Surface x-ray-diffraction study of the $\mathrm{Rh}\mathrm{}\left(111\right)+(2\mathrm{}\times 2)-3\mathrm{CO}$ structure

We have studied the geometry of the high-coverage $\mathrm{Rh}\mathrm{}\left(111\right)+(2\mathrm{}\times 2)-3\mathrm{CO}$ structure by surface x-ray diffraction. Analysis of the in-plane data set reveals three evenly separated CO molecules per $(2\mathrm{}\times 2)$ unit cell. The evaluation of the crystal truncation rods shows that one CO molecule resides in an on-top site while the other two CO molecules occupy hollow sites. The intensity modulations of the out-of-plane fractional order rods provide geometrical information about distances between the C and O atoms and on the buckling of the CO overlayer.