On Discrepancy in Product Branching from Photoexcited and Chemically Activated C₂H₄

Area of Science:

• Chemical Kinetics
• Reaction Dynamics
• Computational Chemistry

Background:

• Significant discrepancies exist in product branching fractions for highly excited ethylene (C2H4).
• These differences arise between ethylene generated by photoexcitation versus chemical activation from methylene (CH2) radical self-reaction.

Purpose of the Study:

• Investigate potential nonstatistical effects in ethylene reactions.
• Determine product branching fractions for H atom producing channels in vibrationally excited and chemically activated C2H4.

Main Methods:

• Classical trajectory calculations on the ground-state potential energy surface.
• New shock tube experiments measuring H atom concentration profiles from CH2 + CH2 reaction.

Main Results:

• Trajectory calculations yielded H atom branching fractions (ϕb) of 0.6-0.7, aligning with photodissociation and statistical theories.
• Shock tube experiments determined a temperature-independent ϕb of 0.15 ± 0.03 for the C2H3 + H channel (1700-2000 K).
• The experimental ϕb is significantly lower than theoretical predictions and photodissociation results.

Conclusions:

• Discrepancies in product branching fractions between experimental methods and theoretical predictions remain unexplained.
• Moderate nonstatistical behavior was suggested by selective vibrational mode excitation but insufficient to resolve differences.
• The study highlights the unresolved origin of these discrepancies, potentially involving ground- and excited-state dynamics coupling.