The physics of photodissociation regions

Photodissociation Regions (PDRs) are gas phases in which ultraviolet radiation plays a role in the heating or chemistry. The physics of PDRs determines the emitted radiation and physical conditions in both the diffuse atomic interstellar medium and the dense molecular phases. High energy laboratory experiments can provide constraints on the survival of small grains which dominate gas heating and in the interaction of X-rays with gas and grains.