Using Molecular Beam Scattering to Study Surfaces

Post-doc: Jim Ferris
Email:
jhferris@princeton.edu

The chemistry and dynamics of surface properties are critical to such diverse fields as catalysis, thin films, composite materials, microelectronics, and corrosion. The collision of a gas molecule with a surface, followed by trapping of the molecule on the surface, must precede any surface reactions. Some knowledge of adsorption and accommodation is essential to a complete understanding of heterogeneous reaction dynamics. Following the initial energy transfer event, the surface adsorbed molecule can then undergo reaction. This reaction step also involves energy transfer processes, and may result in the eventual desorption of a product molecule. From analysis of the energy state of reaction products we gain insight into the dynamics of surface mediated reactions.

Reactive scattering, (MBRS) and thermal energy atom scattering (TEAS) are techniques used to monitor chemical processes on surfaces such as surface coverage, sticking coefficients, and reaction dynamics. The use of MBS to probe surfaces generally takes two forms. The first of these forms focuses on determining mechanisms and kinetics of surface reactions. The second, and more recent application, is the use of reactive scattering to probe dynamic aspects of reactions at the surface. MBRS has the advantage of being able to probe both the surface and the gas phase reaction products. A comprehensive understanding of surface mediated chemistry is the goal of these studies.

Current Work

We have recently used TEAS to study the interaction of a number of molecular reactants on the well characterized Fe(111) and Ga/Fe(111) surfaces. The method is very sensitive to small changes in coverage or adsorption site of the reactants, and can be used in combination with other methods to investigate the adsorption, coadsorption, and reaction of small molecules on surfaces. We have found that the chemistry of a gallium modified iron surface is remarkably different than the clean iron surface. For example, very small amounts of residual gallium markedly decrease the sticking coefficient for the molecular adsorption of water, and modify the degree of dissociation of the water molecule once it adsorbs on the surface.

Presently, we are extending our studies of surface chemical reactions to include chemical kinetics. We are using MBRS to investigate the oxidation of methanol on the model surface of Fe(111). These studies are directed towards understanding the chemistry of this system as it relates to the fields of chemical catalysis and fuel science.

As a group we are continuously pushing our technology forward. Currently, we are working on modifying the existing molecular beam apparatus to include a differentially pumped quadrupole mass spectrometer (QMS) and an additional molecular beam source. These improvements will enable us to expand our research to include experiments investigating complex heterogeneous gas phase-surface reactions.


Figure Captions

This view of the molecular beam scattering instrument shows the molecular beam line. The apparatus consists of three vacuum chambers. 1. A small source chamber where the gas for the molecular beam is introduced; 2. A differentially pumped chamber which collimates the gas from the source into a linear beam and modulates the beam for detection, and 3. The main scattering chamber. The scattering chamber is maintained under ultrahigh vacuum (5x10-11torr) and supports the sample and a complement of surface analysis instruments. In addition to MBS this apparatus is equipped with LEED, AES, and TDS as well as ion sputtering and heating for preparation of pure crystalline surfaces. We are also capable of measuring the angular distribution of scattered atoms and molecules when the QMA is mounted on the rotating chamber lid. This view shows the side of the scattering chamber opposite from the beam inlet. From this side of the instrument one can see the Auger spectrometer, the gas inlet manifold for dosing the sample, and the large gear on the chamber lid which is used to position the quadrupole mass spectrometer for angular dependence measurements.