Surface-controlled deuterium-palladium interactions

Abstract

Measurements were made of the uptake of deuterium (D) into traps within the bulk of palladium, and the release of D back to the gas phase. The study was done with bare metal surfaces under UHV conditions using a novel method based on ion-beam analysis. The D uptake rate was proportional to gas pressure and about a factor of 3 less than the rate of impingement onto the surface from the gas. The release of D was limited by molecular recombination at the surface. A model for surface-limited release of D is presented which shows how the difference in energy between D in traps and recombination sites is a critical parameter in determining the time dependence of the D release. The observed kinetics of the D release from Pd lead to the conclusion that D in recombination sites is less strongly bound than D in traps which in turn is less strongly bound than D in the low-coverage surface chemisorption sites. Molecular recombination must therefore occur from sites with weaker binding, while the low-coverage chemisorption sites with stronger binding are nearly fully occupied, but do not contribute significantly to the release because of the strong binding.