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

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.