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Plasma Chemistry IV Vepfek Venugopalan

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T,K Fig. 34. 2 Catalytic Effects Occurring in the Spatial Afterglow In some discharge-flow systems notably 02 133), H20 65,134,135), H202 1 3 6 , 1 3 7 ) and N2 + H2 13s~ the yields of products were found to depend on reaction time of the dissociated gases in the spatial afterglow rather than in the plasma itself. When the surface-to-volume (S/V) ratio of the spatial afterglow region was increased in experiments using dissociated water vapor 139~, the yield of hydrogen peroxide in a cold trap at 77 K increased to a maximum value and then decreased (Fig.

28). Analysis of the plasma emission before and after the catalyst zone revealed an increase of N H and NH2 concentrations suggesting a heterogeneous radical mechanism leading to ammonia formation: N H + H2 -~ NH~' (6) The low steady-state concentrations of ammonia in the discharge in experiments without a catalyst was taken as evidence that the large excess energy of NH* molecules is removed more effectively on metallic surfaces than homogeneously by triple collisions. Introduction of solid K O H in the space between the electrodes increased the concentration of NH3 by a factor of 18 at U/v = 240 Wh dm -3 112).

Ref. 214)) and on the bias of the substrate. A large negative bias and low deposition temperature favor the formation of the "'diamondlike" films whereas graphitic carbon is obtained at a temperature above ~600 °C and floating potential t63) The physical properties of the "diamond-like carbon'" are intermediate between those of graphite and diamond and such a material contains significant quantities of incorporated hydrogen (I0-25 at%)1). The electron spin resonance study by Gambino and Thompson 211) indicated that a significant quantity of the carbon atoms are in the graphitic sp2 hybrid state, Although the understanding of the structural properties of this material is far from being comprehensive the available data favor a model of hydrogenated, disordered network with a variable mixing of the aromatic, sp2, and diamond-like, sp 3, carbon atoms.

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