英裕 李 - Academia.edu (original) (raw)

Papers by 英裕 李

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2022

Hexagonal boron nitrides (hBNs) have a very high luminescence efficiency and are promising materi... more Hexagonal boron nitrides (hBNs) have a very high luminescence efficiency and are promising materials for deep-UV emitters. Although intense deep-UV emissions have been recorded in various forms of hBN excited by photons or energetic electrons, information on the electronic structure of the conduction band has been derived mainly from theoretical works. Therefore, there is a lack of high-resolution absorption data in the far-UV region. In this study, the far-UV absorption spectra of chemical-vapor-deposition-grown mono- and multilayer hBNs were recorded at 10 and 298 K. In addition to the previously reported band at 6.10 eV, two absorption bands at 6.82 and 8.86 eV were observed for the first time in thin-film hBN. Furthermore, excitation of the hBN thin film samples with 6.89-eV photons revealed intense emission peaks at 6.10 (mono) and 5.98 (multi) eV with a bandwidth of ∼0.7 eV. Comparing the absorption and photoluminescence data, we believe that both direct and indirect transitions occur in the radiative processes.

Dyes and Pigments, 2021

Abstract Fluorescein silica core–shell nanoparticles, gold@silica@fluorescein were synthesized to... more Abstract Fluorescein silica core–shell nanoparticles, gold@silica@fluorescein were synthesized to study the effects of metal-enhanced fluorescence (MEF) on fluorescein in neutral and basic ethanol solutions. The interaction between gold nanoparticles and fluorescein with silica shells of different thicknesses as spacers was investigated by time-resolved fluorescence spectroscopy. Only the emission spectra of fluorescein overlapped with the surface plasmon resonance of gold nanoparticles, which is regarded as an example for the study of the emission-enhancement of MEF. The fluorescence spectra of GNP@silica@FiTC were red-shifted from those of silica@FiTC and FiTC in solution. And, the emission curves had biexponential decay, indicating multiple pathways for relaxation of the excited fluorophore. The values of fluorescence enhancement factor (EF) were slightly greater in neutral ethanol solution than in basic solution. The lowest silica shell thickness synthesized, ≈12 nm, had the largest fluorescence EF≈2; this enhancement decreased as the shell thickness increased. At thickness 12 nm, the lifetime of the short component in the emission decay was 60–70 ps and its amplitude close to 90%. Based on the obtained lifetimes, ratios of component amplitudes, and proposed kinetic model, the rate constants for the process of the energy transfer between fluorophores and gold nanoparticles were attained. These rate constants for energy transfer displayed a dependence on the thickness of the silica shell, d-n, where n = 1.3–2.6.

Optics Express, 2017

Structured illumination microscopy (SIM) was recently adapted to coherent imaging, named structur... more Structured illumination microscopy (SIM) was recently adapted to coherent imaging, named structured oblique-illumination microscopy (SOIM), to improve the contrast and resolution of a light-scattering image. Herein, we present high-resolution laterally isotropic SOIM imaging with 2D hexagonal illuminations. The SOIM is implemented in a SIM fluorescence system based on a spatial-light modulator (SLM). We design an SLM pattern to generate diffraction beams at 0° and ± 60.3° simultaneously to form a 2D hexagonal illumination, and undertake calculations to obtain optimal SLM shifts at 19 phases to yield a reconstructed image correctly. Beams of linear and circular polarizations are used to show the effect of polarization on the resolution improvement. We derive the distributions of the electric field of the resultant hexagonal patterns and work out the formulations of the corresponding coherent-scattering imaging for image reconstruction. The reconstructed images of gold nanoparticles (100 nm) confirm the twofold improvement of resolution and reveal the effect of polarization on resolving adjacent nanoparticles. To demonstrate biological applications, we present the cellular structures of a label-free fixed HeLa cell with improved contrast and resolution. This work enables one to perform high-resolution dualmode − fluorescence and light-scattering − imaging in a system, and is expected to broaden the applications of SOIM.

