The effect of hydrogen-bonding and terminal group structure on the dynamics of Ar collisions with self-assembled monolayers

Abstract

Gas–surface scattering experiments are used to probe the dynamics of energy exchange in collisions of 80 kJ/mol Ar atoms with self-assembled monolayers (SAMs) composed of HS(CH 2) 11OH, HS(CH 2) 10COOH, HS(CH 2) 11CH 3, and HS(CH 2) 9CHCH 2 alkanethiols on gold. Strong similarities in scattering from the methyl and alkene-terminated SAMs indicate that end-group torsional motion and vibration of the terminal C–C bond play only a minor role in the gas–surface energy transfer dynamics. Both of the purely hydrocarbon monolayers proved to be highly effective at dissipating the Ar incident energy. In scattering experiments performed at a specular angle of 30°, over 60% of the incident Ar atoms reached thermal or near-thermal equilibrium before desorbing from the CH 3- and CH 2-terminated monolayers. For those atoms that do scatter directly from the surfaces after an impulsive collision, less than 13% of their incident energy is retained. In comparison, the OH- and COOH-terminated SAMs also yield nearly identical scattering results, but they appear to be much more rigid than the hydrocarbon monolayers. For the OH and COOH surfaces, only 43% of the atoms scatter at 30° off the surface normal after approaching thermal equilibrium and the atoms that scatter impulsively from the surface retain approximately 17% of their collision energy. Since these energy exchange characteristics do not appear to be the result of alterations in the structure or mass of the four monolayers, and the energy modes in the terminus of the monolayers have only minor influences on energy exchange, the differences in the scattering dynamics are attributed to the hydrogen-bonding nature of the COOH- and OH-terminated alkanethiol self-assembled monolayers.