Bio


Fang Liu is an assistant professor of chemistry at Stanford University. Her reserach is focused on the light induced dynamics of solid low dimensional materials and construction of low dimensional artificial structures. Prior to her current position, she was a DOE Office of Energy Efficiency and Renewable Energy (EERE) postdoctoral fellow in the group of Prof. Xiaoyang Zhu at Columbia University. Her postdoctoral research focused on using femtosecond extreme UV in probing time and angle resolved photoemission spectroscopy of 2D materials. Prior to working in Columbia, she worked under the direction of Prof. Marsha I Lester at University of Pennsylvania. She received her Ph.D. in 2015 and worked as a postdoc in the same group in 2016. At UPenn, she used time resolved spectroscopic techniques to study spectroscopy and photochemistry of Criegee intermediates. She received her B.S. in chemistry at Peking University in 2010.

Academic Appointments


Honors & Awards


  • Office of Energy Efficiency & Renewable Energy (EERE) Postdoctoral Research Award, Solar Energy Technologies Office of the Department of Energy (2018-2020)
  • Miller Prize, International Symposium on Molecular Spectroscopy (2019)

Professional Education


  • B.S. in chemistry, Peking University (2010)
  • Ph.D. in chemistry, University of Pennsylvania (2015)

Current Research and Scholarly Interests


The group will develop scalable and controllable processes to produce low dimensional materials and their artificial structures, and unravel their novel static and dynamical properties of broad interest to future photonic, electronic and energy technologies. The topics will include: a) Unraveling time-resolved dynamics in light-induced electronic response of two dimensional (2D) materials artificial structures. b) Fabrication of 1D atomically thin nanoribbon arrays and characterization of the electronic and magnetic properties for the prominent edge states. c) Lightwave manipulation with 2D superlattices. These research projects will provide participating students with broad interdisciplinary training across physics, chemistry, and materials science.

Stanford Advisees


All Publications


  • Dissecting Interlayer Hole and Electron Transfer in Transition Metal Dichalcogenide Heterostructures via Two-Dimensional Electronic Spectroscopy. Nano letters Policht, V. R., Russo, M., Liu, F., Trovatello, C., Maiuri, M., Bai, Y., Zhu, X., Dal Conte, S., Cerullo, G. 2021

    Abstract

    Monolayer transition metal dichalcogenides (ML-TMDs) are two-dimensional semiconductors that stack to form heterostructures (HSs) with tailored electronic and optical properties. TMD/TMD-HSs like WS2/MoS2 have type II band alignment and form long-lived (nanosecond) interlayer excitons following sub-100 fs interlayer charge transfer (ICT) from the photoexcited intralayer exciton. While many studies have demonstrated the ultrafast nature of ICT processes, we still lack a clear physical understanding of ICT due to the trade-off between temporal and frequency resolution in conventional transient absorption spectroscopy. Here, we perform two-dimensional electronic spectroscopy (2DES), a method with both high frequency and temporal resolution, on a large-area WS2/MoS2 HS where we unambiguously time resolve both interlayer hole and electron transfer with 34 ± 14 and 69 ± 9 fs time constants, respectively. We simultaneously resolve additional optoelectronic processes including band gap renormalization and intralayer exciton coupling. This study demonstrates the advantages of 2DES in comprehensively resolving ultrafast processes in TMD-HS, including ICT.

    View details for DOI 10.1021/acs.nanolett.1c01098

    View details for PubMedID 34037406

  • Mechanical exfoliation of large area 2D materials from vdW crystals PROGRESS IN SURFACE SCIENCE Liu, F. 2021; 96 (2)
  • Excitons in strain-induced one-dimensional moire potentials at transition metal dichalcogenide heterojunctions NATURE MATERIALS Bai, Y., Zhou, L., Wang, J., Wu, W., McGilly, L. J., Halbertal, D., Lo, C., Liu, F., Ardelean, J., Rivera, P., Finney, N. R., Yang, X., Basov, D. N., Yao, W., Xu, X., Hone, J., Pasupathy, A. N., Zhu, X. 2020

    Abstract

    The possibility of confining interlayer excitons in interfacial moiré patterns has recently gained attention as a strategy to form ordered arrays of zero-dimensional quantum emitters and topological superlattices in transition metal dichalcogenide heterostructures. Strain is expected to play an important role in the modulation of the moiré potential landscape, tuning the array of quantum dot-like zero-dimensional traps into parallel stripes of one-dimensional quantum wires. Here, we present real-space imaging of unstrained zero-dimensional and strain-induced one-dimensional moiré patterns along with photoluminescence measurements of the corresponding excitonic emission from WSe2/MoSe2 heterobilayers. Whereas excitons in zero-dimensional moiré traps display quantum emitter-like sharp photoluminescence peaks with circular polarization, the photoluminescence emission from excitons in one-dimensional moiré potentials shows linear polarization and two orders of magnitude higher intensity. These results establish strain engineering as an effective method to tailor moiré potentials and their optoelectronic response on demand.

