Bond-breaking without barriers. II. Vibrationally excited products

Abstract

Ketene is photolyzed in a supersonic jet, and the vibrationally excited singlet methylene CH2 (ã 1A1), produced is detected by laser-induced fluorescence. The appearance thresholds and yield curves of individual methylene rovibrational states are obtained by scanning the photolysis laser wavelength. As observed previously by probing the (0,0,0) state at lower photolysis energies, there are no barriers to dissociation and nuclear spin is conserved. Sharp steps are observed just above the energetic threshold in each of these photofragment excitation (PHOFEX) curves. This suggests that the rotational state distributions are given by phase space theory (PST). The quantum yield of the (0,1,0)101 rovibrational state is measured and the quantum yield for (0,1,0) inferred. These values are larger than predicted by PST, and are close to values predicted by variational Rice–Ramsberger–Kassel–Marcus (RRKM) theory and by the separate statistical ensembles (SSE) method. This indicates that near the (0,1,0) energy threshold the (0,0,0) yield is constrained, as by a tight transition state. The appearance of steps spaced by the energies of a free CO rotor in the PHOFEX curves close to the thresholds of each vibrational state probed indicates that the near threshold flux of vibrationally excited products is controlled by a loose ‘‘transition state’’ on a vibrationally adiabatic surface. These observations are consistent with the variational RRKM theory for dissociations without barriers in which each product vibrational state evolves on its own vibrationally adiabatic potential surface and has its own transition state. As the energy increases above the threshold for a vibrational state, its transition state moves in along the reaction coordinate and tightens. Thus total rates increase less rapidly with energy than in PST and vibrational distributions are skewed towards higher levels than in PST.