Self-assembled monolayers (SAMs) of carbonyl-containing alkanethiols on gold are employed to explore the influence of hydrogen-bonding interactions on gas-surface energy exchange and accommodation. H-bonding, COOH-terminated SAMs are found to produce more impulsive scattering and less thermal accommodation than non-H-bonding, COOCH3-terminated monolayers. For carbamate-functionalized SAMs of the form Au/S(CH2)16OCONH(CH2)(n-1)CH3, impulsive scattering decreases and accommodation increases as the H-bonding group is positioned farther below the terminal CH3.
Time-of-flight molecular beam scattering techniques are used to explore the energy exchange, thermal accommodation, and residence time of HCl in collisions with an OH-terminated self-assembled monolayer. The monolayer, consisting of 16-mercapto-1-hexadecanol (HS(CH(2))(16)OH) self-assembled on gold, provides a well-characterized surface containing hydroxyl groups located at the gas-solid interface. Upon colliding with the hydroxylated surface, the gas-phase HCl is found to follow one of three pathways: direct impulsive scattering, thermal accommodation followed by prompt desorption, and temporary trapping through HO--- HCl hydrogen bond formation. For an incident energy of 85 kJ/mol, the HCl transfers the majority, >80%, of its translational energy to the surface. The extensive energy exchange facilitates thermalization, leading to very large accommodation probabilities on the surface. Under the experimental conditions used in this work, over 75% of the HCl approaches thermal equilibrium with the surface before desorption and, for a 6 kJ/mol HCl beam, nearly 100% of the molecules that recoil from the surface can be described by a thermal distribution at the temperature of the surface. For the molecules that reach thermal equilibrium with the surface prior to desorption, a significant fraction appear to form hydrogen bonds with surface hydroxyl groups. The adsorption energy, determined by measuring the HCl residence time as a function of surface temperature, is 24 +/- 2 kJ/mol.
Molecular beam scattering techniques are used to explore the energy exchange and thermal accommodation efficiencies of HCl in collisions with long-chain OH- and CH(3)-terminated self-assembled monolayers (SAMs) on gold. Upon colliding with the nonpolar methyl-terminated SAM, HCl (E(i) = 85 kJ/mol) is found to transfer the majority, 83%, of its translational energy to the surface. The extensive energy loss for HCl helps to bring the molecules into thermal equilibrium with the monolayer. Specifically, 72% of the HCl approaches thermal equilibrium prior to desorption. For the molecules that do not thermally accommodate, but scatter after an impulsive collision with the surface, the final translational energy is observed to be directly proportional to the surface temperature as the thermal surface energy and gas translational energy exchange during the collision. For the OH-terminated SAM, the impulsively scattered HCl escapes from the surface with slightly more average energy. The rigid nature of the OH-terminated SAM is due to the extended intra-monolayer hydrogen-bonding network that restricts some of the low-energy modes of the surface. However, despite the rigid nature of this system, the extent of thermal accommodation for HCl on these two surfaces is remarkably similar. It appears that the potential energy well between the impinging HCl and the polar surface groups is sufficient enough to trap HCl molecules that would otherwise scatter impulsively from this rigid SAM.
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