A new apparatus capable of combining an external laser vaporization supersonic cluster beam source with a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer has been designed and implemented. Cluster ions generated in the supersonic beam source are injected down the bore of the superconducting ICR magnet and trapped in an elongated cylindrical FT-ICR ion trap. Cluster ions can be added into the ICR cell until it is filled to capacity, ensuring a high signal to noise ratio. Using standard FT-ICR techniques, specific cluster masses can be isolated with tailored excitation pulses, reacted with various reagent gases for times as long as several minutes, and examined with a mass resolution of over one million to one.
This apparatus has been used to investigate the gas phase chemistry of silicon cluster ions. A systematic study of the silicon cluster reaction rate as a function of the cluster size has resulted in several remarkable results. Large fluctuations in reactivity are observed even for clusters as large as 50 atoms. Clusters containing 20, 21, 25, 33, 39, and 45 atoms are observed to be particularly inert. This result shows that these magic number clusters must have adapted a unique structure, or possibly set of structures, that exhibit a low reactivity towards ammonia. Kinetic analysis of the rate constants suggests that these unreactive clusters lack reaction sites capable of forming a strong silicon ammonia bond and/or lack sites capable of dissociatively chemisorbing ammonia.
Comparisons of the cluster reactivity with the reactivity of various silicon surface reconstructions has lead to the development of an empirical model for systematically predicting the structure of silicon clusters starting at Si$\sb{20}$. This new model, based on filled pentagonal and hexagonal shell structures similar to the carbon Fullerene molecules and incorporating well established rules for silicon surface reconstructions, predicts covalently bonded clusters with $\pi$-bonded surfaces similar to the Si(111)(2 $\times$ 1) surface. The filled-Fullerene model successfully predicts the unreactive magic number clusters as being exceptionally stable structures in which all of the cluster surface dangling bonds participate in the surface reconstruction.
| Identifer | oai:union.ndltd.org:RICE/oai:scholarship.rice.edu:1911/16316 |
| Date | January 1990 |
| Creators | Alford, John Michael |
| Contributors | Smalley, Richard E. |
| Source Sets | Rice University |
| Language | English |
| Detected Language | English |
| Type | Thesis, Text |
| Format | 197 p., application/pdf |
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