Chemisorption and Absorption of Nitrogen with Tantalum

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Abstract
Sorption of nitrogen on polycrystal tantalum ribbons is studied using a static method in ultra-high vacuum. Saturation coverage is 3.6×1014 molecules/cm2. Sticking probability at room temperature is 0.17 UP to 1.4×1014 molecules/cm2, and abruptly falls to 0.03 at about 2×1014 molecules/cm2. This suggests the existence of multiplex adsorption states in this system. The absorption occurs at temperatures above 500°K, so the desorption from the chemisorption states is not observed. From an analysis of the absorption rate, the nitrogen atoms chemisorbed on the surface of the polycrystal ribbon is shown to be absorbed at the grain boundary. The activation energy of absorption is 1.8 eV/2-atoms, and is much smaller than that of diffusion of nitrogen in tantalum. As the concentration of nitrogen in tantalum is proportional to the square root of equilibrium pressure, nitrogen is absorbed in atomic form into the bulk. Desorption from the absorption state is observed at temperatures above 1800°K. The heat of absorption is 4.4 eV in the pressure range from 10-8 to 10-2 Torr.

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