Issue 4, 2000

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.

Article information

Article type
Paper
Submitted
25 Oct 1999
Accepted
26 Nov 1999
First published
07 Jan 2000

Phys. Chem. Chem. Phys., 2000,2, 461-471

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

C. J. Randall, C. Murray and K. G. McKendrick, Phys. Chem. Chem. Phys., 2000, 2, 461 DOI: 10.1039/A908494K

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