Investigation of Gas-Surface Dynamics Using an Ar Atomic Beam and Functionalized Self-Assembled Monolayers

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2002-04-23

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Virginia Tech

Abstract

Interactions of gas-phase molecules with surfaces are important in many ordinary events, such as ozone depletion, corrosion of metals, and heterogeneous catalysis. These processes are controlled by the bonding, diffusion, and reactivity of the impinging gas species. Our research employs molecular beam techniques and well-characterized surfaces to study these processes.

The goal of this study is to better understand how the physical and chemical nature of the surface interface influences energy transfer dynamics in gas-surface collisions. An atomic beam is used to probe the energy transfer dynamics in collisions of Argon with model surfaces of functionalized self-assembled monolayers (SAMs) (1-dodecanethiol and 11-mercapto-1-undecanol) on gold. The beam is directed towards the surface at an incident angle of 30 degrees and the scattered Ar atoms are detected at the specular angle of 30 degrees. Time-of-flight scans measure the velocity distributions of atoms leaving the surface, which correlate with the energy transfer dynamics of the impinging gas atoms.

Gas-surface energy transfer experiments are accomplished by directing an 80 kJ/mol Ar atomic beam at a clean Au(111) surface and surfaces composed of hydroxyl-terminated or methyl-terminated SAMs on Au(111). The fractional energy transferred to the bare gold surface is 69 %, while it is grater than 77 % for the monolayer-covered surfaces. The extent of thermalization on the surface during the collision is significantly greater for the methyl-terminated surface than for the hydroxyl-terminated surface. Since the two monolayers are similar in structure, packing density, and mass, the differences in scattering dynamics are likely due to a combination of factors that may include differences in the available energy modes between the two terminal groups and the hydrogen-bonding nature of the hydroxyl-terminated SAM.

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self-assembled monolayers (SAMs), molecular beam, ultrahigh vacuum

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