IR laser photochemistry of alkanols and 3,3-dimethyloxetane
In this work, a pulsed CO₂ laser was used to excite methanol, ethanol, propan-2-ol, butan-2-ol, t-butanol, pentan-2-ol, hexan-2-ol, and 3,3-dimethyloxetane in order to study how these moleCUles absorb laser energy and decompose. The dependence of absorbed energy and fractional yield on laser parameters such as irradiating wavelength.,fluence and pulse type, and on reactant molecular size, pressure and diluent, was examined. The absorbed energy was measured using the optoacoustic (OA) technique. A new method for calibrating the OA cell was developed and is described in full in the thesis. The approach enables calibration to be extended to lower absorption levels (about 200µJ for a signal-to-noise ratio of six) than is possible with the more usual transmission methods, and was found to be capable of measuring as little as 5µJ. A particular advantage of the technique, is that it is simple, rapid, and provides an immediate visual indication of the absorption level. It is observed, for all reactants studied, that the absorption cross-section at low pressure was less than at high pressure, but that the difference diminishes with molecular size with the absorption cross-section taking on a value comparable to that of the small signal, broadband cross-section. These findings are consistent with the explanation that rotational hole-burning exists, but decreases in importance as the molecular size, and hence density of states, increases. As the fluence is varied, the absorption cross-section is found to increase with decreasing fluence towards the small signal, broadband value. With increasing molecular size the increase is less noticeable, and the absorption cross-section takes on the value comparable to that of the small signal, broadband cross-section. With increasing alkanol molecular size, it is observed that the major decomposition products can always be explained in terms of a molecular elimination channel i.e. one of dehydration. Also, the number of minor products and their yields both increase. It is believed that most of the minor products arise as a consequence of carbon-carbon fission processes, with minor contributions due to molecular elimination.