Orientational disorder in solid monolayers of tetramethylsilane adsorbed on graphite and MgO (100) surfaces

Incoherent neutron scattering experiments were performed on the tetramethylsilane (TMS) monolayers adsorbed on graphite and MgO (100) surfaces to investigate the dynamics in the orientationally disordered 2-D solid phase. Clear evidence of the order–disorder transition (Ttr=107 K for TMS/graphite) with respect to the reorientation of the TMS molecules is obtained. In the high-temperature phase, the molecules are found to undergo fairly excited rotational motion, and it is proposed that some of the molecules also undergo translational diffusion. Such a situation does not change very much at higher temperatures across the melting point of the 2-D solid (Tm=146 K for TMS/graphite). The activation energies as well as the correlation times are obtained for the relevant molecular motions and compared with the bulk case.