Kinetic Energies of Secondary Ions in MeV and keV Particle-induced Desorption

Abstract

Kinetic energy distributions of secondary ions produced by MeV or keV primary-ion bombardment of molecularsolids were measured in a reflectron time-of-flight mass spectrometer. It was found that the energy distributionsof many ions exhibit tails extending towards energies lower than the accelerating potential. The degree of tailingdepends upon the nature of the desorbed ion and the conditions of the target surface. Some light ions (H' andC' ), ions from inorganic (alkali halide) samples, radical molecular ions (Cg) and pre-formed molecuiar ions (e.g.complexes of valinomycin with alkali metal ions) exhibit almost no tailing. For positive adduct-type molecularions, such as MH', more extensive tailing was observed for MeV compared with keV primary ions. The originof the energy tailing is attributed to gas-phase formation of a fraction of the secondary ions. For molecular ionsof peptides, the characteristic time of formation of that fraction of ions (*lo% of the whole molecular-ionpopulation) was estimated to be of the order of 10 ns, while the characteristic distance from the target surfacewas of the order of 10 pm.