C2N2 photodissociation at 1576 Å. I. CN(A 2Π) radiative lifetimes, nascent vibrational distribution, and C2N2 quenching

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Abstract
Photodissociation of C2N2 at the 1576 Å F2 laser line generates CN(A 2Π) up to the thermodynamic limit of v=5, with a bimodal vibrational distribution peaking at v=0 and v=2. Radiative lifetimes for the six observed levels have been obtained by extrapolating time decays to zero pressure, and the data show rapidly decreasing lifetimes with increasing v, in accord with recent theoretical predictions, and contrary to previous experimental determinations. The range of lifetimes observed is 4.3–8.5 μs. Quenching of these levels by the parent molecule shows a factor of 12 increase in rate coefficient in going from v=0 to v=5, correlated to the energy gap between the A 2Π state and the closest lower X 2Σ+ level, suggesting that the quenching mechanism is one involving cross relaxation between CN(A) and CN(X).