PICOSECOND REAL-TIME STUDY OF THE BIMOLECULAR REACTION O(P-3)+C2H4 AND THE UNIMOLECULAR PHOTODISSOCIATION OF CH3CHO AND H2CO

The bimolecular reaction of O(P-3) with ethylene and the unimolecular photodissociation of acetaldehyde and formaldehyde have been studied u sing a picosecond pump/probe technique. The bimolecular reaction was i nitiated in a van der Waals dimer precursor, C2H4. NO2, and the evolut ion of the vinery radical product monitored by laser-induced fluoresce nce. 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 vinery (CH2CHO) and H. The rise time of vinery radical production was measure d to be 217 (+ 75 - 25) ps. RRKM theory was applied and a late high ex it barrier was invoked in order to fit the measured rise time. The str ucture and binding energy of the van der Waals complex have been model ed using Lennard-Jones type potentials and the results were compared w ith Other systems. The unimolecular side of the potential energy surfa ces of this reaction has been investigated by photodissociating acetal dehyde 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 t ime-of-flight mass :spectrometer.:The similarity in the measured evolu tion times of both radicals indicates the same photodissociation pathw ay of the parent molecule. The photodissociation rate of acetaldehyde is estimated from RRKM theory to be very fast (3x10(12) s(-1)). The; T -1<--S-1 intersystem crossing (ISC) rate is found to be the rate deter mining step to photodissociation and increases with energy. The REMPI mechanism for the production of CH3CO+ is proposed to be the same as t hat of HCO+(2 + 1). The HCO product from the photodissociation of form aldehyde at 266 nm reveals a faster T-1<--S-1 ISC rate than in acetald ehyde. (C) 1998 American Institute of Physics.