The mean value and the distribution of the penetration depth of F atoms is determined from samples composed of three layers with controlled thickness in the monolayer range and with a test of the compactness of the films via the intensity of surface excitons. F atoms with an average kinetic energy of 4.3 eV are generated in the top layer (Ar doped with F-2) by photodissociation of F-2 with 10.15 eV in a spin forbidden repulsive state. The F atoms are injected into an Ar spacer layer of variable thickness, Those reaching the interface to the Kr bottom layer are monitored via the intensity of the Kr2F fluorescence at a wavelength of 444 nm, which allows one to discriminate between F in Ar (439 nm), in Kr (453 nm), and at the Kr/Ar (444 nm) interface. The F content at the interface is kept below 1/20th of a monolayer to suppress recombination, and the detection sensitivity is increased to 1/1000th of a monolayer by excitation via Kr exciton energy transfer. The probability for F atoms to penetrate the Ar spacer layer decreases exponentially with increasing thickness down to 10% for a thickness of 23 monolayers, and an average penetration depth of 10 monolayers is derived. These very large penetration depths exceed those of F+ and F-ions by more than one order of magnitude. They are consistent with those molecular dynamics calculations, which predict a rather rectilinear motion in channels of the Ar lattice. An average length of navel of up to 27 monolayers with a mean-free path (large angle scattering) up to four monolayers is compatible with the results. (C) 1997 American Institute of Physics. [S0021-9606(97)51447-4].