Chaoqun Huang

Isomeric identification of polycyclic aromatic hydrocarbons formed in combustion with tunable vacuum ultraviolet photoionization

Polycyclic aromatic hydrocarbons (PAHs) play important roles in the formation of soot from combustion. The PAHs are formed from incomplete combustion, and are thought to pose a particularly great risk to health. Isomeric identification of PAHs is a big challenge. In this article, we describe an apparatus that combines tunable synchrotron vacuum ultraviolet (VUV) photoionization with molecular-beam mass spectrometry for identifying isomers of PAHs formed in combustion. The isomers of PAHs can be distinguished with measurements of photoionization mass spectrometry and photoionization efficiency spectra. With its unique features, the apparatus provides superior mass and energy resolution and is potentially a powerful tool for the study of formation mechanisms of PAHs and soot in combustion.

Modification of photoionization mass spectrometer with synchrotron radiation as ionization source

In this article we introduce a modification on ion optics of a reflectron time-of-flight mass spectrometer for the study of flame chemistry with synchrotron radiation as a continuous ionization source. A small bias voltage is used in the extraction region to simultaneously reduce the background ion signal, eliminate secondary ionization process caused by photoelectrons, increase the ion detection efficiency, and improve the mass resolution.

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

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.

UV Photoionization Study of the Ethyl Radical

The ethyl radical was observed in a low-pressure premixed gasoline/oxygen/argon flame by using tunable vacuum ultraviolet photoionization mass spectrometry. The ionization energy (IE) of the ethyl radical was derived to be (8.24 ± 0.02) eV from the photoionization efficiency curve. In addition, a high-level ab initio Gaussian-3 (G3) method was used to calculate the energies of the radical and its cation. The calculated adiabatic ionization potential is 8.17 eV, which is in good agreement with the experimental value.

Combustion study with synchrotron radiation single- photon ionization technique

Here we report a combustion endstation at National Synchrotron Radiation Laboratory (NSRL) and some primary experimental results. Synchrotron radiation can provide the tunable vacuum ultraviolet (VUV) photon with the high intensity and the good collimation. VUV photoionization is a single-photon ionization process. Combined with molecular-beam mass spectrometry (MBMS), the VUV single-photon ionization can be applied to detect the combustion products, especially the intermediates and free radicals produced from combustion process. This method is proved to be a powerful tool for combustion study, which could be helpful for developing combustion kinetic models and understanding the mechanism of combustion reactions.