Knowledge Bank

Electronic excitation of an insulator leads in general to a significant rearrangement of the lattice which can induce even a displacement of atoms like in the color center formation. For even stronger changes in the equilibrium coordinates an atom in the excited center can gain significant energy thus being photomobilized. The distance where it comes at rest again due to dissipation of its kinetic energy corresponds to its penetration depth and a mean range can be derived. Exceptionally large penetration depths were predicted for photomobilized F atoms in a rare gas lattice [1] and a new direct and reliable technique will be presented for the determination of the mean range [2]. $F2$ molecules are dissociated in a doped Ar layer of typical 5nm thickness by means of synchrotron radiation with a photon energy of 10.15 eV. F atoms gain a kinetic energy of about 4.2 eV and part of them can cross a spacer layer of pure Ar and some of them will reach the interface between the Ar spacer and the Kr substrate. The thickness of the Ar spacer is varied with monolayer accuracy from 0 up to 10 nm and the penetration depths are derived from the decreasing number of F atoms at the interface with increasing spacer thickness for a complete dissociation of $F2$. The intensity of the $Kr2$ F emission, which is characteristic for the interface, delivers the amount of F atoms and the sensitivity is enhanced considerably by using energy transfer from excitons of the Kr substrate. Indeed, larger penetration depths with mean values corresponding to eight nearest neighbor distances are observed and the results are discussed with respect to the molecular dynamics simulations, angular distributions in the plan as system, homogenity and ordering of the sample structure and temperature effects.