STATIC SECONDARY ION MASS-SPECTROMETRY AS A TOOL FOR STUDYING SURFACE-REACTIONS - THE DECOMPOSITION OF ETHYLENE OVER NI(100) SURFACES

The performance of static secondary ion mass spectrometry (SSIMS) as a tool for characterizing the chemistry of hydrocarbon molecules adsorbed on metal surfaces was tested by studying the thermal decomposition of ethylene on Ni(100). Previous studies have clearly established that ethylene, which chemisorbs molecularly at 90 K, decomposes upon sample heating to form vinyl moieties at a rate that depends strongly on the isotopic composition of the adsorbate. Here we show that trideuterioethylene (C2D3H) only yields vinyl groups with C2D3 stoichiometry, indicating that the C-H bond breaks much faster than any of the C-D bonds. Also, C2D3 formation occurs around 200 K when starting from C2D3H but requires temperatures above 220 K for C2D4. These results illustrate the power of SSIMS as a technique for detecting and characterizing surface intermediates and for distinguishing among compounds with different isotopic substitutions. However, our studies using normal ethylene were inconclusive; they highlight some of the limitations of SSIMS for the study of chemisorbed systems.