Low-Temperature Chemisorption. III. Studies in the Field Emission Microscope

Abstract

The dependence of adsorption upon surface structure has been examined with the field emission microscope. Nitrogen adatoms are found on all the planes of tungsten accessible to observation. Their rate of diffusion is fastest over the regions around the 111 pole; the barrier to migration over this area is only 20 kcal mole—1, compared to 35 kcal mole—1 for diffusion over the sharply stepped region surrounding the 100 pole. A strong structural effect is observed for the weakly bound α state of nitrogen, unstable at T>400°K. This state is concentrated on and around the (111). By contrast, nitrogen molecules held at 79°K in the γ state are distributed without pronounced specificity for surface structure. However, the absence of diffusion in both these states suggests that the nitrogen is held at isolated, discrete sites. The γ state on tungsten somewhat resembles nitrogen adsorbed on nickel; for this system chemisorption is lacking at room temperature, but at 79°K molecules are bound with an energy of at least 5.5 kcal mole—1. At an even lower temperature, T∼20°K, an additional binding state, consisting of nitrogen physically adsorbed on top of the chemisorbed layer, is observed on tungsten.Diffusion out of this state occurs at T∼40°K with the formation of a sharp border, indicating immediate chemisorption of material spilling over the chemisorbed layer. For the adsorption of carbon monoxide on tungsten no structural specificity appears in the field emission microscope. In addition to the primary chemisorbed state, the presence of weakly bound carbon monoxide is confirmed and it is shown that for neither state does dissociation occur. Both carbon monoxide and nitrogen lower the rate of electron emission from an initially clean tungsten surface. CO raises the work function by 0.5 ev at T∼300°K. In the γ state nitrogen also causes an initial work function increase of 0.1 ev. Nitrogen adatoms, however, are found to lower the work function, in contrast with the results obtained for other dissociating gases, as well as with previous work function measurements on nitrogen itself.