A photodissociation region (or
photon-dominated region or PDR or
photodissociation front)
is a region where
photodissociation of molecular hydrogen
is taking place, resulting in atomic hydrogen, along a front where
ultravioletEMR from a nearby source
is sufficiently intense to warm the molecules
to the point where the balance favors atomic hydrogen.
They form a boundary between
an HII region and a molecular cloud.
Other molecules of the molecular cloud can also photodissociate,
providing some of the spectral signatures used to identify
and analyze PDRs.