The mechanism of the laser-induced infrared photolysis of ethylene

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Abstract
The mechanisms for the production of C2(a 3Πu) generated from the laser‐induced ir photolysis of 97.2% 12CH2 13CH2 ethylene has been determined as a function of initial sample pressure. The wavelength resolved laser‐induced fluorescence spectra of 13,13C2, 12,13C2, and 12,12C2 indicate that at pressures greater than ∼0.2 torr, the C2 produced from the ir laser photolysis of C2H4, comes primarily from collisional processes such as C=C scission, followed by recombination of as yet undetermined single carbon fragments or a possible four center reaction between a pair of highly excited ethylene molecules. At pressures less than ∼0.2 torr, C2 is produced primarily by direct elimination of C2 from a single C2H4 molecule. Photolysis of a 0.30 torr sample of C2H4 (in natural abundance) resulted in an isotopic enrichment of ∼100% (or an enrichment factor of ∼2) of 12CH2 12CH2. The delay times between the CO2 photolysis laser pulse and the onset of observed C2 were studied as a function of pressure and extrapolated to the collisionless pressure region to give an estimated threshold energy fluence of 50±12 J/cm2 for direct elimination of C2. Rotational and vibrational temperatures were determined from the rovibronic distributions obtained as a function of sample pressure.