Hydrogen dissociation on high-temperature tungsten

H-2 dissociation on polycrystalline tungsten is measured from 1700 to 3000 K using the filament temperature (T) change and a normalized H-atom density at the chamber surface. The dissociation probability per H2 filament collision (P-diss) saturates at 0.40 at high T and has a 2.25 +/- 0.05 eV apparent activation energy when P-diss << 1. This activation energy is consistent with prior data and models, but the H-2 pressure dependence is not. P-diss is independent of the H-2 pressure for this entire T range and the 1-85 mTorr pressure range studied, contradicting the primary model that has been used to explain H2 dissociation on tungsten and other metals. We show that some apparently contradictory prior measurements are actually consistent with our observations and with each other, once this pressure dependence of P-diss is recognized. (c) 2006 Elsevier B.V. All rights reserved.