Ethane trapping in a secondary-layer state on Pt(111) was studied with supersonic mol. beam techniques in order to investigate the dynamics of extrinsic precursor adsorption. Initial trapping probabilities of ethane on an ethane covered Pt(111) surface were measured directly as a function of incident translational energy and incident angle at a surface temp. of 95 K. At all incident translational energies and angles, the initial trapping probability into the second-layer state is higher than on a clean surface. In addn., the initial trapping probability into the second layer decreases less with incident translational energy than the initial trapping probability onto the clean surface. In contrast to previous findings for non-dissociative weak adsorption on clean surfaces showing the initial trapping probability to increase with incident angle, the initial trapping probability into the second layer is independent of incident angle indicating "total" energy scaling. A dynamical corrugation of the adsorbed layer is postulated to rationalize this strong deviation from the "normal" energy scaling implicit in one-dimensional theories of trapping.