SEQUENTIAL CLUSTERING REACTIONS OF SID+ WITH SID4 - RAPID GROWTH TO KINETIC DEAD-END STRUCTURES

Sequential clustering reactions of SiD+ with SiD4 are monitored in the trapped ion cell of a Fourier transform mass spectrometer. At thermal energies, SiD+ initially clusters by rapid addition of silylene accompanied by elimination of D2. This growth sequence halts after reaching the dead-end structure Si4D7+, which grows further only by a slow termolecular process to form Si5D11+. Nonthermal cluster growth reactions are also observed which generally result in elimination of additional D2 molecules as compared to the thermal reactions. Thus the nonthermal product ions are more silicon rich than the thermal product ions. Some of the resulting nonthermal product ions react further with SiD4, but quickly form dead-end structures which cease to react. Both the forward and back reaction probabilities and products have been determined experimentally for each step of the growth sequence. These are used in combination with phase space theory to model the transition state energies involved in the microscopic pathways that have been elucidated by Raghavachari using ab initio electronic structure theory. The excellent quantitative agreement for these energies, to within 0.12 eV, between the experimentally derived values and those calculated by Raghavachari supports the growth pathway found by ab initio calculations. This pathway also shows why further growth of Si4D7+ can only occur by inefficient bimolecular attachment of SiD4. These experimental results strongly indicate that the sequential growth of SiD+ in reactions with SiD4 will not lead to large hydrogenated silicon particles even under the conditions of higher temperatures, pressures, and ion energies found in silane plasmas.