ORGANIC FILM THICKNESS EFFECT IN SECONDARY ION MASS-SPECTROMETRY AND PLASMA DESORPTION MASS-SPECTROMETRY

The secondary ion emission of superposed Langmuir-Blodgett films of fatty acids has been studied using kiloelectronvolt (secondary ion mass spectrometry, SIMS) and megaelectronvolt (plasma desorption mass spectrometry; PDMS) particle bombardment. It is shown that for thick films (10-12 monolayers), the composition of the ejected volume and the escape depth of the secondary ions are similar using kiloelectron-volt and megaelectronvolt primary ions. A complementary study has been developed to investigate the film thickness effect in SIMS. Contrary to what was observed in PDMS, the molecular ion emission of surface layers decreases when the number of underlayers increases. This decrease most likely results from differences in energy exchanges in the film before ion emission. This indicates that the secondary ion emission originates from an energetically homogeneous region.