Zur Temperaturabhängigkeit der Felddissoziation von Molekülionen

Abstract

The temperature dependence of field dissociation of organic molecular ions as described in this paper can be interpreted qualitatively by means of simple models. Very fast decomposition processes, occurring about 10-12 sec after field ionization, are not only due to the lowering of the dissociation energy by the field, but also due to fluctuation of vibrational energy, especially at higher temperatures. The fragment ion insensities are increased by formation of adsorbed layers on the tip anode at low temperatures, especially with substances having high dipole moments. In the case of molecules containing a polar group and an alkyl chain, only the former is ordered preferrentially, with the dipole moment parallel to the field lines, whereas the latter is relatively mobile in the electric field. The relative fragment ion intensities depend on the orientation of the alkyl chain relatively to the polar group, however, this effect does not depend appreciably on temperature because of the mobility of the alkyl chain. The only exception from this rule was observed with pentanol. The pentanol molecules are adsorbed at the tip anode with the alkyl chains preferrentially parallel to each other. This leads to a preferred formation of C2H5 + fragment ions. With increasing temperature, the adsorbed layer is removed and a statistical orientation of the alkyl chain becomes more important, thus reducing the relative C2H5 + abundance.