State-specific collisional coupling of the CH A 2Δ and B 2Σ- states

Randall, C.J., Murray, C. and McKendrick, K.G. (2000) State-specific collisional coupling of the CH A 2Δ and B 2Σ- states. Physical Chemistry Chemical Physics, 2(4), pp. 461-471. (doi:10.1039/a908494k)

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Electronically excited CH radicals have been prepared in chosen vibrational levels of the A 2Δ and B 2Σ− states by selective laser excitation. The evolution of the populations in the initial and collisionally produced vibronic levels has been followed by time- and wavelength-resolved fluorescence spectroscopy. It is found that CO2, as a model collision partner, efficiently promotes the coupling of the A 2Δ and B 2Σ− states at room temperature (∽295 K). CH A 2Δ, v=1 is reversibly transferred to the near-degenerate B 2Σ−, v=0 level, and irreversibly vibrationally relaxed to A 2Δ, v=0, with comparable probabilities for these competing processes. CH B 2Σ−, v=0 is correspondingly reversibly transferred to A 2Δ, v=1 and irreversibly transferred to A 2Δ, v=0. The branching ratio for these two product vibrational states is ca. 2:1, which contrasts markedly with the predictions of energy-gap scaling laws. The A 2Δ, v=0 level is found to be only weakly quenched by CO2, in agreement with previous measurements. These observations have important consequences for the use of laser-induced fluorescence spectroscopy as a tool for monitoring the density of CH in collisional environments, and in the interpretation of previously measured quenching rate constants for electronically excited CH.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Murray, Dr Craig
Authors: Randall, C.J., Murray, C., and McKendrick, K.G.
College/School:College of Science and Engineering > School of Chemistry
Journal Name:Physical Chemistry Chemical Physics
Journal Abbr.:Phys. Chem. Chem. Phys.
ISSN (Online):1463-9084
Published Online:07 January 2000

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