Time-resolved kinetic studies of electronically excited CH radicals II. Quenching efficiencies for CH(A 2Δ)

Abstract

Quenching of CH(A 2Δ) by the molecules H2, CO, NO, O2, N2O, C2H2, C2H4, C2H6, C3H8 and C4H10 has been studied using direct, time-resolved, techniques. CH(A 2Δ) was produced by ultraviolet laser (193 nm) multiphoton excitation of (CH3)2CO and CHBr3: fluorescence from CH(A 2Δ → X 2Π), at 430 nm, was then monitored as a function of time using a fast (10 ns per channel) transient recorder. The kinetic data obtained with both precursor molecules agree closely. The rate constants for total removal of CH(A 2Δ) are compared with those for the ground state, CH(X 2Π), and are shown to be smaller in every case, except for CO where slow termolecular removal of CH(X 2Π) takes place. A broadly parallel kinetic behaviour for CH(A 2Δ), CH(X 2Π) and the equivalent united atom state, N(2D), is found: the order of reactivity is CH(X 2Π) > CH(A 2Δ) > N(2D).