1. Introduction:
Chemical reactivity at surfaces plays an important role in the modern industrialized age.
Understanding of the interactions between gaseous molecules and solid surfaces is critical from a scientific and technological view. Heterogeneous catalysis is of particular importance due to its importance to a wide range of applications from agro to pharmaceutical, semiconductor, and petrochemical industries. Clearly a better understanding of basic dynamics that govern heterogeneous interactions would be beneficial.
The push for the understanding of molecules at surfaces has led to the development of a number of fields of science such as interfacial chemistry, catalytic chemistry, and surface science. Many of the interactions between molecules and surfaces are similar to those found in gas phase reaction dynamics however, the surface further complicates the dynamics of the interaction. For example the structure of the surface is thought to play a strong role in its reactivity as reactions often occur as step sites or surface anomalies.1
The Born-Oppenheimer approximation (BOA)2 has been successful in modeling gas phase reaction dynamics. This is due to the fact that the excited electronic states for the molecules are significantly high in energy and the nuclear motion sufficiently slow that molecular scattering is typically electronically adiabatic, that is it occurs on only the lowest potential energy surface (PES). This allows theoreticians to utilize the BOA in modeling the scattering dynamics in gas phase reactions. However, non-adiabatic interactions may be substantially more common in molecule surface interactions than in the gas phase. Evidence for non-adiabatic interactions has been seen for molecules adsorbed on surfaces,3-7 as well as molecular beam surface scattering experiments.8-12 Despite the fact that non-adiabatic behavior in molecule surface interactions is predominant enough to warrant several reviews,13-19 the BOA, perhaps due to a lack of alternatives, has traditionally been utilized in modeling gas surface reactions, where it can be successful depending on the system in study.20-23
Most of the interactions between surfaces and gas molecules can be described in the following processes:24
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Figure 1.1 Left panel shows primary interactions that occur in molecule surface scattering. Right panel shows typical chemisorption and physisorption energetic interactions as a function of molecular distance from the surface.24-26
Physisorption: the incoming molecule may be trapped in an energetically shallow physisorption state at the surface, see red curve in Fig. 1.1. This happens if the translational energy of the atom or molecule is efficiently dissipated to the surface. Physisorption forces are usually of van der Waals nature, with corresponding binding energies typically less than 0.2 eV.
Chemisorption: if the molecule surface interaction has enough energy to overcome a dissociation barrier of the molecule, or if no barrier to dissociation exists, the molecule can enter into a chemisorption state, see blue curve in Fig. 1.1. In this state atoms or molecules are strongly bound at the surface, and the bonds are usually of covalent or partially ionic nature, and corresponding binding energies are typically on the order of several electron volts. The dissociation barrier of a molecule at the surface is not as clear as in gas phase chemistry as this barrier may be strongly surface site dependent. The trajectory in which the molecule approaches the surface may also strongly alter the dissociation barrier with the surface.
Additionally, once the molecule undergoes physisorption, they may diffuse to other parts of the surface, which may have increased chemical reactivity. This may also lead to ambiguity in the dissociation mechanism, as the molecule could directly dissociate upon collision or first adsorb in a shallow well, diffuse to a site of high reactivity, and then undergo dissociation.27 This situation is sometimes called precursor mediated trapping.28
Molecule surface interactions involve energy transfer between the molecule and the surface.
These interactions can occur in direct scattering, where the molecule does not adsorb on the surface, or when the molecule is adsorbed on the surface. At the energies typical to molecular beams of light diatomic molecules, the main energy exchange at the surface occurs via lattice
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phonons and surface electron hole pairs. The interactions of the surface electron hole pairs with various degrees of freedom of the scattering molecules are the primary topic of this thesis.
It is difficult to measure the energy change in the surface directly, as once a local excitation or relaxation on a metal surface occurs, it is brought back to equilibrium on a ps time scale. In order to accurately measure the energy transfer I choose to probe the energetics of the molecules before and after they interact with the surface. By understanding the overall energy change of the molecules I am able to determine the amount of energy transferred to the surface. Additionally by knowing what degrees of freedom in the molecule exchanged energy with the surface we are able to have some idea of energy transfer process. Variation of the scattering conditions, such as surface temperature or initial kinetic energy of the molecules, also allows for an elucidation of the dynamics between the molecules and the surface.
Here experiments on the role of translational energy of gas molecules coupling to a metal surface will be discussed. Additionally the role of the molecules initial and final vibrational states will be discussed in regards to their effect on the translational energy of the molecule. Multi-quantum vibrational excitation is also measured over a wide range of initial conditions. The mechanism for vibrational excitation is coupling of the vibrational degree of freedom to electron hole pairs in the metal. A kinetic model for vibrational excitation was developed to draw deeper understanding from these results.
The remainder of this thesis is organized as follows. Section 2 describes previous work which is relevant to this thesis. Section 3 provides information on the experimental techniques utilized in carrying out the experiments here, as well as a short background on some of the techniques.
Section 4 presents results for translational energy transfer between molecular beams and a Au(111) surface. Section 5 gives details for the derivation of vibrational excitation probabilities and shows the results for vibrational excitation. Section 6 outlines a kinetic model which is used to explain the surface temperature dependence of the vibrational excitation probabilities. Section 7 has a theoretical explanation of the incidence energy of translation dependence of vibrational excitation. Section 8 contains the conclusions to this work as well as future outlook in this field.
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