Ion-track model for fast-ion-induced desorption of molecules.

Abstract

Recent data on the desorption of large biomolecules by fast (MeV) ions are examined with use of the ion-track model derived from ``hit'' theory commonly used in radiation biology. In this model, desorption requires that a given molecule be ``hit'' by m secondary electrons produced by the incident ion. Those molecules penetrated by the fast ion, and hence receiving large doses of radiation, will be damaged, whereas those receiving m ``hits'' at a distance from the track may be ejected as whole molecules with probability which varies from about 0.4% to 4% for the molecules considered. The model is shown to describe the nonlinear behavior at small values of dE/dX, the electronic energy loss per unit path length, giving way to a linear behavior (saturation) at large dE/dX. For the best fits to the available data at constant ion velocity, m increases with the size of the molecule and the survival probability tends to decrease with size, although the behavior of the latter quantity is much more susceptible to uncertainties in the model. Furthermore, the dependence of yield on velocity is well described over a broad range of ion velocities. These results suggest this model can be used to unify the data taken for a variety of targets, incident ions, and ion energies. Although the model does not give insight into the exact desorption mechanism, it strongly suggests that desorption is due to the breaking of bonds by the shower of secondary electrons generated by the passing ion.