QUANTITATIVE LEED ANALYSIS OF THE SURFACE-STRUCTURE OF A MGCL2 THIN-FILM GROWN ON PD(111)

An epitaxial, ultrathin single-crystal film (12 Angstrom) of MgCl2 was deposited molecularly onto a Pd single crystal of (111) orientation a t a crystal temperature of similar to 650 K. A detailed surface struct ure determination of this insulating him was performed by low-energy e lectron diffraction (LEED). A low incident LEED beam current (0.36 mu A) was employed to minimize the electron-stimulated desorption (ESD) o f Cl. The best-fit model for the observed hexagonal MgCl2(1x1) pattern corresponds to the unreconstructed (0001) Cl-terminated plane of alph a-MgCl2, whose stacking sequence is Cl-Mg-Cl-Cl-Mg-Cl..., with a Pendr y R-factor of 0.317. Small relaxations of the surface were found as 0. 03+/-0.03, 0.03+/-0.04 and 0.04+/-0.08 Angstrom expansions of the firs t three Mg-Cl interlayer spacings, respectively, and a 0.10+/-0.06 Ang strom contraction of the first Cl-Cl interlayer spacing. These small d eviations were probably driven by the loss of the van der Waals intera ction at the termination plane, since surface structure data for trans ition-metal dichalcogenides, which have different intralayer bonding, but the same intertrilayer bonding, report a similar magnitude of inte rlayer spacing deviations as seen in MgCl2. (C) 1998 Elsevier Science B.V.