Tunnel spectra of carboxylic acids: the transition from physical absorption to chemisorption

Tunnel spectra have been examined for adsorbed formic, acetic, and propionic acids; the vapors were introduced into the junctions by condensation onto aluminum or magnesium films previously oxidized and cooled to 77 °K. The subsequent preparation of the tunnel contact involved no heating, while the characteristics were measured directly on the system after deposition at about 4.2 °K. It is found that HCOOH, CH3COOH, and CH3CH2COOH form dimers under these conditions, which are physically adsorbed on the surface of the base film. If the contact is heated, dissociative chemisorption occurs, with the formation of the corresponding ions. The resulting changes in the tunnel spectra provide direct information on the interaction with the surface, as well as an unambiguous interpretation of various bands.

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