  We have investigated the structure of hydrogen-intercalated
quasi-free-standing monolayer graphene (QFMLG) grown on 6H-SiC(0001) by
employing total-reflection high-energy positron diffraction (TRHEPD). At least
nine diffraction spots of the zeroth order Laue zone were resolved along
<11-20> and three along <1-100>, which are assigned to graphene, SiC and higher
order spots from multiple diffraction on both lattices. We further performed
rocking curve analysis based on the full dynamical diffraction theory to
precisely determine the spacing between QFMLG and the SiC substrate. Our study
yields a spacing of d = 4.18(6)\r{A} that is in excellent agreement with the
results from density-functional theory (DFT) calculations published previously.
