We report a significant increase of the reflectivity of a soft x-ray Mo/Si multilayer mirror after low energy hydrogenion beam bombardment of each of the Si layers after deposition.Cross section transmission electron microscopy pictures indicate no significant qualitative difference in interface roughness between the two samples. Elastic recoil detection and Rutherford backscattering spectrometry reveal a concentration of 22 at. % of H in the ion beam bombarded Silayers and a 12% reduction of the Si atomic density. Calculations using the measured atomic density and a very simple roughness model agree with the measured reflectivities.This is the first report of the modification of atomic density of Si in order to change the x-ray optical constants of the Silayer.