Microelectronic Engineering, 2012

Journal of Molecular Spectroscopy, 2018

We measured the IR spectra of the solid Ar-trapped chloromethylene (HCCl) prepared by electron bo... more We measured the IR spectra of the solid Ar-trapped chloromethylene (HCCl) prepared by electron bombardment of a CH 3 Cl/Ar mixture during matrix deposition. Irradiation of matrix samples at 210 nm decreased the intensity of lines corresponding to HCCl, whereas irradiation at 385 nm had an opposite effect. The above behavior allowed an unambiguous identification of all vibrational modes of HCCl, and the obtained assignments were further confirmed by quantum chemical calculations and 13 C/ 2 H isotopic labeling experiments. Highlights  HCCl generated by electron bombardment of CH 3 Cl/Ar matrix sample.  Comparison of anharmonic vibrational frequencies of isotopomers of ground-state HCCl predicted by B3LYP/aug-cc-pVTZ.  Identification of the CH stretching mode of HCCl in solid Ar.

Journal of the Chinese Chemical Society

The effects of metal‐enhanced fluorescence (MEF) of rhodamine dye by gold sub‐micro hexagonal pla... more The effects of metal‐enhanced fluorescence (MEF) of rhodamine dye by gold sub‐micro hexagonal plates were studied. A multilayered avidin–biotin complex was designed and used as a spacer to connect the fluorophore and the sub‐microplate. Fluorescence lifetime image microscopy (FLIM) combined with time‐correlated single‐photon counting was used to obtain lifetime images and intensities of dye on single particles. Gold hexagon plates ~900 nm were synthesized, and the avidin–biotin complexes were placed layer‐by‐layer for up to 4 layers onto the sub‐microplates. All emission curves of fluorophore displayed biexponential decay with short lifetimes of 18–23 ps and long lifetimes of 249, 271, 304, and 348 ps for 1–4 layers on gold sub‐microplates, respectively. Using the kinetic model of energy transfer between fluorophore and nanoparticles to explain the coupling mechanism, we found that the excited fluorophore transferred energy mostly to the high‐order modes for the 1‐layered avidin–bio...

Journal of the Chinese Chemical Society, 2021

Gold nanotriangles (AuNTs) and nanocubes (NCs) coated with silica shell and fluorescein were synt... more Gold nanotriangles (AuNTs) and nanocubes (NCs) coated with silica shell and fluorescein were synthesized to study the effects of metal‐enhanced fluorescence. The interaction between gold nanoparticles (GNPs) and fluorescein with silica shells of different thicknesses as spacers was investigated by using time‐resolved fluorescence spectroscopy. From the biexponential decay of fluorescence of fluorophore and the kinetic mechanism for the interaction, we obtained the rate constants of energy transfer processes. Both energy transfer rates from fluorophore to the bright and dark modes of gold nanoparticles decreased with distance d (= silica thickness) with a dependence ∝ d−n and n ≈ 2. The rate constant of nanosurface energy transfer kNEST, considering dipolar interaction between fluorophore and metal surface, has an estimated value 1.7 times slower at d = 9 nm and about four times slower at 25 nm than the obtained rate constant for energy transfer from fluorophore to GNP dipolar mode i...

Author Institution: Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan... more Author Institution: Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan; National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan; Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan

Acta Crystallographica Section A Foundations and Advances, 2018

Journal of Synchrotron Radiation, 2019

The covalent electron density, which makes Si(222) measurable, is subject to laser excitation. Th... more The covalent electron density, which makes Si(222) measurable, is subject to laser excitation. The three-wave Si(222)/(13 {\overline 1}) diffraction at 7.82 keV is used for phase measurements. It is found that laser excitation causes a relative phase change of around 4° in Si(222) in the first 100 ps of excitation and this is gradually recovered over several nanoseconds. This phase change is due to laser excitation of covalent electrons around the silicon atoms in the unit cell and makes the electron density deviate further from the centrosymmetric distribution.

Proceedings of the Combustion Institute, 2019

To explore the fuel isomeric effects on the benzene formation pathways, the pyrolysis of two C 6 ... more To explore the fuel isomeric effects on the benzene formation pathways, the pyrolysis of two C 6 H 10 isomers, cyclohexene (cC 6 H 10) and 1,5-hexadiene (C 6 H 10-15), was investigated by using molecular-beam mass spectrometry with tunable synchrotron radiation as the ionization source. The isomer-resolved pyrolysis intermediates, including some key radicals, were clearly identified and quantified at different temperatures for both fuels. A new kinetic model was developed and validated against the experimental results. The fuel-specific intermediates pools, the dominant fuel destruction pathways, as well as specific reactions channels leading towards benzene formations under pyrolysis conditions were revealed through experimental and modeling efforts. The elimination reaction (cC 6 H 10 = C 2 H 4 + C 4 H 6) and the bond fission (C 6 H 10-15 = C 3 H 5-A + C 3 H 5-A) dominate the consumption of cC 6 H 10 and C 6 H 10-15, respectively. Although the fuel structures of cC 6 H 10 and C 6 H 10-15 and their corresponding intermediate pools are quite different, the stepwise dehydrogenation reactions via cyclohexadiene isomers contribute to the majority of the benzene formation in the pyrolysis of both fuels. The recombination between the propargyl radical (C 3 H 3) and allyl radical (C 3 H 5-A) also contributes to benzene formation in the case of C 6 H 10-15, while the C 4 + C 2 pathway provides a small amount of benzene in the case of cC 6 H 10 .

