Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLID-GAS INTERFACE 187complete the experiment. Consider a Maxwellian gas of molecular weight M, attemperature To K and pressure p torr. The number of molecular'collisions Nwith the surface per cmz per second is given byN=3.5 x 1022pJTMFor nitrogen at 298" Kand 1 torrpressure, N = 3.84 x 1OZ0molecules. cm-'. sec-'.Not every molecule which strikes the surface will stick, so the rate of adsorptionwill be given bydN - 3.5 x--1OZ2psdtJTMwhere s is a sticking probability or sticking coefficient. s is dependent on temperatureand the fraction of the surface already covered with adsorbed species. Forthe more common gases (N, , CO, 0,), on clean metal surfaces s - 1 to lo-', butfor less reactive materials, e.g. 0, on germanium6', s - 1O-j. If the rate ofsurface contamination from residual gases is to remain low, say < 1 %, for theduration of the experiment, then the experiment must be completed in a time tseconds wheretsp - 2.5 x lo-*For nitrogen adsorbing on a metal surface at lo-' torr about 100 seconds areavailable for experiment. For oxygen on a germanium surface, a pressure of lo-'torr will allow a similar experimental time. It can be seen that very good vacuumconditions are necessary for many experiments, and for those extending to hoursor days ultra high vacuum is essential. In certain circumstances these conditionscan be relaxed if the composition of the residual ambient is known. For example,a residual pressure of torr, which is mainly nitrogen and carbon monoxide,will not effect experiments on a germanium surface because these gases are notadsorbed.It can be concluded that the experimental conditions must be specified for eachadsorbate-adsorbent pair with regard to the time taken for the experiment ifcontamination is to be avoided and meaningful results obtained.2.1.3 Measurement of adsorbed volumeMeasuring the rate of adsorption by noting changes in the adsorbed volume is inprinciple quite simple. A known mass of gas is admitted to the system and theReferences pp. 270-278