Experiments on the exchange of energy between gas, solid and adsorbed layer in vacno—II. The effect of a deposited monomolecular layer on the efficiency of molecular collisions

1.Introduction . —A previous paper has described a method of detecting small changes in the thermal efficiency of encounters between gas molecules and a solid surface. This efficiency determines the rate at which conductive equilibrium is re-established between filament and walls of a Pirani gauge chamber, when the pressure is altered within a range of 10-1 to 10-4 mm., the medium of thermal exchange being hydrogen. In the paper it was shown that when two such gauges are balanced in a resistance bridge and simultaneously evacuated through long capillaries, the disturbance of balance, due to unequal rates of gas flow and to the finite time for reaching thermal equilibrium, passes through a maximum at a characteristic timet (max.),t (max.) has a lower limiting value of 120 seconds, dependent on the dimensions of the apparatus, and above that its value varies with the thermal efficiency of the gas-solid collisions; it was shown that a decrease int (max.) from 10 minutes to 6 minutes, consequent on outgassing, corresponded to a 17 per cent, increase in the accommodation coefficient “α ,” as the adsorbed layers were removed from the glass. In the present paper the method is extended to determine the change in accommodation coefficient for H2 when a hydrogen layer of monomolecular thickness has been deposited on the glass. Since no chemical reaction can be invoked under the conditions below specified, these experiments isolate the purely physical part of the changes inα which accompany the formation of such active monomolecular layers as are discussed in the chemistry of heterogeneous reactions.