    View details for DOI 10.1038/s41563-020-0730-8

    View details for Web of Science ID 000548166000009

    View details for PubMedID 32661380

  • Direct determination of momentum-resolved electron transfer in the photoexcited van der Waals heterobilayer WS2/MoS2 PHYSICAL REVIEW B Liu, F., Li, Q., Zhu, X. 2020; 101 (20)
  • Strong polaronic effect in a superatomic two-dimensional semiconductor JOURNAL OF CHEMICAL PHYSICS Li, Q., Liu, F., Russell, J. C., Roy, X., Zhu, X. 2020; 152 (17): 171101

    Abstract

    Crystalline solids assembled from superatomic building blocks are attractive functional materials due to their hierarchical structure, multifunctionality, and tunability. An interesting example is Re6Se8Cl2, in which the Re6Se8 building blocks are covalently linked into two-dimensional (2D) sheets that are stacked into a layered van der Waals solid. It is an indirect gap semiconductor that, when heavily doped, becomes a superconductor at low temperatures. Given the finite electronic bandwidths (300-400 meV), carrier properties in this material are expected to be strongly influenced by coupling to phonons. Here, we apply angle-resolved photoemission spectroscopy to probe the valence band edge (VBE) of Re6Se8Cl2. We find that dispersion of the VBE is a strong function of temperature. The bandwidth is W = 120 ± 30 meV at 70 K and decreases by one order of magnitude to W ∼ 10 ± 20 meV as temperature is increased to 300 K. This observation reveals the dominant polaronic effects in Re6Se8Cl2, consistent with the Holstein polaron model commonly used to describe molecular solids.

    View details for DOI 10.1063/5.0006455

    View details for Web of Science ID 000532295500001

    View details for PubMedID 32384833

  • Disassembling 2D van der Waals crystals into macroscopic monolayers and reassembling into artificial lattices SCIENCE Liu, F., Wu, W., Bai, Y., Chae, S., Li, Q., Wang, J., Hone, J., Zhu, X. 2020; 367 (6480): 903-+

    Abstract

    Two-dimensional materials from layered van der Waals (vdW) crystals hold great promise for electronic, optoelectronic, and quantum devices, but technological implementation will be hampered by the lack of high-throughput techniques for exfoliating single-crystal monolayers with sufficient size and high quality. Here, we report a facile method to disassemble vdW single crystals layer by layer into monolayers with near-unity yield and with dimensions limited only by bulk crystal sizes. The macroscopic monolayers are comparable in quality to microscopic monolayers from conventional Scotch tape exfoliation. The monolayers can be assembled into macroscopic artificial structures, including transition metal dichalcogenide multilayers with broken inversion symmetry and substantially enhanced nonlinear optical response. This approach takes us one step closer to mass production of macroscopic monolayers and bulk-like artificial materials with controllable properties.

    View details for DOI 10.1126/science.aba1416

    View details for Web of Science ID 000515235800042

    View details for PubMedID 32079769

  • Broad-Band Near-Infrared Doublet Emission in a Tetrathiafulvalene-Based Metal-Organic Framework JOURNAL OF PHYSICAL CHEMISTRY LETTERS Wang, F., Wang, J., Maehrlein, S. F., Ma, Y., Liu, F., Zhu, X. 2020; 11 (3): 762–66

    Abstract

    The upper limit in LED quantum efficiency from conventional closed-shell molecules is 25% as dictated by singlet and triplet spin statistics. Spin-doublet organic molecules are attractive candidates to exceed this limit, thanks to their 100% theoretical quantum efficiency in radiative recombination. However, examples of stable spin-doublet molecules in the solid state are rare. Here we show broad-band near-infrared emission in the columnar π-π stacked tetrathiafulvalene (TTF) in a metal organic framework (MOF) single crystal. The broad emission is similar to known TTF+• doublet emission and is stabilized in the MOF crystal. This interpretation is supported by the observation of enhanced PL emission following UV oxidation of the MOF crystal to increase the doublet concentration. The findings suggest tetrathiafulvalene-based MOFs as promising materials for near-IR light emission and the MOF structure may be a general strategy to stabilize radical cation species in the solid state.

