Molecular secondary particle emission from molecular overlayers under 10 keV Ar+ primary ion bombardment

In this paper we focus on the matrix dependence of the secondary particle emission of the purine base adenine (C5H5N5) and the aminoacid β-alanine (C3H7NO2) under 10 keV Ar+ primary ion bombardment. We compared the secondary neutral and secondary ion yields of these molecules sputtered from UHV-deposited molecular overlayers with varying surface coverages (sub-monolayer up to a multilayer) on various substrate materials (Si, Cu, Ni, Ag, and Au). Furthermore, we determined disappearance cross-sections of adenine and the mean kinetic energy of sputtered neutral adenine molecules. We found that secondary neutral as well as secondary ion yields for a monolayer coverage are higher than those for a multilayer coverage. The yield enhancement depends on the molecule, the substrate material, and the respective secondary particle species observed in the measurement. These results are explained by a simple model involving different matrix effects. Due to the decoupling of desorption and ionization by using laser postionization for the analysis of sputtered neutral molecules, a classification of the matrix effects into a sputter-induced matrix effect and an ionization matrix effect was possible. Whereas a sputter-induced matrix effect is found for the secondary particle emission of both molecules, adenine and β-alanine, only the secondary ion emission of β-alanine is affected by an ionization matrix effect.