Hydrogen elimination from the dissociation of methyl-substituted silanes on tungsten and tantalum surfaces

Abstract

The elimination of H2 from the dissociation of four methyl-substituted silane molecules, including monomethylsilane (MMS), dimethylsilane (DMS), trimethylsilane (TriMS), and tetramethylsilane (TMS), on a heated tungsten or tantalum filament surface has been studied using laser ionization mass spectrometry. Two complementary ionization methods, i.e., single photon ionization (SPI) using a vacuum ultraviolet wavelength at 118 nm (10.5 eV) and a dual ionization source incorporating both 10.5 eV SPI and laser-induced electron ionization, were employed to detect the production of H2. Examination of the intensity of the H2+ peak from the four molecules has shown that it increases with temperature until reaching a plateau at around 2000−2100 °C on both tungsten and tantalum filaments. These methyl-substituted silanes are dissociatively adsorbed on tungsten and tantalum surfaces by Si−H bond cleavage, and as the temperature is raised, by C−H bond rupture. Experiments with the isotopomers of MMS, DMS, and TriMS have shown that the formation of H2 follows the Langmuir−Hinshelwood mechanism where two adsorbed hydrogen atoms on metal surfaces recombine to produce H2. The determined activation energy (Ea) for H2 formation from MMS, DMS, and TriMS, in the range of 58.2−93.4 kJ mol−1, has been found to increase with the number of methyl substitutions in the precursor molecule. Comparison of these Ea values with the reported values of 51.1−78.8 kJ mol−1 for the methyl radical formation from the same three precursor molecules has led to the conclusion that the initial Si−H bond cleavage in the dissociative adsorption of MMS, DMS, and TriMS is the rate-limiting step for the formation of both H2 molecules and ·CH3 radicals.