Gas–surface energy exchange and thermal accommodation of CO2 and Ar in collisions with methyl, hydroxyl, and perfluorinated self-assembled monolayers

Abstract

Molecular beams of CO₂ and Ar were scattered from long-chain methyl (CH₃–), hydroxyl (OH–), and perfluoro ((CF₂)₇CF₃–) functionalized alkanethiol self-assembled monolayers (SAMs) on gold to study the dynamics of energy exchange and thermal accommodation on model organic surfaces. Ar collisions, for incident energies ranging from 25 to 150 kJ mol⁻¹, exhibit final energy distributions that depend significantly on the terminal functional group of the SAM. The long-chain CH₃-terminated monolayers serve as an excellent energy sink for dissipating the incident translational energy. For example, at 150 kJ mol⁻¹, greater than 90% of the collision energy is transferred to the CH₃–SAM surface for specularly-scattered atoms (θ_i = θ_f = 30° from normal). However, the OH–SAM is a more rigid collision partner due to the formation of an intra-monolayer hydrogen bonding network and the (CF₂)₇CF₃–SAM (F–SAM) provides a high degree of rigidity due to the massive CF₃ groups. The final energies for the triatomic, CO₂, scattering from the three surfaces are remarkably similar to the results for Ar scattering. The only significant difference in the translational energy transfer dynamics for these two gases appears in collisions with the OH–SAM. Strong gas–surface attractive forces between CO₂ and the OH–SAM surface appear to counter the rigidity of the hydrogen-bonding network to help bring the majority of the molecules to thermal equilibrium at all incident energies up to 150 kJ mol⁻¹, resulting in increased energy transfer in comparison to Ar. The similarities in energy transfer for Ar and CO₂ final energy distributions in scattering from the CH₃– and F–SAMs suggest that the internal degrees of freedom in the triatomic play only a small role in determining the outcome of the gas–surface collision under the scattering conditions employed in this work.