To increase the x-ray optical contrast of Mo/Si multilayers, we study low energy hydrogen ion implantation of amorphous Si layers. Using elastic recoil detection and Rutherford backscattering spectrometry, we measure the result of hydrogen implantation on Si atomic density. We find a lowering of Si atomic density, and, thus, an enhancement of x-ray optical contrast, as a result of Himplantation. We find that the Si atomicdensitysaturates at a minimum of 6465% of the crystallinevalue. We have also observed aminorsmoothing effect of H1 ionbombardment. CombinedwithKr1ionbombardment, causing a very much larger smoothing of the Si surface, the atomic densityis found to saturate at a minimum of 7765%of thecrystallinevalue.