DETERMINATION OF THE SINGLET TRIPLET BRANCHING RATIO IN THE PHOTODISSOCIATION OF KETENE

Abstract

The rotational distributions of CO products from the dissociation of ketene at photolysis energies 10 cm-1 below, 56, 110, 200, 325, 425, 1107, 1435, 1720, and 2500 cm-1 above the singlet threshold (30 116.2 cm-1), are measured in a supersonic free jet of ketene. The CO(upsilon" = 0) rotational distributions at 56, 110, 200, 325, and 425 cm-1 are bimodal. The peaks at low J's which are due to CO from the singlet channel, show that the product rotational distribution of CO product from ketene dissociation on the singlet surface is well described kby phase space theory (PST). For CO(upsilon" = 0) rotational distributions at higher excess energies (1107, 1435, 1720, and 2500 cm-1), the singlet and triplet contributions are not clearly resolved, and the singlet/triplet branching ratios are estimated by assuming that PST accurately predicts the CO rotational distribution from the singlet channel and that the distribution from the triplet channel changes little from that at 10 cm-1 below the singlet threshold. The singlet yield shows a rapid increase in the low excess energy region (0-300 cm-1), and a slower increase above. The singlet and triplet rate constants are derived from the directly measured total rate constants using the singlet yields. The triplet rate constant increases monotonically with increasing photolysis energy through the singlet threshold region. The singlet rate constant is accurately established in the threshold region and found to increase much less rapidly than predicted by phase space theory. At 2500 cm-1 excess energy, the CO(upsilon" = 1) rotational distribution is obtained, and the ratio of CO(upsilon" = 1) to CO(upsilon" = 0) products for the singlet channel is measured to be 0.045 +/- 0.017. This ratio is close to the vibrational Rice-Ramsberger-Kassel-Marcus (RRKM) calculation 0.038, and the separate statistical ensembles (SSE) prediction 0.041, but much greater than the PST prediction, 0.016.