Xiaobin Shan

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.

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).

Dissociative photoionization of 1,3-butadiene: experimental and theoretical insights.

The vacuum-ultraviolet photoionization and dissociative photoionization of 1,3-butadiene in a region ∼8.5-17 eV have been investigated with time-of-flight photoionization mass spectrometry using tunable synchrotron radiation. The adiabatic ionization energy of 1,3-butadiene and appearance energies for its fragment ions, C(4)H(5)(+), C(4)H(4)(+), C(4)H(3)(+), C(3)H(3)(+), C(2)H(4)(+), C(2)H(3)(+), and C(2)H(2)(+), are determined to be 9.09, 11.72, 13.11, 15.20, 11.50, 12.44, 15.15, and 15.14 eV, respectively, by measurements of photoionization efficiency spectra. Ab initio molecular orbital calculations have been performed to investigate the reaction mechanism of dissociative photoionization of 1,3-butadiene. On the basis of experimental and theoretical results, seven dissociative photoionization channels are proposed: C(4)H(5)(+) + H, C(4)H(4)(+) + H(2), C(4)H(3)(+) + H(2) + H, C(3)H(3)(+) + CH(3), C(2)H(4)(+) + C(2)H(2), C(2)H(3)(+) + C(2)H(2) + H, and C(2)H(2)(+) + C(2)H(2) + H(2). Channel C(3)H(3)(+) + CH(3) is found to be the dominant one, followed by C(4)H(5)(+) + H and C(2)H(4)(+) + C(2)H(2). The majority of these channels occur via isomerization prior to dissociation. Transition structures and intermediates for those isomerization processes were also determined.

Competitive fragmentation pathways of acetic acid dimer explored by synchrotron VUV photoionization mass spectrometry and electronic structure calculations

In present study, photoionization and dissociation of acetic acid dimers have been studied with the synchrotron vacuum ultraviolet photoionization mass spectrometry and theoretical calculations. Besides the intense signal corresponding to protonated cluster ions (CH(3)COOH)(n)·H(+), the feature related to the fragment ions (CH(3)COOH)H(+)·COO (105 amu) via β-carbon-carbon bond cleavage is observed. By scanning photoionization efficiency spectra, appearance energies of the fragments (CH(3)COOH)·H(+) and (CH(3)COOH)H(+)·COO are obtained. With the aid of theoretical calculations, seven fragmentation channels of acetic acid dimer cations were discussed, where five cation isomers of acetic acid dimer are involved. While four of them are found to generate the protonated species, only one of them can dissociate into a C-C bond cleavage product (CH(3)COOH)H(+)·COO. After surmounting the methyl hydrogen-transfer barrier 10.84 ± 0.05 eV, the opening of dissociative channel to produce ions (CH(3)COOH)(+) becomes the most competitive path. When photon energy increases to 12.4 eV, we also found dimer cations can be fragmented and generate new cations (CH(3)COOH)·CH(3)CO(+). Kinetics, thermodynamics, and entropy factors for these competitive dissociation pathways are discussed. The present report provides a clear picture of the photoionization and dissociation processes of the acetic acid dimer in the range of the photon energy 9-15 eV.

NO+ formation pathways in dissociation of N2O+ ions at the C2Σ+ state revealed from threshold photoelectron-photoion coincidence velocity imaging.

Using the novel threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging technique, the dissociative photoionization of N(2)O molecule via the C(2)Σ(+) ionic state has been investigated. Four fragment ions, NO(+), N(2)(+), O(+), and N(+), are observed, respectively, and the NO(+) and N(+) ions are always dominant in the whole excitation energy range of the C(2)Σ(+) ionic state. Subsequently, the TPEPICO three-dimensional time-sliced velocity images of NO(+) dissociated from the vibrational state-selected N(2)O(+)(C(2)Σ(+)) ions have been recorded. Thus the kinetic and internal energy distributions of the NO(+) fragments have been obtained directly as the bimodal distributions, suggesting that the NO(+) fragments are formed via both NO(+)(X(1)Σ(+)) + N((2)P) and NO(+)(X(1)Σ(+)) + N((2)D) dissociation channels. Almost the same vibrational population reversions are identified for both dissociation pathways. Interestingly, the obtained branching ratios of the two channels exhibit some dependence on the excited vibrational mode for N(2)O(+)(C(2)Σ(+)), in which the excited asymmetrical stretching potentially promotes dissociation possibility along the NO(+)(X(1)Σ(+)) + N((2)D) pathway. In addition, the measured anisotropic parameters of NO(+) are close to 0.5, indicating that the C(2)Σ(+) state of N(2)O(+) is fully predissociative, indeed, with a tendency of parallel dissociation, and therefore, the corresponding predissociation mechanisms for the N(2)O(+)(C(2)Σ(+)) ions are depicted.

Dissociation limit and dissociation dynamic of CF4+: Application of threshold photoelectron-photoion coincidence velocity imaging

Dissociation of internal energy selected CF4(+) ions in an excitation energy range of 15.40-19.60 eV has been investigated using threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging. Only CF3(+) fragment ions are observed in coincident mass spectra, indicating all the X(2)T1, A(2)T2, and B(2)E ionic states of CF4(+) are fully dissociative. Both kinetic energy released distribution (KERD) and angular distribution in dissociation of CF4(+) ions have been derived from three-dimensional TPEPICO time-sliced images. A parallel distribution of CF3(+) fragments along the polarization vector of photon is observed for dissociation of CF4(+) ions in all the low-lying electronic states. With the aid of F-loss potential energy curves, dissociation mechanisms of CF4(+) ions in these electronic states have been proposed. CF4(+) ions in both X(2)T1 and A(2)T2 states directly dissociate to CF3(+) and F fragments along the repulsive C-F coordinate, while a two-step dissociation mechanism is suggested for B(2)E state: CF4(+)(B(2)E) ion first converts to the lower A(2)T2 state via internal conversion, then dissociates to CF3(+) and F fragments along the steep A(2)T2 potential energy surface. In addition, an adiabatic appearance potential of AP0(CF3(+)∕CF4) has also been established to be 14.71 ± 0.02 eV, which is very consistent with the recent calculated values.

Experimental study of photooxidation products of ethylbenzene

Smog chamber experiments were performed to investigate the composition of products formed from photooxidation of aromatic hydrocarbon ethylbenzene. Vacuum ultraviolet photoionization mass spectrometer and aerosol time-of-flight mass spectrometer were used to measure the products in the gas and particle phases in real-time. Experimental results demonstrated that ethylphenol, methylglyoxal, phenol, benzaldehyde, and 2-ethylfurane were the predominant photooxidation products in both the gas and particle phases. However, there were some differences between detected gas phase products and those of particle phase, for example, 2-ethylfurane, ethylglyoxylic acid, nitroethylbenzene, 3,4-dioxopentanal and ethyl-nitrophenol were only existing in the particle-phase. The possible reaction mechanisms leading to these products were also discussed and proposed.