Energy & Fuels, 2017

This work investigates the pyrolysis of n-butylbenzene, which widely exists in transportation fue... more This work investigates the pyrolysis of n-butylbenzene, which widely exists in transportation fuels and their surrogate mixtures. Both reactive and stable pyrolysis products were comprehensively detected with synchrotron vacuum ultraviolet photoionization mass spectrometry. Their mole fractions versus temperature were also evaluated at 30, 150, and 760 Torr. A kinetic model of n-butylbenzene pyrolysis was developed, and new data were used to validate the model. On the basis of the modeling analysis, the benzylic C−C bond dissociation that forms the benzyl radical and the propyl radical was found to be a key decomposition reaction of n-butylbenzene at all investigated pressures, whereas H abstraction provided increasing contributions with increasing pressure. Compared with small alkylbenzenes, such as toluene and ethylbenzene, n-butylbenzene demonstrates different pyrolysis characteristics and chemistry because of the existence of its long alkyl side chain. n-Butylbenzene has a higher pyrolysis reactivity and lower decomposition temperature regions, which inhibit the further decomposition of the benzyl radical and the formation of highly unsaturated C 2 −C 4 products. As a result, conventional combination reactions between aromatic radicals and highly unsaturated C 2 −C 4 species are only minor formation pathways for indene and naphthalene in nbutylbenzene pyrolysis, while fuel-specific pathways become crucial instead. Furthermore, combination reactions involving the benzyl radical and the phenyl radical are crucial for the formation of many PAHs, especially phenanthrene and fluorene. The results in this work reveal the strong influence of side-chain length on the pyrolysis chemistry of alkylbenzenes and indicate a further need for exploring the influences of other structural features.

The Journal of Physical Chemistry C, 2018

The Journal of chemical physics, Jan 21, 2018

Phenol is an important model molecule for the theoretical and experimental investigation of disso... more Phenol is an important model molecule for the theoretical and experimental investigation of dissociation in the multistate potential energy surfaces. Recent theoretical calculations [X. Xu et al., J. Am. Chem. Soc. 136, 16378 (2014)] suggest that the phenoxyl radical produced in both the X and A states from the O-H bond fission in phenol can contribute substantially to the slow component of photofragment translational energy distribution. However, current experimental techniques struggle to separate the contributions from different dissociation pathways. A new type of time-resolved pump-probe experiment is described that enables the selection of the products generated from a specific time window after molecules are excited by a pump laser pulse and can quantitatively characterize the translational energy distribution and branching ratio of each dissociation pathway. This method modifies conventional photofragment translational spectroscopy by reducing the acceptance angles of the de...

The Journal of Physical Chemistry, 1995

The Journal of Physical Chemistry B, 2017

While the significance of aqueous interfaces has been recognized in numerous important fields, it... more While the significance of aqueous interfaces has been recognized in numerous important fields, it can be even more prominent for nanoscaled aqueous aerosols due to their large surfaceto-volume ratios and prevalent existence in nature. Also, considering that organic species are often mixed with aqueous aerosols in nature, a fundamental understanding on the electronic and structural properties of organic species in aqueous nanoaerosols is essential to learn the interplay between water and organic solutes under the nanoscaled size regime. Here we report for the first time the VUV photoelectron spectroscopy of phenol and three dihydroxybenzene (DHB) isomers including catechol, resorcinol and hydroquinone in the aqueous nanoaerosol form. By evaluating two photoelectron features of the lowest vertical ionization energies (VIE) originated from the b 1 (π) and a 2 (π) orbitals for phenolic aqueous nanoaerosols, their interfacial solvation characteristics are unraveled. Phenolic species appear to reside primarily on/near the aqueous nanoaerosol interface, where they appear only partially hydrated on the aqueous interface with the hydrophilic hydroxyl group more solvated in water. An appreciable proportion of phenol is found to coexist with phenolate at/near the nanoaerosol interface even under a high bulk pH of 12.0, indicating that the nanoaerosol interface exhibits a composition distribution and pH drastically different from the bulk. The surface pH of phenol-containing aqueous nanoaerosols is found ~2.2 ± 0.1 units more acidic than the bulk interior. From the photoelectron spectra of DHB aqueous nanoaerosols, the effects of number/arrangements of-OH group are interrogated. This study shows that the hydration extents, pH values, deprotonation status, and numbers/relative arrangments of-OH group are crucial factors affecting the ionization energies of phenolic aqueous nanoaerosols, and thus their redox-based activities. The multi-faceted implications of the present study in the aerosol science, atmospheric/marine chemistry and biological science are also addressed.

