Physisorption of a long-chain eta-alkane on Ag(111) surface: an infra-red reflection absorption spectroscopic study

Adsorption of a long chain n-alkane, tetratetracontane (n-C44H90), on an Ag (111) surface was studied by infrared reflection absorption spectroscopy (I RAS). The first layer physisorbed on the Ag(lll) surface assumes a 'flat on ' structure with the molecular plane formed by carbon atoms parallel to the surface. The structure gave CH stretching vibration bands at 2907 and 2814 cm(-1). With the aid of auxiliary IRAS data of n-C44H90 on Au(111), these peaks were attributed to the CH bonds in the upper side of the molecular pl ane (HC/Ag) and those in the vicinity of the Ag(111) surface (CH/Ag), respe ctively. The difference in frequency was ascribed to the interaction of the latter CH bonds with Ag. This 'softening' of the CH bond was explained by dipole-image dipole interactions with the Ag surface. (C) 1999 Elsevier Sci ence B.V. All rights reserved.