Coherent antihyphen;Stokes Raman Scattering (CARS) is used to monitor the rovibrational population distribution of prompt H2produced from H2CO+hngr;rarr;H2+CO. Photolysis is performed onK=1 lines of the 2141S1larr;S0transition. Scanning CARS spectroscopy is employed, with suppression of the nonresonant background of the H2CO. The detection sensitivity is sim;1012cmminus;3per quantum state. Helium buffer is added to cool the product translation to near room temperature. Statesv=0ndash;4,J=1ndash;9 are detected. The vibrational distribution peaks atv=1. The rotational distributions in the vibrational states are somewhat Boltzmann with lsquo;lsquo;temperaturesrsquo;rsquo; ranging from 1700 to 2500 K forv=3ndash;0. The total energy, 29thinsp;500 cmminus;1, is chosen near the threshold for dissociation. The internal energy of H2(v,thinsp;J) sums with that found earlier for CO(v,thinsp;J) and for translation to this total. An average impact parameter of 0.9 Aring; with a spread of a few tenths Aring; is found. H2pushes away from a point about 0.3 Aring; beyond the C nucleus of CO. The distributions are controlled by the dynamics of the fragments as they separate in the steep, repulsive exit valley of the potential energy surface; they are entirely different from the statistical distributions typical for bond fission without a barrier. Rotational relaxation of H2in collisions with He occurs at 300 K with a rate constant of (9plusmn;3)times;10minus;13cm3thinsp;moleculeminus;1thinsp;sminus;1forJ=3rarr;J=1 and (2.6plusmn;1)times;10minus;13cm3thinsp;moleculeminus;1thinsp;sminus;1forJ=5rarr;J=3.
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