Combustion and Flame, 2017

In this work, oxidation chemistry of methanol was investigated in laminar premixed flames. Lamina... more In this work, oxidation chemistry of methanol was investigated in laminar premixed flames. Laminar burning velocities of methanol/air mixtures at 423 K and 1-10 atm were measured in a spherical combustion vessel, extending the range of equivalence ratio up to 2.1. A stoichiometric premixed flat flame of methanol/O 2 /Ar at 0.04 atm was also conducted using synchrotron VUV photoionization mass spectrometry (SVUV-PIMS) to obtain more detailed kinetic information. Particularly, fuel-derived radicals including methoxy radical and hydroxymethyl radical were observed, while formaldehyde (CH 2 O) and formic acid (HOCHO) were identified as the abundantly produced intermediates in methanol flame. A methanol model was developed and validated against the experimental data obtained in this work, as well as in literature. In the predictions of laminar burning velocities, HO 2 radical plays an important role under very rich conditions. Besides, the measurement of formaldehyde and formic acid in methanol flame is helpful to constrain the rate constant of CH 2 OH + O 2 = CH 2 O + HO 2 , which presents large uncertainties at high temperatures.

The Journal of Chemical Physics, 2017

To facilitate direct spectroscopic observation of hydrogen chloride anions (HCl−), electron bomba... more To facilitate direct spectroscopic observation of hydrogen chloride anions (HCl−), electron bombardment of CH3Cl diluted in excess Ar during matrix deposition was used to generate this anion. Subsequent characterization were performed by IR spectroscopy and quantum chemical calculations. Moreover the band intensity of HCl− decays slowly when the matrix sample is maintained in the dark for a prolonged time. High-level ab inito calculation suggested that HCl− is only weakly bound. Atom-in-molecule charge analysis indicated that both atoms of HCl− are negatively charged and the Cl atom is hypervalent.

The Journal of Chemical Physics, 2017

The original ion imaging technique has low velocity resolution, and currently, photodissociation ... more The original ion imaging technique has low velocity resolution, and currently, photodissociation is mostly investigated using velocity map ion imaging. However, separating signals from the background (resulting from undissociated excited parent molecules) is difficult when velocity map ion imaging is used for the photodissociation of large molecules (number of atoms ≥ 10). In this study, we used the photodissociation of phenol at the S1 band origin as an example to demonstrate how our multimass ion imaging technique, based on modified spatial map ion imaging, can overcome this difficulty. The photofragment translational energy distribution obtained when multimass ion imaging was used differed considerably from that obtained when velocity map ion imaging and Rydberg atom tagging were used. We used conventional translational spectroscopy as a second method to further confirm the experimental results, and we conclude that data should be interpreted carefully when velocity map ion imagi...

The journal of physical chemistry. B, Oct 2, 2016

Glutathione (GSH), the most abundant non-enzymatic antioxidant in living systems actively scaveng... more Glutathione (GSH), the most abundant non-enzymatic antioxidant in living systems actively scavenges various exogenous/endogenous oxidizing species, defending important biomolecules against oxidative damages. Though it is well established that the antioxidant activity of GSH is originated from the cysteinyl thiol (-SH) group, however, the molecular origin that makes the thiol group of GSH a stronger reducing agent than other thiol-containing proteins is unclear. To gain insights into the molecular basis underlying GSH's superior antioxidant capability, here we report the valence electronic structures of solvated GSH for the first time in the aqueous aerosol form via the aerosol VUV photoelectron spectroscopy technique. The pH-dependent electronic evolution of GSH is obtained and the possible correlations between GSH and its constituting amino acids are interrogated. The valence band maximum (VBM) for GSH aqueous aerosols is found at 7.81, 7.61, 7.52 and 5.51 ± 0.10 eV at pH of 1....

