Negative ion-spectra desorbed by Cf252 -fission fragments, called plasma
desorption mass spectra ( PDMS ), of hydrocarbon substances show a strange mass
distribution. They are rather similar for all studied substances such as small
alkanes, benzene as well as big molecules like beta-carotene and even for
fullerene where hydrogen may be present only as a rest gas in the apparatus or
adsorbed on the surface. This leads to the assumption that the ions found in
the low mass region of these spectra are ions formed by a process by which the
primary chemical structure information is lost. We present a model which
describes by a differential equation network the formation of hydrocarbon
clusters out of an expanding atomic gas formed in the inner track of the
primary fast heavy ion.