Circumstellar disks are exposed to intense ultraviolet (UV) radiation from the young star. In the inner disks, the UV radiation can be enhanced by more than seven orders of magnitude compared with the average interstellar radiation field, resulting in a physical and chemical structure that resembles that of a dense photon-dominated region (PDR). This intense UV field affects the chemistry, the vertical structure of the disk, and the gas temperature, especially in the surface layers. The parameters which make disks different from more traditional PDRs are discussed, including the shape of the UV radiation field, grain growth, the absence of PAHs, the gas/dust ratio and the presence of inner holes. Illustrative infrared spectra from the Spitzer Space Telescope are shown. New photodissociation cross sections for selected species, including simple ions, are presented. Also, a summary of cross sections at the Lyman α 1216 Å line, known to be strong for some T Tauri stars, is made. Photodissociation and ionization rates are computed for different radiation fields with color temperatures ranging from 30 000 to 4000 K and grain sizes up to a few μm. The importance of a proper treatment of the photoprocesses is illustrated for the transitional disk toward HD 141569A which includes grain growth.
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