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2022

Hexagonal boron nitrides (hBNs) have a very high luminescence efficiency and are promising materi... more Hexagonal boron nitrides (hBNs) have a very high luminescence efficiency and are promising materials for deep-UV emitters. Although intense deep-UV emissions have been recorded in various forms of hBN excited by photons or energetic electrons, information on the electronic structure of the conduction band has been derived mainly from theoretical works. Therefore, there is a lack of high-resolution absorption data in the far-UV region. In this study, the far-UV absorption spectra of chemical-vapor-deposition-grown mono- and multilayer hBNs were recorded at 10 and 298 K. In addition to the previously reported band at 6.10 eV, two absorption bands at 6.82 and 8.86 eV were observed for the first time in thin-film hBN. Furthermore, excitation of the hBN thin film samples with 6.89-eV photons revealed intense emission peaks at 6.10 (mono) and 5.98 (multi) eV with a bandwidth of ∼0.7 eV. Comparing the absorption and photoluminescence data, we believe that both direct and indirect transitions occur in the radiative processes.

Dyes and Pigments, 2021

Abstract Fluorescein silica core–shell nanoparticles, gold@silica@fluorescein were synthesized to... more Abstract Fluorescein silica core–shell nanoparticles, gold@silica@fluorescein were synthesized to study the effects of metal-enhanced fluorescence (MEF) on fluorescein in neutral and basic ethanol solutions. The interaction between gold nanoparticles and fluorescein with silica shells of different thicknesses as spacers was investigated by time-resolved fluorescence spectroscopy. Only the emission spectra of fluorescein overlapped with the surface plasmon resonance of gold nanoparticles, which is regarded as an example for the study of the emission-enhancement of MEF. The fluorescence spectra of GNP@silica@FiTC were red-shifted from those of silica@FiTC and FiTC in solution. And, the emission curves had biexponential decay, indicating multiple pathways for relaxation of the excited fluorophore. The values of fluorescence enhancement factor (EF) were slightly greater in neutral ethanol solution than in basic solution. The lowest silica shell thickness synthesized, ≈12 nm, had the largest fluorescence EF≈2; this enhancement decreased as the shell thickness increased. At thickness 12 nm, the lifetime of the short component in the emission decay was 60–70 ps and its amplitude close to 90%. Based on the obtained lifetimes, ratios of component amplitudes, and proposed kinetic model, the rate constants for the process of the energy transfer between fluorophores and gold nanoparticles were attained. These rate constants for energy transfer displayed a dependence on the thickness of the silica shell, d-n, where n = 1.3–2.6.

Optics Express, 2017

Structured illumination microscopy (SIM) was recently adapted to coherent imaging, named structur... more Structured illumination microscopy (SIM) was recently adapted to coherent imaging, named structured oblique-illumination microscopy (SOIM), to improve the contrast and resolution of a light-scattering image. Herein, we present high-resolution laterally isotropic SOIM imaging with 2D hexagonal illuminations. The SOIM is implemented in a SIM fluorescence system based on a spatial-light modulator (SLM). We design an SLM pattern to generate diffraction beams at 0° and ± 60.3° simultaneously to form a 2D hexagonal illumination, and undertake calculations to obtain optimal SLM shifts at 19 phases to yield a reconstructed image correctly. Beams of linear and circular polarizations are used to show the effect of polarization on the resolution improvement. We derive the distributions of the electric field of the resultant hexagonal patterns and work out the formulations of the corresponding coherent-scattering imaging for image reconstruction. The reconstructed images of gold nanoparticles (100 nm) confirm the twofold improvement of resolution and reveal the effect of polarization on resolving adjacent nanoparticles. To demonstrate biological applications, we present the cellular structures of a label-free fixed HeLa cell with improved contrast and resolution. This work enables one to perform high-resolution dualmode − fluorescence and light-scattering − imaging in a system, and is expected to broaden the applications of SOIM.

Microelectronic Engineering, 2012

Journal of Molecular Spectroscopy, 2018

We measured the IR spectra of the solid Ar-trapped chloromethylene (HCCl) prepared by electron bo... more We measured the IR spectra of the solid Ar-trapped chloromethylene (HCCl) prepared by electron bombardment of a CH 3 Cl/Ar mixture during matrix deposition. Irradiation of matrix samples at 210 nm decreased the intensity of lines corresponding to HCCl, whereas irradiation at 385 nm had an opposite effect. The above behavior allowed an unambiguous identification of all vibrational modes of HCCl, and the obtained assignments were further confirmed by quantum chemical calculations and 13 C/ 2 H isotopic labeling experiments. Highlights  HCCl generated by electron bombardment of CH 3 Cl/Ar matrix sample.  Comparison of anharmonic vibrational frequencies of isotopomers of ground-state HCCl predicted by B3LYP/aug-cc-pVTZ.  Identification of the CH stretching mode of HCCl in solid Ar.

