The monolayer CO adsorbed on NaCl(001) was studied with helium atom diffraction. At Tsample = 45 K the adlayer is commensurate to the substrate and of (1×1) symmetry. At Tsample ≤ 30 K a commensurate (2×1) superstructure is formed. The unit cell of this low-temperature phase contains two molecules and a glide plane.
It is argued that the growth morphology of ultrathin metal films should fluctuate as a function of film thickness due to the quantum size effect. To verify this, the specularly reflected intensity of x-rays, electrons and He atoms has been measured during the growth of a thin Pb layer on top of an substrate at 100 K. Curiously enough, the expected variation is observed in the He atom and electron scattering data, but not in the x-ray reflectivity. Our explanation is that the differences in the heat of formation for successive atomic layers have a strong effect on the step density, but not on the occupancy of the layers at the low growth temperatures necessary to obtain layer-by-layer growth. This is backed up by the results of a Monte Carlo simulation.
The structure and dynamics of physisorbed carbon dioxide on in situ cleaved single crystal sodium chloride surfaces was studied by means of elastic as well as inelastic helium atom scattering. At Tsurface=80–83.5 K the diffraction patterns indicate a commensurate (2×1) monolayer superstructure on the (001) plane of the substrate, the unit cell containing a glide plane. This is in agreement with results obtained from low energy electron diffraction and infrared spectroscopy. In time-of-flight experiments single phonon low-energy loss and gain features were observed which can be attributed to acoustic and optical modes. Two higher-energy features are probably due to the first combination modes observed by helium atom scattering so far. The growth of solid CO2 adsorbed on NaCl(001) was also studied.
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