Ultraviolet photons also eject electrons from interstellar dust grains; these electrons heat surrounding gas. The energy required to separate a single electron from the neutral grain surface, i. e., the analog of the ionization potential, is known as the work function and is about 6 eV for standard grain composition. The photons actually liberate liberate electrons about 100 A inside the grain (see Figure 7.5). Only about 10 percent of these make their way to the surface. Those that do must still overcome the work function and end up leaving the grain with energies of only about 1 eV. Comparing with the typical photon energy of 10 eV, we see that the net energy efficiency ePE is only about 0.01. The photoelectric process is nevertheless a major heating source for the gas because of the large grain cross section. We may write the associated rate rPE in terms of the grain number density and geometric cross section:
Was this article helpful?