Surface vibrations of a highly ordered low-density alkanethiol monolayer measured using inelastic helium atom scattering

The low-energy surface vibrational structure of the (11.5 x root3) striped phase of 1-decanethiol (C10H21SH) chemisorbed on a reconstructed Au(111) su rface has been studied using a high momentum- and energy-resolution helium atom scattering apparatus. Energy-transfer spectra for this system exhibit a dispersionless inelastic feature at 8 meV. We assign this to the frustrat ed translation of the entire molecule vibrating with polarization perpendic ular to the surface. These results are in contrast to the absence of inelas tic peaks in the time-of-flight spectra of ordered shorter chain alkanethio ls chemisorbed on copper surfaces. Differences in the phonon spectra betwee n these systems are attributed to the longer chain length, the chain orient ation, and the weaker S-Au interaction. These results further the understan ding of the forces that govern nanoscale self-organization. (C) 2001 Publis hed by Elsevier Science B.V.