    View details for DOI 10.1021/acs.jpclett.9b03383

    View details for Web of Science ID 000512223400026

    View details for PubMedID 31935326

  • Variation of Interfacial Interactions in PC61BM-like Electron-Transporting Compounds for Perovskite Solar Cells ACS APPLIED MATERIALS & INTERFACES Fernandez-Delgado, O., Castro, E., Ganivet, C. R., Fosnacht, K., Liu, F., Mates, T., Liu, Y., Wu, X., Echegoyen, L. 2019; 11 (37): 34408–15

    Abstract

    The synthesis, characterization, and incorporation of phenyl-C61-butyric acid methyl ester (PC61BM)-like derivatives as electron transporting materials (ETMs) in inverted perovskite solar cells (PSCs) are reported. These compounds have the same structure except for the ester substituent, which was varied from methyl to phenyl to thienyl and to pyridyl. The three latter derivatives performed better than PC61BM in PSCs, mainly attributed to the specific interactions of the fullerenes with the perovskite layer, as evidenced by X-ray photoelectron spectroscopy (XPS) and steady-state and time-resolved photoluminescence (SS- and TRPL) measurements. The experimental results were fully supported by density functional theory (DFT) calculations, which showed that the strongest interactions were exhibited by the compound possessing the pyridyl substituent.

    View details for DOI 10.1021/acsami.9b09018

    View details for Web of Science ID 000487179900095

    View details for PubMedID 31318519

  • Direct Determination of Band-Gap Renormalization in the Photoexcited Monolayer MoS2 PHYSICAL REVIEW LETTERS Liu, F., Ziffer, M. E., Hansen, K. R., Wang, J., Zhu, X. 2019; 122 (24): 246803

    Abstract

    A key feature of monolayer semiconductors, such as transition-metal dichalcogenides, is the poorly screened Coulomb potential, which leads to a large exciton binding energy (E_{b}) and strong renormalization of the quasiparticle band gap (E_{g}) by carriers. The latter has been difficult to determine due to a cancellation in changes of E_{b} and E_{g}, resulting in little change in optical transition energy at different carrier densities. Here, we quantify band-gap renormalization in macroscopic single crystal MoS_{2} monolayers on SiO_{2} using time and angle-resolved photoemission spectroscopy. At an excitation density above the Mott threshold, E_{g} decreases by as much as 360 meV. We compare the carrier density-dependent E_{g} with previous theoretical calculations and show the necessity of knowing both doping and excitation densities in quantifying the band gap.

    View details for DOI 10.1103/PhysRevLett.122.246803

    View details for Web of Science ID 000473034200015

    View details for PubMedID 31322407

  • Bimodal Bandgaps in Mixed Cesium Methylammonium Lead Bromide Perovskite Single Crystals JOURNAL OF PHYSICAL CHEMISTRY C Liu, F., Wang, F., Hansen, K. R., Zhu, X. 2019; 123 (23): 14865–70
  • Enhanced Open-Circuit Voltage in Perovskite Solar Cells with Open-Cage [60]Fullerene Derivatives as Electron-Transporting Materials MATERIALS Castro, E., Artigas, A., Pla-Quintana, A., Roglans, A., Liu, F., Perez, F., Lledo, A., Zhu, X., Echegoyen, L. 2019; 12 (8)

    Abstract

    The synthesis, characterization, and incorporation of open-cage [60]fullerene derivatives as electron-transporting materials (ETMs) in perovskite solar cells (PSCs) with an inverted planar (p-i-n) structure is reported. Following optical and electrochemical characterization of the open-cage fullerenes 2a-c, p-i-n PSCs with a indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS)/perovskite/fullerene/Ag structure were prepared. The devices obtained from 2a-b exhibit competitive power conversion efficiencies (PCEs) and improved open-circuit voltage (Voc) values (>1.0 V) in comparison to a reference cell based on phenyl-C61-butyric-acid methyl-ester (PC61BM). These results are rationalized in terms of a) the higher passivation ability of the open-cage fullerenes with respect to the other fullerenes, and b) a good overlap between the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) levels of 2a-b and the conduction band of the perovskite.

    View details for DOI 10.3390/ma12081314

    View details for Web of Science ID 000467767400120

    View details for PubMedID 31018500

  • How lasing happens in CsPbBr3 perovskite nanowires NATURE COMMUNICATIONS Schlaus, A. P., Spencer, M. S., Miyata, K., Liu, F., Wang, X., Datta, I., Lipson, M., Pan, A., Zhu, X. 2019; 10: 265

    Abstract

    Lead halide perovskites are emerging as an excellent material platform for optoelectronic processes. There have been extensive discussions on lasing, polariton formation, and nonlinear processes in this material system, but the underlying mechanism remains unknown. Here we probe lasing from CsPbBr3 perovskite nanowires with picosecond (ps) time resolution and show that lasing originates from stimulated emission of an electron-hole plasma. We observe an anomalous blue-shifting of the lasing gain profile with time up to 25 ps, and assign this as a signature for lasing involving plasmon emission. The time domain view provides an ultra-sensitive probe of many-body physics which was obscured in previous time-integrated measurements of lasing from lead halide perovskite nanowires.

