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Electron Spectrometer: HREELS, UHV Surface Chemistry

EMSL's ultrahigh vacuum (UHV) surface chemistry-high-resolution electron energy loss spectroscopy (HREELS) system is designed to study the molecular-level chemistry of adsorbates on metal oxide surfaces. This system is equipped with several spectroscopic tools that follow changes in adsorbate chemistry, including HREELS, secondary ion mass spectrometry, and ultraviolet photoemission and (electronic) electron energy loss spectroscopies. This system also contains an ion gun for sample cleaning, and an Auger electron spectrometer and low-energy electron diffraction system for characterizing sample surface composition and structure, respectively.

Electron-stimulated and temperature-programmed desorption studies are routinely performed using this system. During temperature-programmed desorption studies, the researcher can obtain typical information such as quantity and nature (intact or dissociated molecule) of an adsorbed gas. In addition, they can also estimate the sticking coefficient and activation energy for desorption and/or reaction of the adsorbed molecule.

EMSL researchers have successfully used this system to characterize the surface chemistry of a variety of adsorbates (e.g., water, formic acid, carbon dioxide, methanol, oxygen, and chromyl chloride) on TiO2 and Fe2O3 single crystal substrates.

System Configuration and Operational Overview

Sample Preparation and Handling

While experiments on realistic metal, oxide (ceramic and glass), and polymer materials can be performed using this system, the UHV Surface Chemistry-HREELS System is primarily used to study model (often single-crystal) materials. EMSL researchers who are responsible for this equipment are experienced in mounting conductive metal, semiconductive, and insulating oxide samples as well as in attaching thermocouples to these types of samples for temperature measurements. Typically, square and (nearly) round samples with areas of about 1 cm2 and a thickness of 1 to 2 mm or more are used.

All work in the Surface Science and Catalysis Laboratory and with the UHV Surface Chemistry-HREELS System must be performed in compliance with EMSL practices and permits.

Sample Manipulator

Samples are mounted on a Vacuum Generators x,y,z, manipulator. Heating is accomplished by passing DC current through metallic backing plates used to mount the crystals. Cooling to below 100 Kelvin is accomplished through a gravity-fed liquid nitrogen dewar.

Surface Analysis Chamber

Several techniques typically conducted in the system's surface analysis chamber were chosen due to the system being designed to study molecular-level chemistry of adsorbates on metal oxide surfaces. With HREELS, surface vibrational and electronic dipoles are probed with an electron beam of 3 mV to 5 mV in resolution. For example, the adsorbed state of water (molecular versus dissociative) has been determined on TiO2 single crystal surfaces based on the vibrational features observed in the electron energy loss spectrum.

Other techniques used in this chamber that complement HREELS and provide molecular-level information include:

All of these techniques use an Extrel quadrupole mass spectrometer; a Princeton Scientific reverse-view low-energy electron diffraction apparatus; and a PHI double-pass cylindrical mirror analyzer for Auger electron spectrometer, ultraviolet photoemission, and work function change measurements. In addition, ion and electron beam studies are performed with guns obtained from Kimball Physics, and the light source for ultraviolet photoemission studies is a VSW helium lamp.

The chamber is pumped by a 270 L/s ion pump and a 230 L/s turbomolecular pump, and routinely achieves pressures in the low 10-10 Torr range. Molecules are dosed onto well-characterized surfaces either by backfilling the chamber or through effusive beam sources.

All Related Publications Related Publications

  1. Binding and Direct Electrochemistry of OmcA, an Outer-Membrane Cytochrome from an Iron Reducing Bacterium, with Oxide Electrodes: A Candidate Biofuel Cell System.
  2. NOx Reduction on a Transition Metal-free ?-Al2O3 Catalyst Using Dimethylether (DME).
  3. Transient Mobility of Oxygen Adatoms upon O2 Dissociation on Reduced TiO2 (110).
  4. Effect of Coadsorbed Water on the Photodecomposition of Acetone on TiO2(110).
  5. Preface.
Henderson, Mike | , 509-371-6527