Penetration depths of atoms with kinetic energy provided by photodissociation of parent molecules in the top layer of a multilayer sample are determined from the probability to cross a spacer layer of thickness d and to arrive at the interface to a substrate. Top layer growth up to a final thickness s corresponds to a continuous increase of the effective spacer layer thickness. Modeling of growth and comparison with sample-to-sample variation of d allows us to determine separately and in a consistent way the precursors' dissociation cross section q.sigma and the mean penetration depth d(0) of the fragments together with elimination of contaminated samples. For F atoms with 4.3 eV kinetic energy from F-2 dissociation values of q.sigma =3x10(-17) cm(2) and d(0)=2.1 nm (8 to 9 monolayers) are derived for Ar spacers. A strong increase of d(0) in the case of unintentional multistep excitation of F fragments is demonstrated.