Tunneling reactions on solid surfaces

For the last few years numerous electron tunneling processes on the surface of highly dispersed (specific surface area of about 100-200m2g x) oxides have been discovered and investigated [67-70]. The processes studied represent reactions of recombination of radiation defects. In highly dispersed oxides a considerable portion of these defects is known to be stabilized on the surface. 

Recombination of the surface electron Fs+ -centres and of the bulk hole V -centres in y-irradiated highly dispersed oxide CaO has been studied [69]. The recombination kinetics is weakly dependent on temperature in the range 4.2-77 K. The formal activation energy has a value of only 30 cal mol l. At small irradiation doses (less than 2 x 1019eVcm-3) the recombination appears to be of geminate character, i.e. it occurs only in the parent donor-acceptor pairs, the process kinetics being well described by the linear dependence of the concentration of centres on the logarithm of observation 

At temperatures above 77 K the recombination rate increases markedly, but the process kinetics at high irradiation doses is still described well by eqn. (30) of Chap. 4 up to 240 K. Analysis of the kinetic data by Aristov [69] has made it possible to find the process activation energies Ea = 1.6 0.2kcalmol 1 in the range 100-137 K and Ea = 2.4 0.4kcal mol i in the range 137-240 K. 

Similar results on the kinetics and temperature dependence of the recombination of electron and hole centres have been obtained [68-71] in studying highly dispersed samples of magnesium oxide MgO. As distinct from CaO, however, in MgO the hole centres are mainly stabilized on the surface (so-called Vs -centres) while the electron centres are stabilized both on the surface (Fs+ -centres) and in the volume (F1 -centres). After irradiation is over a slow recombination of radiation defects is observed 

The kinetics of these processes in the time range 102-106s is well described by equations of the type illustrated by eqn. (30) of Chap. 4 [69, 70]. With a decrease in temperature from 77 to 4.2 K the rate of recombination of electron and hole centres decreases by less than a factor of 100, which corresponds to a formal activation energy of less than 40 cal mol i.e. the reaction proceeds practically without activation. In the absence of contact between the F+-centres stabilized in the volume and the Yr-centres stabilized on the surface these data point to the tunneling mechanism of recom- 

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