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.