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 23 asymmetric
stretch vibration. Preliminary experiments for methane adsorption
on Ni(111) also indicate that the vibrational energy of the 23
mode is less effective than an equivalent amount of translational
energy for the promotion of chemisorption.
This thesis is dedicated to Irving Langmuir and Gerhard Herzberg.
For whom, all our novel discoveries are just extensions
of what they started fifty years earlier.
Thanks go out to all of the members of the numerous collaborations that made the experiments feasible, the results understandable, and the papers readable. But special thanks...
...to Dave for fixing far more on Surf 2 than you'll ever admit to breaking.
...to John, James, and Matt for a daily dose of sanity over the lunch hour.
...to André, Andrea, and Hemant for trusting a surface scientist with their lasers.
...to Pete, Paul, and Becky for seemingly endless days of disulfide hunting.
...to Larry and Werner for always getting jobs done ahead of schedule.
...to Ilse, without whom, my daily caloric intake would be significantly lower.
...to my advisors, for showing me how to "think", "do", and "write" science (and grammar).
...to my wife, for not killing me while we were both stressfully finishing our theses.
...to my parents, for not asking, "Aren't you
done yet?" while I was stressfully finishing my thesis.
...and to all the members of the Scoles and Bernasek
groups for your help and support.
Table of Contents
2.1. Helium Atom Reflectivity
2.2. Basic Apparatus
2.3. Alkane, Alkanethiol Experiments
3. Physisorption of Hydrocarbons on Au(111)
4. Physisorption and Chemisorption of Alkanethiols and Alkylsulfides on Au(111)
5. Activated Methane Chemisorption on Pt(111)
6. Conclusions and Future Work