Über den Einfluß der Adsorption auf die Feldionisierung organischer Substanzen

Abstract

A new effect was observed: The overall field ion current generated by thin wires or tips changes with the average work function of the metal if different organic substances are adsorbed. The shift of the tunneling probability versus voltage curve with change of work function and of ionization potential for molecules such as heptane, and heptene is calculated. This is compared with the shift of the experimental current-voltage curves with change of work function and ionization potential when supplying different organic compounds into the field ionization chamber and after addition of small amounts of another organic substance to the substance under discussion. Physically adsorbed substances on platinum, like n-paraffins, are always shifting these curves to higher voltages. This corresponds to a lowering of the average work function. The shift is more pronounced with alcohols on platinum because of the large work function change due to the polar groups. The higher aliphatic alcohols — starting with pentanol — are forming two dimensional condensed phases as postulated by KEMBALL. This was proven by detection of stepwise increase of dimeric ions in the FI mass spectra of pentanol as compared with the lower alcohols. The hydrocarbon chains of the alcohol molecules are ordered perpendicular to the metal surface in these phases. Benzene, ethylene and acetylene additives are shifting the FI-curves of paraffins to lower voltages, thus indicating a chemisorption process of these unsaturated compounds increasing the average work function of the platinum surface.