Molecular beam investigation of adsorption kinetics on bulk metal targets: Nitrogen on tungsten
Abstract
An ultra high vacuum molecular beam technique for the study of the kinetics of the interaction of reactive gases with well-defined metal surfaces is described. Well collimated beams of nitrogen molecules, with an accurately measured intensity of 10 12 to 10 13 molecules cm -2 sec -1 over a target area of ~ 4 × 10 -2 cm 2, are obtained using a series of five chambers differentially pumped with a combination of high speed diffusion pumps and strategically deposited titanium getter films. Out-of-beam leakage into the adsorption cell was < 10 8 molecules sec -1. A method is described for obtaining absolute values of sticking probabilities and surface coverages with a high degree of accuracy. For nitrogen on a polycrystalline tungsten foil, sticking probabilities and their coverage profiles were found to be independent of incident molecular beam angle, over a range of 60°, with an initial sticking probability, s0, at 300 K of 0.61 ± 0.02. The sticking probability profile is monotonie over the range examined, and is theoretically described in terms of a modified form of Kisliuk's precursor state model, with self-consistent parameters. On a linear plot, the sticking probability extrapolates to zero at a coverage of 3.0(± 0.5) × 10 14 molecules cm 2. On the W(111) plane s0 was found to be 0.08 ± 0.01, and the sticking probability falls linearly with increasing coverage, with a saturation coverage of 1.2(± 0.2) × 10 14 molecules cm -2. It is argued that adsorption sites on the (111) plane are created by the presence of surface vacancies or randomly distributed W atoms on the perfect (111) surface. Within the limits of detection of the molecular beam technique no nitrogen adsorption could be detected on the (110) plane of tungsten, in agreement with previous work.
- Publication:
-
Surface Science
- Pub Date:
- February 1972
- DOI:
- 10.1016/0039-6028(72)90232-4
- Bibcode:
- 1972SurSc..29..454K