Berlin 2008 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 51: Metal Substrates: Adsorption of Organic/Bio Molecules III

O 51.13: Vortrag

Mittwoch, 27. Februar 2008, 18:15–18:30, MA 041

Surface-selection rule for infrared spectra of adsorbate molecules violated by metal ad-atoms — •Olaf Skibbe¹, Martin Binder¹, Andreas Otto², and Annemarie Pucci¹ — ¹Kirchhoff-Institut für Physik, Universität Heidelberg, Im Neuenheimer Feld 227, D-69120 Heidelberg — ²Institut für Physik der kondensierten Materie, Heinrich-Heine Universiät, Düsseldorf

Copper adatoms on Cu(111) significantly modify the infrared reflection-absorption spectrum of ethylene (C₂H₄) on Cu(111). The infrared spectral changes do not involve significant shifts of vibration frequencies as they would be expected for strong distortions. But originally non-infrared active Raman modes of the centrosymmetric molecule appear in the spectra and the infrared active CH₂-wagging mode disappears already below 0.2 monolayers of Cu adatoms. High-resolution electron energy loss spectra of adsorbed ethylene for various Cu-adatom pre-coverage show the same vibration lines and indicate the change in their dipole character. Since the molecules are obviously unchanged, the Raman lines in the infrared spectra must get their dynamic dipole moment from transient electron transfer favored by atomic disorder on the metal surface, which is an experimental proof of a previous hypothesis. The completely new and surprising result of this work is the disappearing infrared active mode at sub-monolayer Cu-adatom coverage, which we explain by screening due to the background polarizability increased by Cu adatoms.