Journal of the Chinese Chemical Society

The effects of metal‐enhanced fluorescence (MEF) of rhodamine dye by gold sub‐micro hexagonal pla... more The effects of metal‐enhanced fluorescence (MEF) of rhodamine dye by gold sub‐micro hexagonal plates were studied. A multilayered avidin–biotin complex was designed and used as a spacer to connect the fluorophore and the sub‐microplate. Fluorescence lifetime image microscopy (FLIM) combined with time‐correlated single‐photon counting was used to obtain lifetime images and intensities of dye on single particles. Gold hexagon plates ~900 nm were synthesized, and the avidin–biotin complexes were placed layer‐by‐layer for up to 4 layers onto the sub‐microplates. All emission curves of fluorophore displayed biexponential decay with short lifetimes of 18–23 ps and long lifetimes of 249, 271, 304, and 348 ps for 1–4 layers on gold sub‐microplates, respectively. Using the kinetic model of energy transfer between fluorophore and nanoparticles to explain the coupling mechanism, we found that the excited fluorophore transferred energy mostly to the high‐order modes for the 1‐layered avidin–bio...

Journal of the Chinese Chemical Society, 2021

Gold nanotriangles (AuNTs) and nanocubes (NCs) coated with silica shell and fluorescein were synt... more Gold nanotriangles (AuNTs) and nanocubes (NCs) coated with silica shell and fluorescein were synthesized to study the effects of metal‐enhanced fluorescence. The interaction between gold nanoparticles (GNPs) and fluorescein with silica shells of different thicknesses as spacers was investigated by using time‐resolved fluorescence spectroscopy. From the biexponential decay of fluorescence of fluorophore and the kinetic mechanism for the interaction, we obtained the rate constants of energy transfer processes. Both energy transfer rates from fluorophore to the bright and dark modes of gold nanoparticles decreased with distance d (= silica thickness) with a dependence ∝ d−n and n ≈ 2. The rate constant of nanosurface energy transfer kNEST, considering dipolar interaction between fluorophore and metal surface, has an estimated value 1.7 times slower at d = 9 nm and about four times slower at 25 nm than the obtained rate constant for energy transfer from fluorophore to GNP dipolar mode i...

Author Institution: Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan... more Author Institution: Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan; National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan; Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan

Acta Crystallographica Section A Foundations and Advances, 2018

Journal of Synchrotron Radiation, 2019

The covalent electron density, which makes Si(222) measurable, is subject to laser excitation. Th... more The covalent electron density, which makes Si(222) measurable, is subject to laser excitation. The three-wave Si(222)/(13 {\overline 1}) diffraction at 7.82 keV is used for phase measurements. It is found that laser excitation causes a relative phase change of around 4° in Si(222) in the first 100 ps of excitation and this is gradually recovered over several nanoseconds. This phase change is due to laser excitation of covalent electrons around the silicon atoms in the unit cell and makes the electron density deviate further from the centrosymmetric distribution.

Proceedings of the Combustion Institute, 2019

To explore the fuel isomeric effects on the benzene formation pathways, the pyrolysis of two C 6 ... more To explore the fuel isomeric effects on the benzene formation pathways, the pyrolysis of two C 6 H 10 isomers, cyclohexene (cC 6 H 10) and 1,5-hexadiene (C 6 H 10-15), was investigated by using molecular-beam mass spectrometry with tunable synchrotron radiation as the ionization source. The isomer-resolved pyrolysis intermediates, including some key radicals, were clearly identified and quantified at different temperatures for both fuels. A new kinetic model was developed and validated against the experimental results. The fuel-specific intermediates pools, the dominant fuel destruction pathways, as well as specific reactions channels leading towards benzene formations under pyrolysis conditions were revealed through experimental and modeling efforts. The elimination reaction (cC 6 H 10 = C 2 H 4 + C 4 H 6) and the bond fission (C 6 H 10-15 = C 3 H 5-A + C 3 H 5-A) dominate the consumption of cC 6 H 10 and C 6 H 10-15, respectively. Although the fuel structures of cC 6 H 10 and C 6 H 10-15 and their corresponding intermediate pools are quite different, the stepwise dehydrogenation reactions via cyclohexadiene isomers contribute to the majority of the benzene formation in the pyrolysis of both fuels. The recombination between the propargyl radical (C 3 H 3) and allyl radical (C 3 H 5-A) also contributes to benzene formation in the case of C 6 H 10-15, while the C 4 + C 2 pathway provides a small amount of benzene in the case of cC 6 H 10 .

