State-specific collisional coupling of the CH A (2)Delta and B (2)Sigma(-)states

Electronically excited CH radicals have been prepared in chosen vibrational levels of the A (2)Delta and B (2)Sigma(-) states by selective laser excit ation. The evolution of the populations in the initial and collisionally pr oduced vibronic levels has been followed by time- and wavelength-resolved f luorescence spectroscopy. It is found that CO2, as a model collision partne r, efficiently promotes the coupling of the A (2)Delta and B (2)Sigma(-) st ates at room temperature (similar to 295 K). CH A (2)Delta, upsilon = 1 is reversibly transferred to the near-degenerate B (2)Sigma(-), upsilon = 0 le vel, and irreversibly vibrationally relaxed to A (2)Delta, upsilon = 0, wit h comparable probabilities for these competing processes. CH B(2)Sigma(-), upsilon = 0 is correspondingly reversibly transferred to A (2)Delta, upsilo n = 1 and irreversibly transferred to A (2)Delta, upsilon = 0. The branchin g ratio for these two product vibrational states is ca. 2:1, which contrast s markedly with the predictions of energy-gap scaling laws. The A (2)Delta, upsilon = 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 monit oring the density of CH in collisional environments, and in the interpretat ion of previously measured quenching rate constants for electronically exci ted CH.