Spectroscopic studies of chemical reactions using carbon dioxide lasers
The laser powered homogeneous pyrolysis (LPHP) technique was used to promote chemical reactions. The temperature and the geometry of the temperature produced in the reaction cell by a continuous wave IR CO2 laser were photographed and measured using chemiluminescence, spectroscopic (IR diode laser source spectrometer) and chemical standard techniques. The distribution of the temperature over the volume of the cell was found to be inhomogeneous, and the diffusion effects on the reaction rate were negligible. The mechanism of the decomposition of CH3I, d3-CH3I and their mixture was investigated using the LPHP technique. The rate of the decomposition of d3-CH3I was faster than that of CH3I. No significant amount of mixed isotopes of the products were observed during the irradiation of the mixture. The reactions of (CH3)4Sn (TMT) and (CH3)3SnSn(CH3)3 (HMDT) showed they decomposed, and the major products were CH4 and metallic layer deposited on the walls of the reaction cell. A significant amount of CH3D was observed during the reaction of TMT in presence of D2 at a relatively high laser power. The reaction of (CH3)3Al (TMAL) was investigated using LPHP technique and the mechanism of its reaction was presented. A monomeric TMAL form was detected during the reaction using FTIR spectrometer scanning of the reaction zone perpendicularly with the incident CO2 laser beam. A (CH3)2A1 (DMAL) radical was found as a result of the decomposition of TMAL. The DMAL radical was trapped and isolated as stable compounds: (CH3)2AIF (DMAF) in dimeric and tetrameric forms, and (CH3)2AICI (DMAC) in dimeric form, using SF6 and CCI4, CHCI3 and CDCI3 as radical scavengers, respectively. The presence of H2 and D2, which are widely used as carrier gases in the metal organic chemical vapour deposition (MOCVD) technique, did not have major effect on the mechanism of the reaction, but were shown to have isotopic effects on the thermal conductivity of the reaction cell, slowing down the reaction. A new photoacoustic spectroscopic technique, based on the change of the resonance frequency of the reaction cell (r.f.), was introduced to follow and examine the chemical reactions, revealing the change of the cell composition. The results obtained by this technique were fairly comparable with those obtained by IR spectroscopic methods.