NEGATIVELY AND POSITIVELY POLARIZED HYDROGEN ADSPECIES ON THIN PLATINUM FILMS

Hydrogen absorption on thin platinum films under isothermal conditions within the temperature and pressure intervals 78-298 K and 10(-10)-10(-2) Torr has been studied measuring simultaneously surface potential (SP) and pressure (P), by means of a sensitive, short response time apparatus. Two forms of hydrogen deposit of different electrical character have been distinguished. The first one arising at the beginning of adsorption decreases the surface potential, the second one following it increases the SP due to positively polarized adspecies formation. This positively polarized form is inhomogeneous as concerns binding energy. Hydrogen uptake associated with the decrease of SP becomes larger with increase of temperature. It seems that this beta- form of the adsorbate stabilizes the positively polarized adspecies on the surface.