    View details for DOI 10.1038/s41467-018-07972-7

    View details for Web of Science ID 000455762900009

    View details for PubMedID 30651537

    View details for PubMedCentralID PMC6335413

  • Three-Dimensional Graphene Nanostructures JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Peurifoy, S. R., Castro, E., Liu, F., Zhu, X., Ng, F., Jockusch, S., Steigerwald, M. L., Echegoyen, L., Nuckolls, C., Sisto, T. J. 2018; 140 (30): 9341–45

    Abstract

    This Communication details the implementation of a new concept for the design of high-performance optoelectronic materials: three-dimensional (3D) graphene nanostructures. This general strategy is showcased through the synthesis of a three-bladed propeller nanostructure resulting from the coupling and fusion of a central triptycene hub and helical graphene nanoribbons. Importantly, these 3D graphene nanostructures show remarkable new properties that are distinct from the substituent parts. For example, the larger nanostructures show an enhancement in absorption and decreased contact resistance in optoelectronic devices. To show these enhanced properties in a device setting, the nanostructures were utilized as the electron-extracting layers in perovskite solar cells. The largest of these nanostructures achieved a PCE of 18.0%, which is one of the highest values reported for non-fullerene electron-extracting layers.

    View details for DOI 10.1021/jacs.8b04119

    View details for Web of Science ID 000440877000004

    View details for PubMedID 29799738

  • Competition Between Hot-Electron Cooling and Large Polaron Screening in CsPbBr3 Perovskite Single Crystals JOURNAL OF PHYSICAL CHEMISTRY C Evans, T. S., Miyata, K., Joshi, P. P., Maehrlein, S., Liu, F., Zhu, X. 2018; 122 (25): 13724–30
  • Superatomic Two-Dimensional Semiconductor NANO LETTERS Zhong, X., Lee, K., Choi, B., Meggiolaro, D., Liu, F., Nuckolls, C., Pasupathy, A., De Angelis, F., Batail, P., Roy, X., Zhu, X. 2018; 18 (2): 1483–88

    Abstract

    Structural complexity is of fundamental interest in materials science because it often results in unique physical properties and functions. Founded on this idea, the field of solid state chemistry has a long history and continues to be highly active, with new compounds discovered daily. By contrast, the area of two-dimensional (2D) materials is young, but its expansion, although rapid, is limited by a severe lack of structural diversity and complexity. Here, we report a novel 2D semiconductor with a hierarchical structure composed of covalently linked Re6Se8 clusters. The material, a 2D structural analogue of the Chevrel phase, is prepared via mechanical exfoliation of the van der Waals solid Re6Se8Cl2. Using scanning tunneling spectroscopy, photoluminescence and ultraviolet photoelectron spectroscopy, and first-principles calculations, we determine the electronic bandgap (1.58 eV), optical bandgap (indirect, 1.48 eV), and exciton binding energy (100 meV) of the material. The latter is consistent with the partially 2D nature of the exciton. Re6Se8Cl2 is the first member of a new family of 2D semiconductors whose structure is built from superatomic building blocks instead of simply atoms; such structures will expand the conceptual design space for 2D materials research.

    View details for DOI 10.1021/acs.nanolett.7b05278

    View details for Web of Science ID 000425559700120

    View details for PubMedID 29368934

  • Cove-Edge Nanoribbon Materials for Efficient Inverted Halide Perovskite Solar Cells ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Castro, E., Sisto, T. J., Romero, E. L., Liu, F., Peurifoy, S. R., Wang, J., Zhu, X., Nuckolls, C., Echegoyen, L. 2017; 56 (46): 14648–52

    Abstract

    Two cove-edge graphene nanoribbons hPDI2-Pyr-hPDI2 (1) and hPDI3-Pyr-hPDI3 (2) are used as efficient electron-transporting materials (ETMs) in inverted planar perovskite solar cells (PSCs). Devices based on the new graphene nanoribbons exhibit maximum power-conversion efficiencies (PCEs) of 15.6 % and 16.5 % for 1 and 2, respectively, while a maximum PCE of 14.9 % is achieved with devices based on [6,6]-phenyl-C61 -butyric acid methyl ester (PC61 BM). The interfacial effects induced by these new materials are studied using photoluminescence (PL), and we find that 1 and 2 act as efficient electron-extraction materials. Additionally, compared with PC61 BM, these new materials are more hydrophobic and have slightly higher LUMO energy levels, thus providing better device performance and higher device stability.