Energy & Fuels, 2017

This work investigates the pyrolysis of n-butylbenzene, which widely exists in transportation fue... more This work investigates the pyrolysis of n-butylbenzene, which widely exists in transportation fuels and their surrogate mixtures. Both reactive and stable pyrolysis products were comprehensively detected with synchrotron vacuum ultraviolet photoionization mass spectrometry. Their mole fractions versus temperature were also evaluated at 30, 150, and 760 Torr. A kinetic model of n-butylbenzene pyrolysis was developed, and new data were used to validate the model. On the basis of the modeling analysis, the benzylic C−C bond dissociation that forms the benzyl radical and the propyl radical was found to be a key decomposition reaction of n-butylbenzene at all investigated pressures, whereas H abstraction provided increasing contributions with increasing pressure. Compared with small alkylbenzenes, such as toluene and ethylbenzene, n-butylbenzene demonstrates different pyrolysis characteristics and chemistry because of the existence of its long alkyl side chain. n-Butylbenzene has a higher pyrolysis reactivity and lower decomposition temperature regions, which inhibit the further decomposition of the benzyl radical and the formation of highly unsaturated C 2 −C 4 products. As a result, conventional combination reactions between aromatic radicals and highly unsaturated C 2 −C 4 species are only minor formation pathways for indene and naphthalene in nbutylbenzene pyrolysis, while fuel-specific pathways become crucial instead. Furthermore, combination reactions involving the benzyl radical and the phenyl radical are crucial for the formation of many PAHs, especially phenanthrene and fluorene. The results in this work reveal the strong influence of side-chain length on the pyrolysis chemistry of alkylbenzenes and indicate a further need for exploring the influences of other structural features.

The Journal of Physical Chemistry C, 2018

The Journal of chemical physics, Jan 21, 2018

Phenol is an important model molecule for the theoretical and experimental investigation of disso... more Phenol is an important model molecule for the theoretical and experimental investigation of dissociation in the multistate potential energy surfaces. Recent theoretical calculations [X. Xu et al., J. Am. Chem. Soc. 136, 16378 (2014)] suggest that the phenoxyl radical produced in both the X and A states from the O-H bond fission in phenol can contribute substantially to the slow component of photofragment translational energy distribution. However, current experimental techniques struggle to separate the contributions from different dissociation pathways. A new type of time-resolved pump-probe experiment is described that enables the selection of the products generated from a specific time window after molecules are excited by a pump laser pulse and can quantitatively characterize the translational energy distribution and branching ratio of each dissociation pathway. This method modifies conventional photofragment translational spectroscopy by reducing the acceptance angles of the de...

The Journal of Physical Chemistry, 1995

The Journal of Physical Chemistry B, 2017

While the significance of aqueous interfaces has been recognized in numerous important fields, it... more While the significance of aqueous interfaces has been recognized in numerous important fields, it can be even more prominent for nanoscaled aqueous aerosols due to their large surfaceto-volume ratios and prevalent existence in nature. Also, considering that organic species are often mixed with aqueous aerosols in nature, a fundamental understanding on the electronic and structural properties of organic species in aqueous nanoaerosols is essential to learn the interplay between water and organic solutes under the nanoscaled size regime. Here we report for the first time the VUV photoelectron spectroscopy of phenol and three dihydroxybenzene (DHB) isomers including catechol, resorcinol and hydroquinone in the aqueous nanoaerosol form. By evaluating two photoelectron features of the lowest vertical ionization energies (VIE) originated from the b 1 (π) and a 2 (π) orbitals for phenolic aqueous nanoaerosols, their interfacial solvation characteristics are unraveled. Phenolic species appear to reside primarily on/near the aqueous nanoaerosol interface, where they appear only partially hydrated on the aqueous interface with the hydrophilic hydroxyl group more solvated in water. An appreciable proportion of phenol is found to coexist with phenolate at/near the nanoaerosol interface even under a high bulk pH of 12.0, indicating that the nanoaerosol interface exhibits a composition distribution and pH drastically different from the bulk. The surface pH of phenol-containing aqueous nanoaerosols is found ~2.2 ± 0.1 units more acidic than the bulk interior. From the photoelectron spectra of DHB aqueous nanoaerosols, the effects of number/arrangements of-OH group are interrogated. This study shows that the hydration extents, pH values, deprotonation status, and numbers/relative arrangments of-OH group are crucial factors affecting the ionization energies of phenolic aqueous nanoaerosols, and thus their redox-based activities. The multi-faceted implications of the present study in the aerosol science, atmospheric/marine chemistry and biological science are also addressed.

