The diffusion of 1H and 2H on the (001) plane of a field emitter was studied (tables 192 and 193) by using the field emission fluctuation method, with a rectangular probe for the determination of anisotropy. The diffusion was found to be temperature-dependent at temperatures below 140K, followed by a slightly activated regime at temperatures of between 140 and 220K. Within this range, the activation energies were between 1 and 2kcal/mol and the pre-exponential factors ranged from 10-9 to 10-10cm2/s. At temperatures above 220K, a third regime was found within which the activation energies ranged from 4 to 7kcal/mol and the pre-exponential factors were between 10-5 and 10-7cm2/s. An anisotropy was found in the case of both tunnelling and weakly activated diffusion, and also at high temperatures and low coverages. This was suggested to correspond to a surface reconstruction in which a single domain predominated. At temperatures below 200K, evidence was found for the coexistence of 2 diffusion regimes. This was tentatively attributed to the effect of fluctuations which were caused by single atoms and by collective modes, respectively. The isotope effect was small under all conditions, including tunnelling. The mean square fluctuations were temperature-dependent, except at temperatures below 140K. The resultant activation energies suggested that much of this dependence involved substrate compression. A small isotope effect during tunnelling, and small values of the activation energies and pre-exponential factors in the first activated regime, were attributed to the effect of substrate distortions upon diffusion. The self-diffusion of thermally generated W atoms at temperatures of between 660 and 775K was also investigated, and was found to be described by an activation energy of about 18kcal/mol and a pre-exponential factor of about 10-7cm2/s.

Diffusion of 1H and 2H on W(001). E.A.Daniels, R.Gomer: Surface Science, 1995, 336, 245-61

Table 191

Diffusion of D and H on the (001) Plane of W

at Temperatures of between 78 and 200K

continued

Table 191 (continued)

Diffusion of D and H on the (001) Plane of W

at Temperatures of between 78 and 200K

Table 192

Parameters for the Diffusion of H Isotopes on (111) W Surfaces