    View details for DOI 10.1002/anie.201706895

    View details for Web of Science ID 000414764600056

    View details for PubMedID 28950414

  • Hydroxyacetone Production From C-3 Criegee Intermediates JOURNAL OF PHYSICAL CHEMISTRY A Taatjes, C. A., Liu, F., Rotavera, B., Kumar, M., Caravan, R., Osborn, D. L., Thompson, W. H., Lester, M. I. 2017; 121 (1): 16–23

    Abstract

    Hydroxyacetone (CH3C(O)CH2OH) is observed as a stable end product from reactions of the (CH3)2COO Criegee intermediate, acetone oxide, in a flow tube coupled with multiplexed photoionization mass spectrometer detection. In the experiment, the isomers at m/z = 74 are distinguished by their different photoionization spectra and reaction times. Hydroxyacetone is observed as a persistent signal at longer reaction times at a higher photoionization threshold of ca. 9.7 eV than Criegee intermediate and definitively identified by comparison with the known photoionization spectrum. Complementary electronic structure calculations reveal multiple possible reaction pathways for hydroxyacetone formation, including unimolecular isomerization via hydrogen atom transfer and -OH group migration as well as self-reaction of Criegee intermediates. Varying the concentration of Criegee intermediates suggests contributions from both unimolecular and self-reaction pathways to hydroxyacetone. The hydroxyacetone end product can provide an effective, stable marker for the production of transient Criegee intermediates in future studies of alkene ozonolysis.

    View details for DOI 10.1021/acs.jpca.6b07712

    View details for Web of Science ID 000392035800003

    View details for PubMedID 28001404

  • Deep tunneling in the unimolecular decay of CH3CHOO Criegee intermediates to OH radical products JOURNAL OF CHEMICAL PHYSICS Fang, Y., Liu, F., Barber, V. P., Klippenstein, S. J., McCoy, A. B., Lester, M. I. 2016; 145 (23): 234308

    Abstract

    Unimolecular decay of Criegee intermediates produced in alkene ozonolysis is known to be a significant source of OH radicals in the troposphere. In this work, unimolecular decay of the methyl-substituted Criegee intermediate, syn-CH3CHOO, to OH products is shown to occur at energies significantly below the transition state barrier for a 1,4 hydrogen transfer that leads to these products [Y. Fang et al., J. Chem. Phys. 144, 061102 (2016)]. The rate of appearance of OH products arising from tunneling through the barrier is obtained through direct time-domain measurements following the vibrational activation of syn-CH3CHOO. IR excitation of syn-CH3CHOO at energies nearly 2000 cm-1 below the barrier is achieved through combination bands involving CH stretch and another lower frequency mode, and the resultant OH products are detected by UV laser-induced fluorescence. The observed syn-CH3CHOO combination bands in the 4100-4350 cm-1 region are identified by comparison with the computed IR absorption spectrum. The experimental decay rates are found to be ca. 106 s-1 in this deep tunneling regime, which is approximately 100-times slower than that in the vicinity of the barrier.The experimental results are consistent with statistical Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of the microcanonical decay rates with tunneling through the barrier, and notable deviations may originate from the sparsity in the density of states for syn-CH3CHOO at lower energies. Thermal unimolecular decay of syn-CH3CHOO is predicted to have significant contribution from microcanonical rates at energies that are much below the barrier.

    View details for DOI 10.1063/1.4972015

    View details for Web of Science ID 000391688900020

    View details for PubMedID 28010089

  • Direct observation of unimolecular decay of CH3CH2CHOO Criegee intermediates to OH radical products JOURNAL OF CHEMICAL PHYSICS Fang, Y., Liu, F., Klippenstein, S. J., Lester, M. I. 2016; 145 (4): 044312

    Abstract

    The unimolecular decay of carbonyl oxide intermediates, known as Criegee intermediates, produced in alkene ozonolysis is a significant source of OH radicals in the troposphere. Here, the rate of appearance of OH radical products is examined directly in the time-domain for a prototypical alkyl-substituted Criegee intermediate, CH3CH2CHOO, following vibrational activation under collision-free conditions. Complementary statistical Rice-Ramsperger-Kassel-Marcus calculations of the microcanonical unimolecular decay rate for CH3CH2CHOO are also carried out at energies in the vicinity of the barrier for 1,4 hydrogen atom transfer that leads to OH products. Tunneling through the barrier, derived from high level electronic structure calculations, contributes significantly to the decay rate. Infrared transitions of CH3CH2CHOO are identified in the CH stretch overtone region, which are detected by ultraviolet laser-induced fluorescence of the resultant OH products. The features observed are attributed to CH vibrational excitations and conformational forms utilizing insights from theory. Both experiment and theory yield unimolecular decay rates for CH3CH2CHOO of ca. 10(7) s(-1), which are slower than those obtained for syn-CH3CHOO or (CH3)2COO reported previously [Fang et al., J. Chem. Phys. 144, 061102 (2016)] at similar energies. Master equation modeling is also utilized to predict the thermal decay rate of CH3CH2CHOO under atmospheric conditions, giving a rate of 279 s(-1) at 298 K.

