Helium atom reflectivity study of physisorption and chemisorption on single crystal metal surfaces

A new helium atomic beam reflectivity apparatus has been developed to measure adsorption and desorption rates for hydrocarbons and alkanethiols on Au(111) and to study the internal energy dependence of the chemisorption of methane on transition metal surfaces.;Twenty-five hydrocarbons (alkanes, alkenes, and related cyclic molecules) were measured and found to be only capable of physisorption on Au(111) with an adsorption energy which is proportional to bulk properties of the molecule such as heat of vaporization or polarizability. Since polarizability is an additive property, an empirical additive model has been developed to predict the adsorption energy based on the composition of the molecule. In addition, the sticking coefficients of the linear hydrocarbons were measured as a function of surface temperature. As the surface temperature is increased and desorption becomes significant, the sticking coefficient decreases due to incomplete accommodation of the translational energy of the incident molecule.;On a Au(111) surface, alkanethiols are capable of both physisorption and chemisorption. Although the physisorption energy of the 1-alkanethiols increases linearly as a function of chain length, the activation energy for desorption from the chemisorbed state is constant at 124 kJ/mol. As a result, at very long chain lengths (tetradecanethiol and above), the activation energy for desorption from the physisorbed state exceeds that for chemisorption. The rate of chemisorption of the thiols from their physisorbed state was also found to be a function of the chain-length of the molecule. While the energy of activation for chemisorption was found to be the same for all alkanethiols studied, the pre-exponential factor (attempt frequency) decreases with increasing alkanethiol chain length.;With the development and installation of an external resonant cavity system, high fluxes of vibrationally excited methane could be generated for mode-specific adsorption experiments. Although the rate, of activated methane chemisorption on Pt(111) increases with increasing impinging translational energy, the rate appears to be independent of excitation of the 2nu 3 asymmetric stretch vibration. Preliminary experiments for methane adsorption on Ni(111) also indicate that the vibrational energy of the 2nu 3 mode is less effective than an equivalent amount of translational energy for the promotion of chemisorption.