGNs) and in a soon to be published book on Masers by Elitzur. Many of the theoretical aspects dealt with in this chapter have been discussed in several review papers, where a more complete lists of references can also be found. The situation prior to 1979 is summarized by Davidson and Netzer (1979). Later, exhaustive reviews are by Kwan and Krolik (1981), Ferland and Shields (1985) and Osterbrock and Mathews (1986). Some detailed papers, on more specific topics, are:
X-ray related processes: Halpern and Steiner (1983), Kallman and McCray (1982), Shull and Van Steenberg (1985, heating and ionization by secondary electrons ) and Ferland and Rees (1988).
Chemical composition: Shields (1976, broad lines, C/N and O/C), Uomoto (1984), Gaskell et al. (1981, metal deficiency in dusty BLRs), Wills, Netzer and Wills (1985, iron abundance), Binnette (1985, narrow lines in LINERs).
H- and molecules: The most recent and complete paper is by Ferland and Persson (1989), where references to earlier papers are given.
Very high densities: See Ferland and Rees (1988) and Rees, Netzer and Ferland (1989).
Continuum transfer: See Netzer and Ferland (1984) and references therein for a modified on-the-spot approximation. See Collin-Souffrin and Dumont (1989) for treatment of hydrogen n > 1 levels. Puetter and Hubbard (1985) describe the formal solution of the free-free radiation transfer.
The escape probability method: The basic formalism is given by Elitzur (1984). The local escape probability is discussed in Kwan and Krolik (1981), and in many later papers. Useful approximations for and more references can be found in Rees, Netzer and Ferland (1989). For Stark broadening see Puetter (1981). Continuum destruction is discussed in Netzer et al. (1985). Their treatment is however different from the one suggested by Hummer (1968) and at this stage it is not clear which is more applicable to AGN clouds. Puetter and collaborators (see references in Hubbard and Puetter 1985) discussed various aspects of the escape probability method, as well as some more complete transfer methods, as applied to AGN clouds. Most of their calculations are for a pure hydrogen case. A similar approach, including more elements, is described in Collin-Souffrin and Dumont (1986), where earlier references to such work can be found. Avrett and Loeser (1988) described an attempt to perform a complete transfer calculation for some lines, combined with a simplified treatment for many others. Numerical calculation of the scattering of resonance line photons, and references to earlier papers on the subject, can be found in Hummer and Kunasz (1980).
Line and continuum fluorescence: For the escape probability approximation of this process see Elitzur and Netzer (1984, lines), Netzer, Elitzur and Ferland (1985, line and continuum) and Lockett and Elitzur (1989, a critical discussion of the escape probability approximation). A more complete transfer approach to this problem is described by Weymann and Williams (1969) and Eastman and MacAlpine (1985). Line fluorescence in FeII is described in Netzer and Wills (1983), Wills, Netzer and Wills (1985) and Elitzur and Netzer (1984). For fluorescence with L see Penston (1987).
Complete photoionization calculations: There are quite a few of these, starting from the works of Davidson (1972) and MacAlpine (1972). Some recent ones are: Kwan (1984, BLR, dependences on density and ionization parameter), Mushotzky and Ferland (1984, BLR, dependence on ionization parameter), Netzer, Elitzur and Ferland (1985, BLR and NLR), Ferland and Osterbrock (1986, NLR), Netzer (1987, BLR, continuum shape, angle dependence and large ionization parameter), Krolik and Kallman (1988, continuum shape), Rees, Netzer and Ferland (1989, BLR, including very high densities), Ferland and Persson (1989, BLR, large ionization parameters and extreme column densities), Collin-Souffrin and Dumont (1990) and Dumont and Collin-Souffrin (1990, very low ionization parameter and high densities) and Korista and Ferland (1989, interstellar high ionization lines).