Brownian friction of gas molecules on the graphite surface

A calculation of the Brownian phononic friction coefficient of an isolated adatom on a crystal surface is formulated and then performed for Kr and N-2 molecules on the {0001} graphite surface. The fluctuating forces on the ad atom are expressed in terms of the normal modes of vibration of the substra te atoms. Then, the force autocorrelation functions and friction coefficien ts are calculated. The graphite phonon frequencies and polarization vectors are calculated for a thick slab using a generalized bond-charge model. The re is a strong spatial dependence of the friction coefficient. [S0163-1829( 99)13015-7].