The dynamics of gas-surface energy exchange in collisions of Ar atoms with ω-functionalized self-assembled monolayers

Atomic-beam scattering experiments using n-alkanethiol and omega-functionalized alkanethiol self-assembled monolayers (SAMs) on gold are employed to explore the dynamics of gas-surface energy exchange in collisions with model organic surfaces. The studies are performed by directing a nearly monoenergetic beam of 80 kJ/mol Ar atoms onto a particular SAM at an incident angle of 30degrees with respect to the surface normal and recording the time-of-flight distributions for the atoms as they scatter from the surface at a final angle of 30degrees. Among the monolayers studied, long-chain CH3-terminated SAMs are found to be the most effective at dissipating the translational energy of impinging atoms. For alkanethiols with greater than seven total carbon atoms (HS(CH2)(n>6)CH3), we find that, for specular scattering conditions, over 80% of the incident energy is transferred to the surface and that over 60% of the impinging atoms approach thermal equilibrium with the surface before scattering back into the gas phase. In contrast to CH3-terminated monolayers, SAMs constructed from hydrogen-bonding alkanethiols: HS(CH2)(11)OH, HS(CH2)(10)COOH, and HS(CH2)(11)NH2, exhibit characteristics of more rigid collision partners. The Ar atoms transfer about 77% of their energy to these surfaces with only 43% of the atoms reaching thermal or near thermal equilibrium before recoiling. Further comparisons of mixed OH- and CH3-terminated SAMs and alkene-terminated SAMs suggest that intramonolayer hydrogen bonding of terminal functional groups may play an important role in determining the extent of energy transfer and thermalization. (C) 2003 American Institute of Physics.