ABSTRACT
In the interstellar medium, gas atoms and molecules continually collide with the dust particles. They may either rebounce from the grain surface or stick. In equilibrium, the number of accreted atoms per unit time is equal to the number of atoms leaving the grain. Consequently, large grains hardly grow any bigger through accretion, although the mean grain size, which is the average over the total distribution, increases. To describe the collision between an atom on the grain surface and an approaching gas atom, we first derive their interaction potential. The absolute value of the van der Waals potential increases rapidly at short distances, much quicker than for monopoles. But when the two dipoles are very close, a repulsive interaction sets in because the electrons tend to overlap and Pauli’s exclusion principle forbids them to occupy the same quantum state.
