KINETIC ENERGIES OF SECONDARY IONS IN MEV AND KEV PARTICLE-INDUCED DESORPTION

Kinetic energy distributions of secondary ions produced by MeV or keV primary-ion bombardment of molecular solids sere measured in a reflect ron time-of-flight mass spectrometer. It was found that the energy dis tributions of many ions exhibit tails extending towards energies lower than the accelerating potential. The degree of tailing depends upon t he nature of the desorbed ion and the conditions of the target surface . Some light ions (H+ and C+), ions from inorganic (alkali halide) sam ples, radical molecular ions (C-60(+.)) and pre-formed molecular ions (e.g. complexes of valinomycin with alkali metal ions) exhibit almost no tailing. For positive adduct-type molecular ions, such as MH(+), mo re extensive tailing was observed far MeV compared with keV primary io ns. The origin of the energy tailing is attributed to gas-phase format ion of a fraction of the secondary ions. For molecular ions of peptide s, the characteristic time of formation of that fraction of ions (appr oximate to 10% of the whole molecular-ion population) was estimated to be of the order of 10 ns, while the characteristic distance from the target surface was of the order of 10 mu m.