AVS 46th International Symposium
    Applied Surface Science Division Monday Sessions
       Session AS1-MoA

Paper AS1-MoA10
Molecular Secondary Neutral Emission from Molecular Overlayers under SF@sub 5@@super +@-Bombardment

Monday, October 25, 1999, 5:00 pm, Room 610

Session: Molecular Mass Spectrometry including Interpretation
Presenter: A. Schnieders, Universität Münster, Germany
Authors: A. Schnieders, Universität Münster, Germany
M. Schröder, Universität Münster, Germany
D. Stapel, Universität Münster, Germany
H.F. Arlinghaus, Universität Münster, Germany
A. Benninghoven, Universität Münster, Germany
Correspondent: Click to Email
It is well known that TOF-SIMS and in favourable cases laser postionization of sputtered neutrals (Laser-SNMS) are powerful tools for the analysis of organic surface species. Many efforts have been made to enhance the sensitivity and the efficiency of these techniques. Especially the use of polyatomic primary ions for the analysis of molecular overlayers results in an enhanced secondary ion emission compared to the use of atomic primary ions. We have investigated the influence of polyatomic primary ions on the secondary neutral emission with the intention to improve our understanding of the processes of molecular sputtering and ion formation. We used a reflectron-type time-of-flight mass spectrometer equipped with an electron impact gas ion source for sputtering. As primary ion species we chose Ar@super +@, Xe@super +@, and SF@sub 5@@super +@. For postionization of sputtered neutral molecular species a subpicosecond excimer laser system operating at 248 nm was available. As model systems we used molecular layers of adenine and alanine prepared on liquid nitrogen cooled substrates (Si, Au, ...). They were produced by evaporation of the molecules from a Knudsen cell under UHV condition. During overlayer formation the flux of sputtered secondary neutrals and secondary ions was continuously monitored under static sputtering conditions. We determined the influence of the substrate and of the layer thickness on the secondary ion and neutral yields under bombardment with the different primary ions. We found that neutral as well as ion yields from a submonolayer coverage are higher than for a multilayer. The yield enhancement depends on the sputtering conditions. We also determined kinetic energy distributions and disappearance cross sections as a function of layer thickness. A detailed discussion with special emphasis on the sputter process - including results on the internal excitation of sputtered molecules - will be presented.