    View details for DOI 10.1063/1.4958992

    View details for Web of Science ID 000381679800027

    View details for PubMedID 27475366

  • UV plus VUV double-resonance studies of autoionizing Rydberg states of the hydroxyl radical JOURNAL OF CHEMICAL PHYSICS Green, A. M., Liu, F., Lester, M. I. 2016; 144 (18): 184311

    Abstract

    The hydroxyl radical (OH) is a key oxidant in atmospheric and combustion chemistry. Recently, a sensitive and state-selective ionization method has been developed for detection of the OH radical that utilizes UV excitation on the A(2)Σ(+)-X(2)Π transition followed by fixed 118 nm vacuum ultraviolet (VUV) radiation to access autoionizing Rydberg states [J. M. Beames et al., J. Chem. Phys. 134, 241102 (2011)]. The present study uses tunable VUV radiation generated by four-wave mixing to examine the origin of the enhanced ionization efficiency observed for OH radicals prepared in specific A(2)Σ(+) intermediate levels. The enhancement is shown to arise from resonant excitation to distinct rotational and fine structure levels of two newly identified (2)Π Rydberg states with an A(3)Π cationic core and a 3d electron followed by ionization. Spectroscopic constants are derived and effects due to uncoupling of the Rydberg electron are revealed for the OH (2)Π Rydberg states. The linewidths indicate a Rydberg state lifetime due to autoionization on the order of a picosecond.

    View details for DOI 10.1063/1.4948640

    View details for Web of Science ID 000377711900025

    View details for PubMedID 27179488

  • Communication: Real time observation of unimolecular decay of Criegee intermediates to OH radical products JOURNAL OF CHEMICAL PHYSICS Fang, Y., Liu, F., Barber, V. P., Klippenstein, S. J., McCoy, A. B., Lester, M. I. 2016; 144 (6): 061102

    Abstract

    In the atmosphere, a dominant loss process for carbonyl oxide intermediates produced from alkene ozonolysis is also an important source of hydroxyl radicals. The rate of appearance of OH radicals is revealed through direct time-domain measurements following vibrational activation of prototypical methyl-substituted Criegee intermediates under collision-free conditions. Complementary theoretical calculations predict the unimolecular decay rate for the Criegee intermediates in the vicinity of the barrier for 1,4 hydrogen transfer that leads to OH products. Both experiment and theory yield unimolecular decay rates of ca. 10(8) and 10(7) s(-1) for syn-CH3CHOO and (CH3)2COO, respectively, at energies near the barrier. Tunneling through the barrier, computed from high level electronic structure theory and experimentally validated, makes a significant contribution to the decay rate. Extension to thermally averaged unimolecular decay of stabilized Criegee intermediates under atmospheric conditions yields rates that are six orders of magnitude slower than those evaluated directly in the barrier region.

    View details for DOI 10.1063/1.4941768

    View details for Web of Science ID 000371607800003

    View details for PubMedID 26874475

  • Direct observation of vinyl hydroperoxide PHYSICAL CHEMISTRY CHEMICAL PHYSICS Liu, F., Fang, Y., Kumar, M., Thompson, W. H., Lester, M. I. 2015; 17 (32): 20490–94

    Abstract

    Many alkyl-substituted Criegee intermediates are predicted to undergo an intramolecular 1,4-hydrogen transfer to form isomeric vinyl hydroperoxide species (C[double bond, length as m-dash]COOH moiety), which break apart to release OH and vinoxy radicals. We report direct detection of stabilized vinyl hydroperoxides formed via carboxylic acid-catalyzed tautomerization of Criegee intermediates. A doubly hydrogen-bonded interaction between the Criegee intermediate and carboxylic acid facilitates efficient hydrogen transfer through a double hydrogen shift. Deuteration of formic or acetic acid permits migration of a D atom to yield partially deuterated vinyl hydroperoxides, which are distinguished from the CH3CHOO, (CH3)2COO, and CH3CH2CHOO Criegee intermediates by mass. Using 10.5 eV photoionization, three prototypical vinyl hydroperoxides, CH2[double bond, length as m-dash]CHOOD, CH2[double bond, length as m-dash]C(CH3)OOD, and CH3CH[double bond, length as m-dash]CHOOD, are detected directly. Complementary electronic structure calculations reveal several reaction pathways, including the barrierless acid-catalyzed tautomerization reaction predicted previously and a barrierless addition reaction that yields hydroperoxy alkyl formate.

