Regensburg 2004 – wissenschaftliches Programm

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O: Oberflächenphysik

O 14: Postersitzung (Adsorption an Oberflächen, Epitaxie und Wachstum, Organische Dünnschichten, Oxide und Isolatoren, Phasenübergänge, Rastersondentechniken, Struktur und Dynamik reiner Oberflächen)

O 14.6: Poster

Montag, 8. März 2004, 18:00–21:00, Bereich C

Physisorption systems with moderate lattice mismatch: The structure of CO₂/KCl(100) — •Jochen Vogt¹, Milica Hadnadev¹, J.-Peter Toennies², Franziska Traeger², and Helmut Weiss¹ — ¹Chemisches Institut, Universität Magdeburg, Universitätsplatz 2, 39106 Magdeburg, Germany — ²Max-Planck-Institut für Strömungsforschung, Bunsenstr. 10, 37073 Göttingen, Germany

Weakly bound adsorbates with a moderate lattice mismatch between the solid adsorptive and the substrate are examples of self-organizing systems in which short-range forces may induce complicated long-range order. The lattice constant of solid CO₂ is 12 % smaller than that of KCl(100). On the latter CO₂ initially adsorbs in an unsaturated phase with (2√2×√2)R45^∘ symmetry which is barely visible at 80 K and well-ordered at 20 K. The saturated 2D phase forms a (6√2×√2)R45^∘ lattice, verified both with helium atom scattering (HAS) and low-energy electron diffraction (LEED). Moreover, we use polarization infrared spectroscopy (PIRS) and show that dense phase IR spectra in the region of the CO₂ asymmetric stretch vibration are consistent with dynamic dipole-dipole coupling of 12 molecules in the corresponding unit cell. The spectra reveal an aging of the freshly prepared layer within short time. This is attributed to the heat of adsorption (25±1 kJ/mol) being slightly lower than the heat of sublimation of solid CO₂ (27 kJ/mol). Hence the 2D layer should be metastable and form 3D clusters. The 2D phase collapses immediately to the latter upon exposure to acetylene, which has no lattice mismatch to KCl(100) and a heat of adsorption of 27 kJ/mol.