Combustion and Flame, 2017

In this work, oxidation chemistry of methanol was investigated in laminar premixed flames. Lamina... more In this work, oxidation chemistry of methanol was investigated in laminar premixed flames. Laminar burning velocities of methanol/air mixtures at 423 K and 1-10 atm were measured in a spherical combustion vessel, extending the range of equivalence ratio up to 2.1. A stoichiometric premixed flat flame of methanol/O 2 /Ar at 0.04 atm was also conducted using synchrotron VUV photoionization mass spectrometry (SVUV-PIMS) to obtain more detailed kinetic information. Particularly, fuel-derived radicals including methoxy radical and hydroxymethyl radical were observed, while formaldehyde (CH 2 O) and formic acid (HOCHO) were identified as the abundantly produced intermediates in methanol flame. A methanol model was developed and validated against the experimental data obtained in this work, as well as in literature. In the predictions of laminar burning velocities, HO 2 radical plays an important role under very rich conditions. Besides, the measurement of formaldehyde and formic acid in methanol flame is helpful to constrain the rate constant of CH 2 OH + O 2 = CH 2 O + HO 2 , which presents large uncertainties at high temperatures.

The Journal of Chemical Physics, 2017

To facilitate direct spectroscopic observation of hydrogen chloride anions (HCl−), electron bomba... more To facilitate direct spectroscopic observation of hydrogen chloride anions (HCl−), electron bombardment of CH3Cl diluted in excess Ar during matrix deposition was used to generate this anion. Subsequent characterization were performed by IR spectroscopy and quantum chemical calculations. Moreover the band intensity of HCl− decays slowly when the matrix sample is maintained in the dark for a prolonged time. High-level ab inito calculation suggested that HCl− is only weakly bound. Atom-in-molecule charge analysis indicated that both atoms of HCl− are negatively charged and the Cl atom is hypervalent.

The Journal of Chemical Physics, 2017

The original ion imaging technique has low velocity resolution, and currently, photodissociation ... more The original ion imaging technique has low velocity resolution, and currently, photodissociation is mostly investigated using velocity map ion imaging. However, separating signals from the background (resulting from undissociated excited parent molecules) is difficult when velocity map ion imaging is used for the photodissociation of large molecules (number of atoms ≥ 10). In this study, we used the photodissociation of phenol at the S1 band origin as an example to demonstrate how our multimass ion imaging technique, based on modified spatial map ion imaging, can overcome this difficulty. The photofragment translational energy distribution obtained when multimass ion imaging was used differed considerably from that obtained when velocity map ion imaging and Rydberg atom tagging were used. We used conventional translational spectroscopy as a second method to further confirm the experimental results, and we conclude that data should be interpreted carefully when velocity map ion imagi...

The journal of physical chemistry. B, Oct 2, 2016

Glutathione (GSH), the most abundant non-enzymatic antioxidant in living systems actively scaveng... more Glutathione (GSH), the most abundant non-enzymatic antioxidant in living systems actively scavenges various exogenous/endogenous oxidizing species, defending important biomolecules against oxidative damages. Though it is well established that the antioxidant activity of GSH is originated from the cysteinyl thiol (-SH) group, however, the molecular origin that makes the thiol group of GSH a stronger reducing agent than other thiol-containing proteins is unclear. To gain insights into the molecular basis underlying GSH's superior antioxidant capability, here we report the valence electronic structures of solvated GSH for the first time in the aqueous aerosol form via the aerosol VUV photoelectron spectroscopy technique. The pH-dependent electronic evolution of GSH is obtained and the possible correlations between GSH and its constituting amino acids are interrogated. The valence band maximum (VBM) for GSH aqueous aerosols is found at 7.81, 7.61, 7.52 and 5.51 ± 0.10 eV at pH of 1....