By modeling diffusion-controlled exciton energy transfer to atoms at the interface or surface of rare gas films it is shown that about 10% of the available light flux can be funneled to a coverage of the order of 1/100 of a monolayer. Analytical expressions for the transfer efficiency with respect to absorption coefficient, diffusion length, boundary condition, and the linear range of detection probability are presented and applied to F atoms in a Kr/Ar interface. Transfer efficiencies, exciton diffusion lengths and densities are derived from the measured spectral and thickness dependences of the Kr-2* and Kr2F fluorescence. This system provides nearly optimal parameters, and the potential for penetration depth measurements of atoms is illustrated.