Stokes formula and density perturbances for driven tracer diffusion in an adsorbed monolayer

We study the intrinsic friction of monolayers adsorbed on solid surfaces fr om a gas phase or vapor. Within the framework of the Langmuir model of delo calized adsorption, we calculate the resistance offered by the mobile adsor bate's particles to some impure tracer molecule, whose diffusive random mot ion is biased by a constant external force. We find that for sufficiently s mall driving forces the force exerted on the tracer shows viscouslike behav ior. We derive then the analog of the Stokes formula for two-dimensional ad sorbates, calculate the corresponding friction coefficient, and determine t he stationary particle distribution in the monolayer as seen from the drive n impurity. PACS numbers: 66.30.Lw, 05.40.-a, 68.45.Da.