Molecular desorption and secondary ion mass spectrometry

Experiments and molecular dynamics computer simulations have been conducted to gain insight into the mechanism of desorption of molecules on surfaces subjected to bombardment by energetic particles. The model system is a monolayer of benzene on Ag{111}. The computer simulations are performed using a many body potential function based on the Brenner potential. This function allows interactions between desorbing benzene molecules to be accurately modeled. The experiments utilize multiphoton resonance ionization of desorbed benzene molecules to determine energy, angle, and vibrational state-selected information. The results show there is close agreement between the model and the measurements, providing a confirmation that the physical basis of the calculations, are indeed correct. The benzene molecules are desorbed by direct collisions with substrate atoms and many of the molecules are internally excited enough to be in excited vibrational levels or to undergo fragmentation. For multilayers of benzene on a metal, the desorbed molecules have much lower kinetic energy, suggesting that exothermic chemical reactions in the film may play a role in producing a thermal-like event. The results also show that the excited molecules arise mainly from the surface layer since molecules originating from below the surface tend to be cooled by multiple collisions with other molecules in the interfacial region. The model calculations are extended to show that cooperative uplifting of surface molecules is observed even for species of molecular weight of several thousand Dalton. The context of these experiments and calculations is considered as a consequence of a vision of the mechanism of molecular desorption in secondary ion mass spectrometry presented 20 years ago by Cooks.