A Classical Dynamics Model of Plasma Desorption Mass Spectrometry Experiments

A classical dynamics procedure is developed to model the desorption of particles from a surface due to heavy particle bombardment. The theoretical approach is to utilize a metallic (Ni) microcrystallite with up to 8 atomic layers covered with an organic monolayer such as benzene. This ensemble of atoms is then bombarded from both the front and the back of the microcrystallite to test whether there are fundamental differences in mechanisms by which the organic molecule is ejected between the two configurations. The model calculation is performed in order to evaluate the importance of collision cascades in the desorption of large molecules using plasma desorption mass spectrometry (PDMS) where a ~ 100-MeV incident particle bombards a thin foil from behind. This geometry contrasts that utilized in SIMS or FAB mass spectrometry experiments where the molecular layer is bombarded directly by a 1-5-keV heavy particle. The calculated results show that most of the predicted observables for the ejected benzene molecules, including their mass spectrum and their energy and polar angle distributions, are similar. The energy distributions of the benzene molecules are Maxwell-Boltzmann-like in character even though the desorption is a consequence of a nonequilibrium energetic collision cascade. The details of the collision events that lead to desorption of particles, however, are quite different between the two calculations. These collisional differences are most apparent in the angular distributions of the ejected Ni atoms. They also indicate that molecular ejection is slightly more favorable when bombarding from behind the microcrystallite than when bombarding the sample directly, since the direction of momentum need not be reversed.