Direct identification of propargyl radical in combustion flames by vacuum ultraviolet photoionization mass spectrometry
- Department of Chemistry, University of California at Davis, Davis, California 95616 (United States)
We have developed an effusive laser photodissociation radical source, aiming for the production of vibrationally relaxed radicals. Employing this radical source, we have measured the vacuum ultraviolet (VUV) photoionization efficiency (PIE) spectrum of the propargyl radical (C{sub 3}H{sub 3}) formed by the 193 nm excimer laser photodissociation of propargyl chloride in the energy range of 8.5-9.9 eV using high-resolution (energy bandwidth=1 meV) multibunch synchrotron radiation. The VUV-PIE spectrum of C{sub 3}H{sub 3} thus obtained is found to exhibit pronounced autoionization features, which are tentatively assigned as members of two vibrational progressions of C{sub 3}H{sub 3} in excited autoionizing Rydberg states. The ionization energy (IE=8.674{+-}0.001 eV) of C{sub 3}H{sub 3} determined by a small steplike feature resolved at the photoionization onset of the VUV-PIE spectrum is in excellent agreement with the IE value reported in a previous pulsed field ionization-photoelectron study. We have also calculated the Franck-Condon factors (FCFs) for the photoionization transitions C{sub 3}H{sub 3}{sup +}(X-tilde;{nu}{sub i},i=1-12)<-C{sub 3}H{sub 3}(X-tilde). The comparison between the pattern of FCFs and the autoionization peaks resolved in the VUV-PIE spectrum of C{sub 3}H{sub 3} points to the conclusion that the resonance-enhanced autoionization mechanism is most likely responsible for the observation of pronounced autoionization features. We also present here the VUV-PIE spectra for the mass 39 ions observed in the VUV synchrotron-based photoionization mass spectrometric sampling of several premixed flames. The excellent agreement of the IE value and the pattern of autoionizing features of the VUV-PIE spectra observed in the photodissociation and flames studies has provided an unambiguous identification of the propargyl radical as an important intermediate in the premixed combustion flames. The discrepancy found between the PIE spectra obtained in flames and photodissociation at energies above the IE(C{sub 3}H{sub 3}) suggests that the PIE spectra obtained in flames might have contributions from the photoionization of vibrationally excited C{sub 3}H{sub 3} and/or the dissociative photoionization processes involving larger hydrocarbon species formed in flames.
- OSTI ID:
- 20783228
- Journal Information:
- Journal of Chemical Physics, Vol. 124, Issue 7; Other Information: DOI: 10.1063/1.2168448; (c) 2006 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 0021-9606
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
AUTOIONIZATION
COMBUSTION
DISSOCIATION
EV RANGE 01-10
EXCIMER LASERS
FAR ULTRAVIOLET RADIATION
MASS SPECTRA
MASS SPECTROSCOPY
PHOTOIONIZATION
PHOTOLYSIS
PHOTON-MOLECULE COLLISIONS
PROPARGYL RADICALS
REACTION KINETICS
RYDBERG STATES
SYNCHROTRON RADIATION
VIBRATIONAL STATES