Even-odd orientation and chain-length effects in the energy exchange of argon collisions with self-assembled monolayers

Self-assembled monolayers (SAMs) of alkanethiols, HS(CH2)(n-1)CH3 adsorbed on gold have been used to explore the dynamics of energy exchange and thermalization in high-energy gas-surface collisions. We find that the extent of thermalization and the fractional energy transferred to the surfaces during collisions with 80 kJ/mol Ar atoms are directly proportional to the alkyl chain length for 2 less than or equal to n less than or equal to 6. The results suggest that long-range molecular motions involving up to 6 carbon atoms play the dominate role in controlling the dynamics of the gas-surface impact. In addition to the chain-length effect, alkanethiol monolayers; with n > 6 show an oscillation in the amount of impulsively scattered atoms that depends on whether the chains consist of an even or an odd number of carbon atoms. The even-odd effect on the amount of impulsive scattering is associated with differences in the orientation of the terminal methyl group for the two classes of SAMs.