TEA Laser Induced Multiphoton Dissociation of Ethylene in a Collisional Regime: Model and Experiment

The focused beam of a pulsed CO₂ TEA laser was used to photodissociate ethylene gas at total pressures of 100-300 torr. The laser pulse induces molecular hydrogen elimination to yield primarily C₂H₂ and H₂ in an large excess of helium to eliminate heating. An invesigation was made of the dependence of product yield on wavelength, inert gas pressure, ethylene pressure and intensity. Photochemical yield was followed over a 10,000 fold range. Its dependence was investigated employing alternate methods of focusing the laser beam, one of which results in a constant volume geometry. A photometric method of characterizing this focal zone is described. A "structureless" computer model of the laser pumping process involving coupled rate equations and phenomenological absorption and stimulated emission coefficients is described. The model assumes rapid intramolecular energy transfer between the various vibrational modes throughout the entire vibrational manifold, and rapid equilibration of rotational states. The model includes stimulated emission and deactivating collisions, and predicts a product yield versus intensity dependence that does not exhibit threshold behavior in agreement with experiment. Calculated dependences of product yield on laser power for various parameters are given in order to relate the model to various photolysis experiments.