    View details for DOI 10.1039/c5cp02917a

    View details for Web of Science ID 000359237800004

    View details for PubMedID 26199999

  • Direct production of OH radicals upon CH overtone activation of (CH3)(2)COO Criegee intermediates JOURNAL OF CHEMICAL PHYSICS Liu, F., Beames, J. M., Lester, M. I. 2014; 141 (23): 234312

    Abstract

    Ozonolysis of alkenes, a principle non-photolytic source of atmospheric OH radicals, proceeds through unimolecular decay of energized carbonyl oxide intermediates, known as Criegee intermediates. In this work, cold dimethyl-substituted Criegee intermediates are vibrationally activated in the CH stretch overtone region to drive the 1,4 hydrogen transfer reaction that leads to OH radical products. IR excitation of (CH3)2COO reveals the vibrational states with sufficient oscillator strength, coupling to the reaction coordinate, and energy to surmount the effective barrier (≤ 16.0 kcal mol(-1)) to reaction. Insight on the dissociation dynamics is gleaned from homogeneous broadening of the spectral features, indicative of rapid intramolecular vibrational energy redistribution and/or reaction, as well as the quantum state distribution of the OH X(2)Π (v = 0) products. The experimental results are compared with complementary electronic structure calculations, which provide the IR absorption spectrum and geometric changes along the intrinsic reaction coordinate. Additional theoretical analysis reveals the vibrational modes and couplings that permit (CH3)2COO to access to the transition state region for reaction. The experimental and theoretical results are compared with an analogous recent study of the IR activation of syn-CH3CHOO and its unimolecular decay to OH products [F. Liu, J. M. Beames, A. S. Petit, A. B. McCoy, and M. I. Lester, Science 345, 1596 (2014)].

    View details for DOI 10.1063/1.4903961

    View details for Web of Science ID 000346662700029

    View details for PubMedID 25527940

  • Infrared-driven unimolecular reaction of CH3CHOO Criegee intermediates to OH radical products SCIENCE Liu, F., Beames, J. M., Petit, A. S., McCoy, A. B., Lester, M. I. 2014; 345 (6204): 1596–98

    Abstract

    Ozonolysis of alkenes, an important nonphotolytic source of hydroxyl (OH) radicals in the troposphere, proceeds through energized Criegee intermediates that undergo unimolecular decay to produce OH radicals. Here, we used infrared (IR) activation of cold CH3CHOO Criegee intermediates to drive hydrogen transfer from the methyl group to the terminal oxygen, followed by dissociation to OH radicals. State-selective excitation of CH3CHOO in the CH stretch overtone region combined with sensitive OH detection revealed the IR spectrum of CH3CHOO, effective barrier height for the critical hydrogen transfer step, and rapid decay dynamics to OH products. Complementary theory provides insights on the IR overtone spectrum, as well as vibrational excitations, structural changes, and energy required to move from the minimum-energy configuration of CH3CHOO to the transition state for the hydrogen transfer reaction.

    View details for DOI 10.1126/science.1257158

    View details for Web of Science ID 000342164500041

    View details for PubMedID 25258077

  • 1+1 ' resonant multiphoton ionisation of OH radicals via the A(2)sigma(+) state: insights from direct comparison with A-X laser-induced fluorescence detection MOLECULAR PHYSICS Beames, J. M., Liu, F., Lester, M. I. 2014; 112 (7): 897–903
  • UV Spectroscopic Characterization of Dimethyl- and Ethyl-Substituted Carbonyl Oxides JOURNAL OF PHYSICAL CHEMISTRY A Liu, F., Beames, J. M., Green, A. M., Lester, M. I. 2014; 118 (12): 2298–2306

    Abstract

    Dimethyl- and ethyl-substituted Criegee intermediates, (CH3)2COO and CH3CH2CHOO, are photolytically generated from diiodo precursors, detected by VUV photoionization at 118 nm, and spectroscopically characterized via UV-induced depletion of the m/z = 74 signals under jet-cooled conditions. In each case, UV excitation resonant with the B-X transition results in significant ground-state depletion, reflecting the large absorption cross section and rapid dynamics in the excited B state. The broad UV absorption spectra of both (CH3)2COO and CH3CH2CHOO peak at ~320 nm with absorption cross sections approaching ~4 × 10(-17) cm(2) molec(-1). The UV absorption spectra for (CH3)2COO and CH3CH2CHOO are similar to that reported previously for syn-CH3CHOO, suggesting analogous intramolecular interactions between the α-H and terminal O of the COO groups. Hydroxyl radical products generated concurrently with the Criegee intermediates are detected by 1 + 1' resonance enhanced multiphoton ionization. The OH signals, scaled relative to those for the Criegee intermediates, are compared with prior studies of OH yield from alkene ozonolysis. The stationary points along the reaction coordinates from the alkyl-substituted Criegee intermediates to vinyl hydroperoxides and OH products are also computed to provide insight on the OH yields.

