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)

Abstract

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
• Status: Published
• Refereed: Yes
• 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: 1463-9076
• ISSN (Online): 1463-9084
• Published Online: 07 January 2000