The mechanism of the laser-induced infrared photolysis of ethylene

Abstract

The mechanisms for the production of C₂(a ³Π_u) 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.