Picosecond real time study of the bimolecular reaction O(³P)+C₂H₄ and the unimolecular photodissociation of CH₃CHO and H₂CO

The bimolecular reaction of O(³P) with ethylene and the unimolecular photodissociation of acetaldehyde and formaldehyde have been studied using a picosecond pump/probe technique. The bimolecular reaction was initiated in a van der Waals dimer precursor, C₂H₄̇NO₂, and the evolution of the vinoxy radical product monitored by laser-induced fluorescence. The NO₂ constituent of the complex was photodissociated at 266 nm. The triplet oxygen atom then attacks a carbon atom of C₂H₄ to form a triplet diradical (CH₂CH₂O) which subsequently dissociates to vinoxy (CH₂CHO) and H. The rise time of vinoxy radical production was measured to be 217 (+75-25) ps. RRKM theory was applied and a late high exit barrier was invoked in order to fit the measured rise time. The structure and binding energy of the van der Waals complex have been modeled using Lennard-Jones type potentials and the results were compared with other systems. The unimolecular side of the potential energy surfaces of this reaction has been investigated by photodissociating acetaldehyde at the same pump energy of 266 nm. The resulting photoproducts, acetyl radical (CH₃CO) and formyl radical (HCO), have been monitored by resonance enhanced multiphoton ionization (REMPI) combined with a time-of-flight mass spectrometer. The similarity in the measured evolution times of both radicals indicates the same photodissociation pathway of the parent molecule. The photodissociation rate of acetaldehyde is estimated from RRKM theory to be very fast (3×10¹²s^-1). The T₁←S₁ intersystem crossing (ISC) rate is found to be the rate determining step to photodissociation and increases with energy. The REMPI mechanism for the production of CH₃CO⁺ is proposed to be the same as that of HCO⁺(2+1). The HCO product from the photodissociation of formaldehyde at 266 nm reveals a faster T₁←S₁ ISC rate than in acetaldehyde.