Heating of Interstellar H i Clouds by Ultraviolet Photoelectron Emission from Grains

The physical processes and consequences of photoemission from interstellar grains in H 1 regions are investigated. For likely grain materials except ice, the photoemission is expected from laboratory data to be relatively efficient for photon energies of 10-13.6 eV. It causes these interstellar grains to be positively charged in H 1 regions if the galactic ultraviolet flux is that indicated by recent observations. The ejected photoelectrons represent an energy input into interstellar H I clouds that is comparable to that from proposed fluxes of low energy cosmic rays or X-rays. Photoemission alone can maintain gas temperatures near 50 K in "standard" H I clouds. The exact value of the temperature depends on the grain density in the gas and the possible underabundance of carbon.