Liusi Sheng

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.

Breakdown of the Born-Oppenheimer approximation in the F+ o-D2 -> DF + D reaction.

The reaction of F with H2 and its isotopomers is the paradigm for an exothermic triatomic abstraction reaction. In a crossed-beam scattering experiment, we determined relative integral and differential cross sections for reaction of the ground F(2P3/2) and excited F*(2P1/2) spin-orbit states with D2 for collision energies of 0.25 to 1.2 kilocalorie/mole. At the lowest collision energy, F* is ∼1.6 times more reactive than F, although reaction of F* is forbidden within the Born-Oppenheimer (BO) approximation. As the collision energy increases, the BO-allowed reaction rapidly dominates. We found excellent agreement between multistate, quantum reactive scattering calculations and both the measured energy dependence of the F*/F reactivity ratio and the differential cross sections. This agreement confirms the fundamental understanding of the factors controlling electronic nonadiabaticity in abstraction reactions.

A threshold photoelectron-photoion coincidence spectrometer with double velocity imaging using synchrotron radiation

A novel threshold photoelectron-photoion coincidence (TPEPICO) imaging spectrometer at the U14-A beamline of the Hefei National Synchrotron Radiation Laboratory is presented. A set of open electron and ion lenses are utilized to map velocity imaging of photoelectrons and photoions simultaneously, in which a repelling electric field using an extra lens is applied to magnify images of photoelectrons instead of traditional accelerating electric field in order to suppress the contribution of energetic electrons in the threshold photoelectron spectroscopy (TPES) and the mass-selected TPEPICO spectroscopy. The typical energy resolution of TPES is measured to be 9 meV (full width at half maximum), as shown on the (2)P(1/2) ionization of argon. The measured mass resolving power for the present TPEPICO imaging spectrometer is above 900 of M/DeltaM. Subsequently as a benchmark, oxygen molecule is photoionized by monochromatic synchrotron radiation at 20.298 eV and dissociates to an oxygen atomic ion and a neutral oxygen atom, and the translation energy distribution of oxygen atomic ion is measured by the time-sliced imaging based on mass-selected TPEPICO experiment. The kinetic energy resolution of the present ion velocity imaging is better than 3% of DeltaE/E.

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.

Performance of the atomic and molecular physics beamline at the National Synchrotron Radiation Laboratory

At the National Synchrotron Radiation Laboratory, The University of Science and Technology of China, an atomic and molecular physics beamline with an energy range of 7.5-124 eV has been constructed for studying the spectroscopy and dynamics of atoms, molecules and clusters. The undulator-based beamline, with a high-resolution spherical-grating monochromator (SGM), is connected to the atomic and molecular physics end-station. This end-station includes a main experimental chamber for photoionization studies and an additional multi-stage photoionization chamber for photoabsorption spectroscopy. A mid-photon flux of 5 x 10(12) photons s(-1) and a high resolving power is provided by this SGM beamline in the energy range 7.5-124 eV. The size of the synchrotron radiation beam spot at the sample is about 0.5 mm in the vertical direction and 1.0 mm in the horizontal direction. Some experimental results of photoionization efficiency spectroscopy and photoabsorption spectroscopy of atoms and molecules are also reported.

Thermal desorption/tunable vacuum-ultraviolet time-of-flight photoionization aerosol mass spectrometry for investigating secondary organic aerosols in chamber experiments.

This paper describes thermal desorption/tunable vacuum-ultraviolet photoionization time-of-flight aerosol mass spectrometry (TD-VUV-TOF-PIAMS) for the real-time analysis of secondary organic aerosols (SOAs) in smog chamber experiments. SOAs are sampled directly from atmospheric pressure and are focused through an aerodynamic lens assembly into the mass spectrometer. Once the particles have entered the source region, they impact on a heater and are vaporized. The nascent vapor is then softly ionized by tunable VUV synchrotron radiation. TD-VUV-TOF-PIAMS was used in conjunction with the smog chamber to study SOA formation from the photooxidation of toluene with hydroxyl radicals. The ionization energies (IEs) of these SOA products are sometimes very different with each other. As the ideal photon source is tunable, its energy can be adjusted for each molecular to be ionized. The mass spectra obtained at different photon energies are then to be useful for molecular identification. Real-time analysis of the mass spectra of SOAs is compared with previous off-line measurements. These results illustrate the potential of TD-VUV-TOF-PIAMS for direct molecular characterization of SOAs in smog chamber experiments.

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.

Studies of cluster anions C n X− (X=N, P, As, Sb, Bi) produced by laser ablation

A series of cluster anions CnX− are produced from laser ablation of appropriate samples, where X is selected as a group-VB element. The recorded mass spectra of these cluster anions display a drastic even/odd alternation on ion intensities: For CnN−, only anions with oddn can be observed; For CnP− and CnAs−, cluster anions with evenn are produced but with lower signal intensities; For CnSb−, the signal intensity of clusters does not show even/odd alternation; Finally, for CnBi−, the intensities of cluster anions with evenn are higher than those with oddn. This parity effect can be attributed to the linear structure of the cluster anions, and the parity reversal of CnX− from CnN− to CnBi− can be explained from the electronegativity decreasing of the heteroatom X as it descends in the group. The Hückel model was applied to account the structural feature of these clusters.

Dissociative photoionization of isoprene : experiments and calculations

Vacuum ultraviolet (VUV) dissociative photoionization of isoprene in the energy region 8.5-18 eV was investigated with photoionization mass spectroscopy (PIMS) using synchrotron radiation (SR). The ionization energy (IE) of isoprene as well as the appearance energies (AEs) of its fragment ions C(5)H(7) (+), C(5)H(5) (+), C(4)H(5) (+), C(3)H(6) (+), C(3)H(5) (+), C(3)H(4) (+), C(3)H(3) (+) and C(2)H(3) (+) were determined with photoionization efficiency (PIE) curves. The dissociation energies of some possible dissociation channels to produce those fragment ions were also determined experimentally. The total energies of C(5)H(8) and its main fragments were calculated using the Gaussian 03 program and the Gaussian-2 method. The IE of C(5)H(8), the AEs for its fragment ions, and the dissociation energies to produce them were predicted using the high-accuracy energy model. According to our results, the experimental dissociation energies were in reasonable agreement with the calculated values of the proposed photodissociation channels of C(5)H(8).

Experimental and theoretical study of the dissociation energies D0(H2NH) and D0(H2N+H) and other related quantities

Abstract Combining the techniques of molecular beam and vacuum ultraviolet synchrotron radiation photoionization mass spectroscopy, we have measured the ionization energy of NH3 (10.16±0.02 eV) and the appearance potentials of H+ (18.57±0.05 eV) and NH2+ (15.75±0.02 eV). From these data, we have obtained dissociation energy values for D0(H2NH) (4.97±0.05 eV), D0(H2N+H) (5.59±0.02 eV) and D0(H2NH+) (8.41±0.05 eV) and the ionization energy IE(NH2) (10.78±0.05 eV). High-level ab initio calculations lead to results that are in good agreement with the experimental findings.