By virtue of being immersed in a plasma and radiative environment, dust grains in a dusty plasma are necessarily electrically charged. While it has long been recognized that secondary and photoelectric emission play an important role in determining the equilibrium grain potential, the fact that the grain size plays a crucial role in this potential (when either of the two charging processes is important) has not been widely appreciated. We will show that the secondary electron yield and the photoelectric yield both increase as die size of the grain decreases. It will also be shown that this size effect is most pronounced when the grain size is on the order of the penetration depth of the incoming electrons (for secondary electron emission due to electrons) or photons (for photoemission). Our theoretical results will be compared with experimental data (where available) concerning grain charging.
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