Grain Dynamics in Photodissociation Regions

We discuss the forces on grains exposed to anisotropic radiation fields, including the usual ``radiation pressure’’ force and also recoil forces due to photoemitted electrons, photodesorbed hydrogen atoms, and hydrogen molecules which form on the grain surface. We show that these forces can lead to grain dynamics in photodissociation regions which result in enhanced dust-to-gas ratios. Since the gas heating is probably dominated by photoelectric emission from dust, this might explain the unusually high gas temperatures inferred from ISO observations of molecular hydrogen in photodissociation regions.