Reflection and Dissociation of H2 on Tungsten

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Abstract
Through the use of modulated atomic beam techniques, the reflection and dissociation of H2 at a tungsten surface above 2500°K has been studied. Two experimental configurations were employed. In the first configuration, a collimated beam of molecular hydrogen was directed at the tungsten surface, and the angular distribution of particles evaporating from the surface was examined mass spectrometrically. In the second configuration, the tungsten test surface was located in the first of three differentially pumped vacuum chambers. H2 was admitted into this chamber, thereby providing an isotropic source of incident particles. Particles evaporating from the surface were collimated and examined mass spectrometrically in the third chamber. From these experiments it is observed that at high temperatures the angular distribution of atomic hydrogen evaporating from the target surface obeys the cosine law. However, the angular distribution of reflected H2 displays a shift toward the specular ray. Above 2500°K the sticking probability of H2 on tungsten approaches a limiting value of 0.3.

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