An atomic beam diffraction study of a rare gas monolayer of square symmetry: Kr on (100) NaCl

Using cold helium atomic beam diffraction and atomic beam overlayer deposition, we have measured the structure of krypton physisorbed on the (100) face of NaCl crystals at temperatures below 50 K. Our data show that this system provides the first example of a rare gas overlayer growing with fourfold nearest neighbor coordination up to a complete monolayer coverage and beyond. The lattice mismatch with the substrate is 1.65%, resulting in the formation of coherent patches of about 30 atoms in diameter as derived from the width of the diffraction peaks. By means of close coupling intensity calculations, we have also determined the average location of the atoms in the direction perpendicular to the substrate. Because of the lattice mismatch this distance results to be equal to the position of the minimum of the laterally averaged Kr-substrate potential (3.4 Å) as opposed to the position of the absolute minimum of the potential which is located 2.7 Å above the center of the unit cell.