Secondary ion formation from Langmuir-Blodgett films: Studies of positive molecular ions

Langmuir-Blodgett mono- and multi-layer (1-15 layers) systems consisting of stearic acid (SA) or barium stearate (BS) deposited on RF plasma glow discharge prepared Ag, Au, or Ge were studied by secondary ion mass spectrometry (SIMS). For the monolayer samples, substrate signals were not completely obscured (e.g. Ag⁺, AgCl^-₂) even though sample related molecular ions persisted for extended periods. Ion beam induced damage and channeling of the primary and secondary ions may be responsible for these observations. (M + H)⁺ and (M + H - H₂O)⁺ were observed for SA on Au and Ge but not Ag. An explanation involves the deprotonation of the SA by Ag oxide as opposed to the less active Au and oxidic Ge surfaces. For the analogous BS monolayers, no (M + H)⁺ or (M + H - H₂O)⁺ were observed. Barium cationization ((M -- H + Ba²⁺)⁺) was observed for all BS layers on any of the substrates. The only substrate cationization observed, (M + Ag)⁺ was for BS samples prepared on Ag. For SA and BS multilayers, attenuation of substrate signals increased with the number of layers; the attenuation is greater for equivalent numbers of BS layers than SA. In addition, the substrate has an effect, the attenuation being greatest for Ag, then Ge, then Au. Examination of molecular ion yields versus time under static sputtering conditions shows that initial ion beam damage-extends to approximately 250 Å. Studies of molecular ion yields versus time (while under ion beam irradiation) indicate that the substrate, packing density, and added cations can have an effect on the ion yield.