Time-resolved Tunable Diode Laser Detection of the Products of the Infrared Multiphoton Dissociation of CF₂HCl, (CF₃)₂CO, and CHCl₃

Abstract

<p>In the experiments described in thesis, tunable u diode lasers are used to detect infrared absorption by the transient species produced in the infrared multiphoton dissociation (IRMPD) of CF₂HCl, (CF₃)₂CO, and CHCl₃.</p> <p>The time-resolved infrared absorptions of CF₂ and HCl are detected following the IRMPD of CF₂HCl. Quantification of the HCl allows one to determine a CF₂ infrared absorption Iinestrength, (9±2) X 10ˉ²¹ cm moleculeˉ¹ for the ʳQ₃(9) line. From this linestrength, the CF₂ ν₁ bandstrength is found to be (3.4±0.8) X 10ˉ¹⁸ cm moleculeˉ¹. The rate constant for CF₂ recombination to form C₂F₄ is found to be (2.3±0.7) X10ˉ¹⁴ cm³ moleculeˉ¹ sˉ¹.</p> <p>Time-resolved detection of CF₃, C₂F₆, and CO following the IRMPD of (CF₃)₂CO establishes that the dissociation mechanism involves the production of one molecule of CO and two of CF₃. From this stoichiometry, the linestrength of the ʳR₁₆(20) CF₃ transition is found to be (1.4±0.3) X 10ˉ²⁰ cm moleculeˉ¹ and the CF₃ ν₃ bandstrength is then calculated to be (8±2) X 10ˉ¹⁷ cm moleculeˉ¹. The rate constant for CF₃ recombination is measured to be (2.2±0.5) X 10ˉ¹² cm³ moleculeˉ¹ sˉ¹ at or near the high pressure limit, while the rate constant for CF₃ reacting with O₂ and NO are found to be (2.1±0.5) X 10ˉ²⁹ and (2.8±0.7) X 10ˉ²⁹ cm⁶ moleculeˉ² sˉ¹, respectively.</p> <p>The use of TDLs in determining the isotopic selectivity of IRMPD processes is then analysed. The sensitivity of the technique is demonstrated in the detection of the DCI produced in the IRMPD of natural abundance CDCl₃ in CHCl₃. However, complications due to wall adsorption and desorption effects necessitate monitoring the DCI and HCl on a short timescaIe (tens of us) to obtain meaningful quantitative information. It is shown that the CCl₄ and much of the HCI produced in the photolysis of CDCl₃/CHCl₃ mixtures is the result of isotopically non-selective reactions involving Cl atoms.</p>