Direct identification of propargyl radical in combustion flames by vacuum ultraviolet photoionization mass spectrometry

Abstract

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(3)H(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(3)H(3) thus obtained is found to exhibit pronounced autoionization features, which are tentatively assigned as members of two vibrational progressions of C(3)H(3) in excited autoionizing Rydberg states. The ionization energy (IE = 8.674 +/- 0.001 eV) of C(3)H(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(3)H(3) (+)(X;nu(i),i = 1-12)<--C(3)H(3)(X). The comparison between the pattern of FCFs and the autoionization peaks resolved in the VUV-PIE spectrum of C(3)H(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(3)H(3)) suggests that the PIE spectra obtained in flames might have contributions from the photoionization of vibrationally excited C(3)H(3) and/or the dissociative photoionization processes involving larger hydrocarbon species formed in flames.

High temperature reaction kinetics of CN(v = 0) with C2H4 and C2H6 and vibrational relaxation of CN(v = 1) with Ar and He

“…12 More recently, an experimental study employing vacuum ultraviolet photoionization mass spectrometry suggested the presence of vibrationally excited propargyl C 3 H 3 in a low pressure fuel/O 2 /Ar premixed flame. 13 Vibrationally excited species may also be found in fast flow reactors employed for kinetics studies. In some of these reactors, the radical is produced by photolysis of a molecular precursor.…”

Identification of combustion intermediates in isomeric fuel-rich premixed butanol–oxygen flames at low pressure

“…Because of the high vacuum ultraviolet (VUV) energy resolution and the physics of photoionization, the ionization energies (IEs) of molecular species with different isomeric structures can readily be obtained by VUV photoionization efficiency (PIE) measurements [20]. Recently, flame-sampling photoionization mass spectrometry has been successfully applied for the identification of combustion species using broadly tunable synchrotron radiation [21][22][23].…”

Valence-Shell Photoionization of C4H5: The 2-Butyn-1-yl Radical

“…As a result, the photoionization spectra of many larger molecules, as well as a significant number of smaller ones, often show very little resonant autoionization structure. Interestingly, the high-resolution photoionization spectrum of the propargyl radical shows considerable intense structure converging to electronically excited states of the C 3 H 3 + cation. − While the assignments of these resonances are currently tentative, their stability with respect to competing decay processes may be due to the relatively stiff structure of the radical, which could reduce the potential for radiationless transitions, or to the relative stability of the closed-shell propargyl C 3 H 3 + cation. The extent to which this resonance structure is preserved or modified in the more flexible radicals produced by the addition of a methyl rotor to the propargyl radical (HCC– • CH 2 ), depending on which H atom is substituted, is thus of some interest.…”