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 H implantation. We find that the Si atomic density saturates at a minimum of 64±5% of the crystalline value. We have also observed a minor smoothing effect of H+ion bombardment. Combined with Kr+ion bombardment, causing a very much larger smoothing of the Si surface, the atomic density is found to saturate at a minimum of 77±5% of the crystalline value. ©1996 American Institute of Physics.