Abstract
Starting from a master equation with the transition probabilities calculated quantum mechanically for one-phonon cascades, we derive via a Kramers-Moyal expansion the classical Kramers's equation and also van Kampen's linear Fokker-Planck equation for physisorption. The friction coefficient is calculated in terms of the microscopic parameters of the gas-solid system; it agrees with the result from a classical model. The macroscopic laws of friction for an adsorbing or desorbing gas particle are derived and solved establishing the time scales for the intermediate quasiequilibration and for the overall adsorption and desorption processes.