Photodissociation dynamics of ethyl ethynyl ether: A new ketenyl radical precursor

Abstract

The work presented here investigates the dynamics of the photodissociation of ethyl ethynyl ether at 193.3 nm with photofragment translational spectroscopy and laser-induced fluorescence. The data from two crossed laser-molecular beam apparatuses, one with vacuum ultraviolet photoionization detection and one with electron bombardment detection, showed that only cleavage of the C–O bond to form a C2HO radical and a C2H5 (ethyl) radical occurs. We observed neither cleavage of the other C–O bond nor molecular elimination to form C2H4+CH2CO (ketene). The C2HO radical is formed in two distinct product channels, with 37% of the radicals formed from a channel with recoil kinetic energies extending from about 10 to 70 kcal/mole and the other 63% formed from a channel with lower average recoil energies ranging from 0 to 40 kcal/mole. The measurements using photoionization detection reveal that the C2HO radical formed in the higher recoil kinetic-energy channel has a larger ionization cross section for photon energies between 10.3 and 11.3 eV than the radical formed in the lower recoil kinetic-energy channel, and that the transition to the ion is more vertical. The radicals formed in the higher recoil kinetic-energy channel could be either X̃(2A″) or Ã(2A′) state ketenyl (HCCO) product and the shape of the recoil kinetic-energy distribution fitting this data does not vary with ionization energy between 10.3 and 11.3 eV. The C2HO formed in the channel with the lower kinetic-energy release is likely the spin forbidden ã(4A″) state of the ketenyl radical, reached through intersystem crossing. The B̃ state of ketenyl is energetically inaccessible. We also consider the possibility that the lower kinetic-energy channel forms two other C2HO isomers, the CCOH (hydroxyethynyl) radical or the cyclic oxiryl radical. Signal from laser-induced fluorescence of the HCCO photofragment was detected at the electronic origin and the 510 band. The fluorescence signal peaks after a 20 μs delay, indicating that HCCO is formed with a significant amount of internal energy and then subsequently relaxes to the lowest vibrational level of the ground electronic state. The data show that the photodissociation of ethyl ethynyl ether produces C2HO with unit quantum yield, establishing it as the first clean photolytic precursor of the ketenyl radical, a key species in combustion reactions.