Electronic to Vibrational Energy Transfer from Cl* (^2P1/2) to CH4 and CD4
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 ?4 asymmetric bending mode as observed by infrared (IR) fluorescence from the vibrationally excited products. Time-resolved IR fluorescence observations of CH4(?4) and CD4(?4) are consistent with a simple kinetic scheme involving direct E-V excitation of CH4(?4) or CD4(?4) followed by a slower collisional relaxation. The total quenching rate of Cl* is reflected in the rise of the ?4 fluorescence signal. The Cl* total bimolecular quenching rate coefficients (± 2?) obtained in this study at 298 ± 2 K are (1.9 ± 0.5) x 10-11 cm^3 molecule-1 s-1 for quenching by CH4 and (1.4 ± 0.9) x 10-10 cm^3 molecule-1 s-1 for CD4. Intensity measurements interpreted within this kinetic scheme indicate that the E-V channel for ?4 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 ?4 – Cl* energy differences span a four-fold range from approximately ½kT (CD4) to 2kT (CH4).
School:Wright State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:e v energy transfer spin orbit excited chlorine time resolved ir fluorescence
Date of Publication:05/15/2009