Using the 002 reflection of a LiF single crystal, we have observed phase-matched parametric down conversion of 8-keV ($\mathrm{Cu}-K\alpha$) x rays into single photons at 7.7 keV; the corresponding idler photons are in the extreme vacuum ultraviolet at 335 eV. Agreement well within the estimated experimental uncertainty is found with theory, which predicts that this process can provide a means for the direct determination of valence-electron charge distributions.

• Received 15 December 1980

DOI:https://doi.org/10.1103/PhysRevLett.46.1127