A Photoelectric Technique for the Study of Adsorption: the Aluminum–Oxygen and Aluminum–Water Systems

Gas adsorption on clean metal surfaces may be studied by observing the change in work function of the surface as adsorption proceeds. A photoelectric technique for following these changes by measuring the photocurrent excited by two spectral lines is developed, and a graphical method of analysis of the results is given. This method has the advantage of being much quicker than other photoelectric techniques such as the determination of the retarding-potential characteristic, and may thus be applied to the study of more rapid adsorption processes. As examples of its use, the method has been applied to a study of the adsorption of oxygen and water vapor on films of aluminum evaporated under conditions of ultrahigh vacuum. The work function of the clean aluminum films was 4.27 eV. For small oxygen exposures (up to about 10(18) molecules cm(-2)) the work function of the contaminated aluminum decreased by about 0.05 eV; for larger exposures (up to about 10(20) molecules cm(-2)) the work function increased by about 0.05 eV. Water vapor adsorption causes a reduction in the work function by more than 1 eV for an exposure of about 10(19) molecules cm(-2).