David Dolson (Advisor), Steven Higgins (Committee Member), Paul Seybold (Committee Member)
Master of Science (MS)
Electronic-to-vibrational (E-V) energy transfer is a significant kinetic channel in the collisional quenching of spin-orbit excited chlorine atoms, Cl* (2P1/2, 882 cm-1), by molecular collision partners. In the present study Cl* atoms are prepared in the presence of CH4 or CD4, under pseudo first-order conditions, by photolysis of ICl at 532 nm with a pulsed Nd:YAG laser. Quenching of Cl* by CH4 or CD4 results in E-V excitation of the ν4 asymmetric bending mode as observed by infrared (IR) fluorescence from the vibrationally excited products. Time-resolved IR fluorescence observations of CH4(ν4) and CD4(ν4) are consistent with a simple kinetic scheme involving direct E-V excitation of CH4(ν4) or CD4(ν4) followed by a slower collisional relaxation. The total quenching rate of Cl* is reflected in the rise of the ν4 fluorescence signal. The Cl* total bimolecular quenching rate coefficients (± 2σ) obtained in this study at 298 ± 2 K are (1.9 ± 0.5) x 10-11 cm3 molecule-1 s-1 for quenching by CH4 and (1.4 ± 0.9) x 10-10 cm3 molecule-1 s-1 for CD4. Intensity measurements interpreted within this kinetic scheme indicate that the E-V channel for ν4 mode excitation accounts for ≈30% of the total quenching of Cl* by CH4 and CD4. It is remarkable that the E-V branching ratios are the same in both systems even though the ν4 -- -Cl* energy differences span a four-fold range from approximately ½kT (CD4) to 2kT (CH4).
Department or Program
Department of Chemistry
Year Degree Awarded
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