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

摘要

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_3H_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_3H_3 thus obtained is found to exhibit pronounced autoionization features,which are tentatively assigned as members of two vibrational progressions of C_3H_3 in excited autoionizing Rydberg states.The ionization energy (IE=8.674+-0.001 eV) of C_3H_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_3H_3(X;v_i,i= 1-12)<-C_3H_3(X).The comparison between the pattern of FCFs and the autoionization peaks resolved in the VUV-PIE spectrum of C_3H_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_3H_3) suggests that the PIE spectra obtained in flames might have contributions from the photoionization of vibrationally excited C_3H_3 and/or the dissociative photoionization processes involving larger hydrocarbon species formed in flames.