| Abstract
| - The tightening of the transition state as energy increases abovethe dissociation threshold is studied for thedissociation of singlet ketene. Rate constants and quantum yieldshave been determined for the photodissociation of ketene to produceCH2(ãA1)(0,0,0)+ CO(X̃1Σ+)(v=1). At 57,110, 200, 357, and 490 cm-1above this threshold, vibrational branching ratios for the singletproducts were measured and compared totheory. Above 100 cm-1, the experimentalvalues are consistent with the separate statistical ensembles(SSE)and variational RRKM models.CO(v=1,J) photofragment excitation(PHOFEX) spectra were observed upto 300 cm-1 over the threshold for productionof CO(v=1) and used to calculate the total yield ofthe stateprobed. The J dependence of these yields isstatistical, consistent with the observed1CH2 rotationaldistributions. Thus, the total CO(v=1) singletyield and rate constant are determined as a continuousfunctionof energy up to 300 cm-1. Rate constantsare given accurately by phase space theory (PST) up to 35 ±5cm-1. The ab initio rateconstants of Klippenstein, East, and Allen match the experimental rateconstantsfrom 10 to 6000 cm-1 well within experimentaluncertainty without any adjustment of parameters for theunified statistical model of Miller. Thus the rate appears to becontrolled by an inner transition state near 3Å and the outer PST transition state acting in series with the outertransition state dominating for energiesbelow 50 cm-1 and the inner for energiesabove a few hundred cm-1. From themeasured rate constants, anexperimental density of states is calculated to be 0.94 times theanharmonic ab initio density of states.Thisallows the degeneracy gt of the coupled tripletchannels to be determined from the rates at the tripletthreshold,gt = 1.0 ± 0.1. The vibrationalbranching ratios and product yields for the vibrationally excitedtriplet productswere also estimated and found to be nearly constant over this energyregion. These values are about 17% ofthose predicted for vibrationally adiabatic dynamics in the exit valleyafter passage through the triplet transitionstate.
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