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 CO bond to form a C 2 HO radical and a C 2 H 5 ͑ethyl͒ radical occurs. We observed neither cleavage of the other CO bond nor molecular elimination to form C 2 H 4 ϩCH 2 CO ͑ketene͒. The C 2 HO 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 C 2 HO 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 X (2 AЉ) or à (2 AЈ) state ketenyl ͑HCCO͒ 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 C 2 HO formed in the channel with the lower kinetic-energy release is likely the spin forbidden ã (4 AЉ) state of the ketenyl radical, reached through intersystem crossing. The B state of ketenyl is energetically inaccessible. We also consider the possibility that the lower kinetic-energy channel forms two other C 2 HO isomers, the CCOH ͑hydroxyethynyl͒ radical or the cyclic oxiryl radical. Signal from laser-induced fluorescence of the HCCO photofragment was detected at the electronic origin and the 5 1 0 band. The fluorescence signal peaks after a 20 s 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 C 2 HO with unit quantum yield, establishing it as the first clean photolytic precursor of the ketenyl radical, a key species in combustion reactions.

On the mechanism of the HCCO + O2reaction: Probing multiple pathways to a single product channel

“…Ethyl ethynyl ether is photolyzed at 193 nm to produce HCCO þ C 2 H 5 with near unit quantum yield. 34 The 193 nm photons from an unfocused ArF excimer laser are introduced into the cell about 1 cm below the gas mixture entrance. Typical laser pulsewidth is 20 ns with a fluence of 24-30 mJ cm À2 pulse À1 at a 50 Hz repetition rate.…”

Characterization of Kemer L4 meteorite using Raman spectroscopy, X-ray diffraction, magnetization measurements and Mössbauer spectroscopy

“…The dynamics of the photodissociation of EE at 193 nm has been previously reported, namely describing EE as an efficient photolytic precursor of ketenyl radical which acts as an intermediate in a wide range of combustion reactions. [53][54][55] Though irradiation in the present study has been carried on at longer wavelengths, it appears still possible that the initially photoproduced EE subsequently fragments into ketene (H2C=C=O; K) and ethene (H2C=CH2) (Figure 5). Under the conditions of our experiments, the process has very low efficiency, as inferred from the low experimental infrared intensity of the anti-symmetric C=C=O stretching vibration of the ketene (the band at 2151.5 cm -1 observed in Figure 4).…”

Kinetic parameters for gas-phase reactions: Experimental and theoretical challenges

“…An even cleaner, and perhaps more promising HCCO source than ketene is the photo-dissociation at 193 nm of ethyl ethynyl ether, [33][34][35] as recently employed by Meyer and Hershberger in their study of the HCCO + NO 2 reaction, in which HCCO was monitored by transient infrared absorption. 36…”