STRUCTURE AND DYNAMICS OF A MONOLAYER OCS ON NACL(001) - A COMBINED FTIRS, HAS AND LEED STUDY

Monolayer films of carbonyl sulfide (OCS) on well-defined NaCl(001) si ngle-crystal surfaces have been studied using Fourier-transform infrar ed 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 x 10(-8) mbar proceeds via large islands which have the structure of the completed monolayer. For the l atter, both HAS and LEED indicate a (2 x 1) superstructure, most proba bly with two molecules in each adsorbate unit cell. In the infrared sp ectrum of the OCS asymmetric stretching vibration nu(3), three intense and three weak absorptions are observed (FWHM <1 cm(-1)). Due to the non-degeneracy of the nu(3) mode this observation would suggest at lea st three molecules in each unit cell, and challenges the conclusions d rawn from HAS, LEED and the integrated infrared absorption. HAS time-o f-flight spectroscopy was used to investigate the adsorbate phonon dis persion curves. Eight modes with energies below 80 cm(-1) were observe d along the [100] direction. The observability of some phonon modes in certain parts of reciprocal space suggests a close relation between p honon intensity and phonon symmetry.