    View details for DOI 10.1021/jp412726z

    View details for Web of Science ID 000333578100014

    View details for PubMedID 24621008

  • UV spectroscopic characterization of an alkyl substituted Criegee intermediate CH3CHOO JOURNAL OF CHEMICAL PHYSICS Beames, J. M., Liu, F., Lu, L., Lester, M. I. 2013; 138 (24): 244307

    Abstract

    Ozonolysis of alkenes in the troposphere proceeds through a Criegee intermediate, or carbonyl oxide, which has only recently been detected in the gas phase. The present study focuses on the production of an alkyl-substituted Criegee intermediate, CH3CHOO, in a pulsed supersonic expansion, and then utilizes VUV photoionization at 118 nm and UV-induced depletion of the m∕z = 60 signal to probe the B (1)A(') ← X (1)A(') transition. The UV-induced depletion approaches 100% near the peak of the profile at 320 nm, indicating rapid dynamics in the B state, and corresponds to a peak absorption cross section of ∼5 × 10(-17) cm(2) molecule(-1). The electronic spectrum for CH3CHOO is similar to that reported recently for CH2OO, but shifted 15 nm to shorter wavelength, which will result in a longer tropospheric lifetime for CH3CHOO with respect to solar photolysis. Complementary electronic structure calculations (EOM-CCSD) are carried out for the B and X potentials of these Criegee intermediates along the O-O coordinate. An intramolecular interaction stabilizes the ground state of the syn-conformer of CH3CHOO relative to anti-CH3CHOO, and indicates that the syn-conformer will be the more abundant species in the expansion. The excited B electronic state of syn-CH3CHOO is also predicted to be destabilized relative to that for anti-CH3CHOO and CH2OO, in accord with the shift in the B-X transition observed experimentally. Hydroxyl radicals produced concurrently with the generation of the Criegee intermediates are detected by 1+1(') resonance enhanced multiphoton ionization. The OH yield observed with CH3CHOO is 4-fold larger than that from CH2OO, consistent with prior studies of OH generation from alkene ozonolysis.

    View details for DOI 10.1063/1.4810865

    View details for Web of Science ID 000321148200025

    View details for PubMedID 23822244

  • Ultraviolet Spectrum and Photochemistry of the Simplest Criegee Intermediate CH2OO JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Beames, J. M., Liu, F., Lu, L., Lester, M. I. 2012; 134 (49): 20045–48

    Abstract

    Ozonolysis of alkenes in the troposphere produces Criegee intermediates, which have eluded detection in the gas phase until very recently. This laboratory has synthesized the simplest Criegee intermediate within a quartz capillary tube affixed to a pulsed valve to cool and isolate CH(2)OO in a supersonic expansion. UV excitation resonant with the B (1)A' ← X (1)A' transition depletes the ground-state population of CH(2)OO, which is detected by single-photon ionization at 118 nm. The large UV-induced depletion (approaching 100%) near the peak of the profile at 335 nm is indicative of rapid dissociation, consistent with the repulsive B (1)A' potential along the O-O coordinate computed theoretically. The experimental spectrum is in very good accord with the absorption spectrum calculated using the one-dimensional reflection principle. The B ← X spectrum is combined with the solar actinic flux to estimate an atmospheric lifetime for CH(2)OO at midday on the order of ∼1 s with respect to photodissociation.

    View details for DOI 10.1021/ja310603j

    View details for Web of Science ID 000312351000024

    View details for PubMedID 23206289

  • Communication: A new spectroscopic window on hydroxyl radicals using UV plus VUV resonant ionization JOURNAL OF CHEMICAL PHYSICS Beames, J. M., Liu, F., Lester, M. I., Murray, C. 2011; 134 (24): 241102

    Abstract

    A 1 + 1' multiphoton ionization (MPI) detection scheme for OH radicals is presented. The spectroscopic approach combines initial excitation on the well-characterized A(2)Σ(+)-X(2)Π band system with vacuum ultraviolet (VUV) ionization via autoionizing Rydberg states that converge on the OH(+) A(3)Π ion state. Jet-cooled MPI spectra on the (1,0) and (2,0) bands show anomalous rotational line intensities, while initial excitation on the (0,0) band does not lead to detectable OH(+) ions. The onset of ionization with the (1,0) band is attributed to an energetic threshold; the combined UV + VUV photon energies are above the first member of the autoionizing (A(3)Π)nd Rydberg series. Comparison of the OH 1 + 1' MPI signal with that from single photon VUV ionization of NO indicates that the cross section for photoionization from OH A(2)Σ(+), v' = 1 is on the order of 10(-17) cm(2).

    View details for DOI 10.1063/1.3608061

    View details for Web of Science ID 000292331900002

    View details for PubMedID 21721604

  • Liquid-phase Fischer-Tropsch synthesis over Fe nanoparticles dispersed in polyethylene glycol (PEG) GREEN CHEMISTRY Fan, X., Tao, Z., Xiao, C., Liu, F., Kou, Y. 2010; 12 (5): 795–97

    View details for DOI 10.1039/b926397g

    View details for Web of Science ID 000277563500010