Structure and dynamics of a monolayer OCS on NaCl(001): a combined FTIRS, HAS and LEED study

Abstract

Monolayer films of carbonyl sulfide (OCS) on well-defined NaCl(001) single-crystal surfaces have been studied using Fouriertransform infrared spectroscopy (FTIRS) with polarized light, helium atom scattering (HAS) and low-energy electron diffraction (LEED). The monolayer growth at T sample = 77–80 K and p(OCS) = 1 × 10 −8mbar proceeds via large islands which have the structure of the completed monolayer. For the latter, both HAS and LEED indicate a (2 × 1) superstructure, most probably with two molecules in each adsorbate unit cell. In the infrared spectrum of the OCS asymmetric stretching vibration v 3, three intense and three weak absorptions are observed (FWHM < 1 cm −1). Due to the non-degeneracy of the v 3 mode this observation would suggest at least three molecules in each unit cell, and challenges the conclusions drawn from HAS, LEED and the integrated infrared absorption. HAS time-of-flight spectroscopy was used to investigate the adsorbate phonon dispersion curves. Eight modes with energies below 80 cm −1 were observed along the 〈100〉 direction. The observability of some phonon modes in certain parts of reciprocal space suggests a close relation between phonon intensity and phonon symmetry.