On Discrepancy in Product Branching from Photoexcited and Chemically Activated C 2 H 4

Significant discrepancies have been observed in the product branching fractions of highly excited ethylene (C2H4) generated either by photoexcitation or by chemical activation through the self-reaction of methylene (CH2) radicals. In this study, to investigate possible nonstatistical effects, classical trajectory calculations were performed on the ground-state potential energy surface for vibrationally excited and chemically activated C2H4. The calculated branching fractions for the H atom producing channels, ϕb, were in the range of 0.6-0.7 at excitation energies corresponding to ethylene photodissociation and methylene recombination. These values are close to those reported in previous photodissociation experiments as well as predictions by statistical rate theory but are significantly higher than those observed in shock tube experiments on the CH2 + CH2 reaction. Selective excitation of certain vibrational modes of C2H4 suggested moderate nonstatistical behavior, though insufficient to account for the observed discrepancies. Additionally, new shock tube experiments were conducted to measure the concentration profiles of H atoms produced in CH2 + CH2, with CH2 radicals generated from the thermal decomposition of 3-5 ppm of CH2I2 in an Ar bath at a pressure of 2 bar. The rate constant for the CH2 + CH2 reaction and the branching fraction for the C2H3 + H channel were determined to be k = (1.57 ± 0.39) × 10-10 cm3 molecule-1 s-1 and ϕb = 0.15 ± 0.03, respectively, both temperature-independent over 1700-2000 K. This branching fraction is consistent with a previous shock tube study, but is substantially smaller than statistical and trajectory predictions, as well as photodissociation results. The origin of these discrepancies in the product branching fractions remains unsolved. Potential roles of coupling between the ground- and excited-state dynamics in the product branching are discussed.