Infrared (sim;950 cmminus;1) predissociation of ethylene clusters has been studied using a crossed laser beamndash;molecular beam apparatus equipped with a moveable detector. van der Waals molecules undergo dissociation following absorption of a single infrared photon. Angular distributions, obtained for product molecules (C2H4)n,n=1ndash;3, all show nearly exponentially decreasing product flux with increasing scattering angle. A product flux contour map has been generated for the photolysis reaction (C2H4)2thinsp;rarr;thinsp;C2H4+C2H4. Twoisotropiccenterhyphen;ofhyphen;mass distribution functions yield excellent agreement with experimental results. One is a function of reaction kinetic energyE, withP(E)=exp(minus;E/80 cmminus;1); the other is a function of product velocity (momentum)u, withU(u) =uthinsp;exp(minus;u/9times;103cm/s). The latter distribution is characteristic of a dissociation pathway with a barrier in the exit channel. Such a barrier could result from centrigufal effects. It is argued that isotropic product scattering can be consistent with a direct dissociation reaction occurring on the subpicosecond time scale. It is not possible to determine, unambiguously, if dissociation is direct or if the reaction proceeds through a longhyphen;lived intermediate. Experimental results indicate that only a small fraction of the energy available to products appears as translation, with the remainder appearing as rotational excitation of C2H4. Results are compared with other experimental studies on infrared predissociation of van der Waals cluster as well as a recent theoretical analysis of (C2H4)2dissociation.
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