Cathode ray oscillography of gas adsorption phenomena I-A method for measuring high-velocity approach to certain physical and chemical equilibria

Abstract

The essential feature of many experiments on adsorption, evaporation, surface diffusion, and surface chemistry consists of observing a progress towards equilibrium after altering A the pressure of gas which has access to a solid, or B the temperature of a solid exposed to gas or gas mixture. But such observations only become adequate for investigating probabilities of transition between the relevant physical and chemical states if the true rate of approach to equilibrium is not obscured or distorted by time lags in the experimental methods employed. If the term "reaction velocity" be generalized to include rate of simple phase change, owing to the similarity of the latter to chemical change in its thermodynamic treatment, then the limits to accessible range of such velocity are set by the following: ( a ) the maximum rate at which A or B can be made to stimulate reaction, ( b ) the rate at which resulting energy exchanges affect some observable quantity chosen as indicator, and ( c ) the rate at which the indicator can record itself. Such limitations become serious in the study of reactions occurring in interfacial layers of monomolecular thickness. These present uniquely simple kinetics owing to the elimination of transmission delays in the actual reaction, since all the atoms may be exposed simultaneously to any agency of change; but reaction velocity becomes for that reason so large, in the interesting cases where intrinsic probabilities of transition are not extremely small, that detailed investigation by ordinary means becomes impossible. The difficulty is most acute when the solid surface is reduced to the dimensions of a filament capable of high-temperature flashing, to obey modern needs of reproducible cleanliness; the gas content is then so minute that all optical methods are inapplicable and no micromanometric or thermal observation can keep pace with the reaction. Time constants for the faster of these would seem essentially inaccessible except electron emission from a surface offers an observable quantity indicative of the state of surface layers, and responding instantaneously to any reactions which modify that state. Such thermionic acid photoelectric emission has not hitherto been combined with fast enough recording to take full advantage of this rapidity of response; Langmuir and others have used auxiliary vapours to avoid temperature ranges in which gas reaction would be rapid, while Oliphant and Moon, and Evans, have used mechanical oscillography for alkali ions only, whose behaviour is important in a low-velocity range. The extension which we here put forward, to phenomena rapid enough to justify introducing a cathode ray oscillographic technique, requires firstly certain improvements in the timing, etc., controls of such instruments not needed for their normal use; secondly, it requires theoretical and experimental determinations of the limits imposed on observable velocity by the time taken in attaining pressure equilibria and thermal and electrical steady state, and by the speed of photography. These requirements in establishing the method for rapid surface processes are completed in Part I, in relation to the two standard way A and B of initiating reaction by pressure and temperature. The conclusions are exhibited in photographic examples from phenomena well known but previously inaccessible to exact analysis, before proceeding, Part II, to apply them in the solution of particular problems.