Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

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Probing the Actinide-Ligand Binding and the Electronic Structure of Gaseous Actinide Molecules and Clusters Using Anion Photoelectron Spectroscopy PI: Lai-Sheng Wang Department of Chemistry Brown University 324 Brook Street Providence, RI 02912 Email: lai-sheng_wang@brown.edu Program Scope The broad scope of this program is to better understand actinide chemistry using new spectroscopic techniques and to provide accurate spectroscopic data for the validation of new theoretical methods aimed at actinide chemistry. This program contains three thrust areas: ∗ probing ligand-uranyl (UO2 2+ ) interactions in gaseous anionic complexes in the form of [UO2Lx] n– produced by electrospray ionization ∗ probing the electronic structure and bonding of inorganic and organometallic compounds of actinides in the gas phase using electrospray and anion photoelectron spectroscopy ∗ probing the metal-metal bonding and size-dependent electronic structures in Ux – and UxOy – clusters as a function of size and composition. Understanding the chemistry of the actinide elements, in particular, uranium, is of critical importance to the mission of DOE. We have developed a number of experimental apparatuses, including a magnetic-bottle photoelectron spectroscopy instrument equipped with a laser vaporization supersonic cluster source, a photoelectron imaging apparatus with a laser vaporization cluster source, and a combined photoelectron imaging and magnetic-bottle instrument equipped with an electrospray ionization source and low temperature ion trap. This suite of state-of-the-art instruments are uniquely suitable to investigate the three classes of actinide compounds. Photodetachment involves removal of electrons from occupied molecular orbitals and probes directly the electronic structure and chemical bonding properties of the underlying molecular species. Photoelectron spectroscopy of anions yields electron affinities, vibrational information, and low-lying electronic state information for the corresponding neutral species. The application of anion photoelectron spectroscopy to actinide molecules opens up new research opportunities and yields systematic electronic structure and spectroscopic information that can be used to verify computational methods and advance our understanding of the reactivity, structure, and bonding of actinide molecules. Recent Progress (FY2011) This program commenced in August 2011. During the first funding year, we have focused on the first two thrust areas and have studied [UO2F4] 2– and its H2O and CH3CN solvated species, UF5 – and UF6 – complexes, and [UO2Cl4] 2– . Preliminary data have been obtained on a number of other uranyl complexes with different ligands. Observation and investigation of the uranyl tetrafluoride dianion (UO2F4 2– ) and its solvation complexes with water and acetonitrile. Bare uranyl tetrafluoride (UO2F4 2– ) and its solvation complexes by one and two water or acetonitrile molecules have been observed in the 195
gas phase using electrospray ionization and investigated by photoelectron spectroscopy and ab initio calculations. The isolated UO2F4 2– dianion is found to be electronically stable with an adiabatic electron binding energy of 1.10 ± 0.05 eV and a repulsive Coulomb barrier of ~2 eV. Photoelectron spectra of UO2F4 2– display congested features due to detachment from U–O bonding orbitals and F 2p lone pairs. Solvated complexes by H2O and CH3CN, UO2F4(H2O)n 2– and UO2F4(CH3CN)n 2– (n = 1, 2), are also observed and their photoelectron spectra are similar to those of the bare UO2F4 2– dianion, suggesting that the solvent molecules are coordinated to the outer sphere of UO2F4 2– with relatively weak interactions between the solvent molecules and the dianion core. Both DFT and CCSD(T) calculations are performed on UO2F4 2– and its solvated species to understand the electronic structure of the dianion core and solute-solvent interactions. The strong U–F interactions with partial (d-p)π bonding are shown to weaken the U=O bonds in the [O=U=O] 2+ unit. Each H atom in the water molecules forms a H-bond to a F atom in the equatorial plane of UO2F4 2– (Fig. 1), while each CH3CN molecule forms H-bonds to two F ligands and one axial oxygen. Photoelectron spectroscopy and theoretical studies of UF5 − and UF6 − . The UF5 − and UF6 − anions are produced using electrospray ionization (Fig. 2) and investigated by photoelectron spectroscopy and relativistic quantum chemistry. An extensive vibrational progression is observed in the spectra of UF5 – , indicating significant geometry changes between the anion and the neutral ground states. Franck-Condon factor simulations of the observed vibrational progression yield an accurate electron affinity of 3.82 ± 0.05 eV for UF5, which is an important thermodynamic value, but was not accurately known before. Relativistic quantum calculations using density functional and ab initio theories are performed on UF5 − and UF6 − and their neutrals. The ground states of UF5 − and UF5 are found to have C4v symmetry, but with a large U−F bond Fig. 1. Optimized structures of (a) UO2F4(H2O) 2– (C2v) and (b) UO2F4(H2O)2 2– (D2d) using density functional theory. The H-bond lengths are in Å. − − Fig. 2. Electrospray mass spectrum for UO2F3 , UF5 , and UF6 − . length change. The ground state of UF5 − is a triplet state ( 3 B2) with the two 5f electrons occupying a 5fz3-based 8a1 highest occupied molecular orbital (HOMO) and the 5fxyz-based 2b2 HOMO-1 orbital. The detachment cross section from the 5fxyz orbital is observed to be extremely small and the detachment transition from the 2b2 orbital is more than ten times weaker than that from the 8a1 orbital at the photon energies available. The UF6 – anion is found to be octahedral, similar to neutral UF6 with the extra electron occupying the 5fxyz-based a2u orbital. Surprisingly, no photoelectron spectrum could be observed for UF6 − due to the extremely low detachment cross section from the 5fxyz-based HOMO of UF6 − . Photoelectron spectroscopy and the electronic structure of the uranyl tetrachloride dianion: UO2Cl4 2− . The uranyl tetrachloride dianion (UO2Cl4 2– ) is one of the most important uranyl complexes in solution. We observed it in the gas phase using electrospray ionization and investigated its electronic structure using photoelectron spectroscopy and 196
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Eighth Condensed Phase and Interfac
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Potential energy landscape of the (
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Agenda
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Session III Chair: Jay A. LaVerne,
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Table of Contents
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Transition Metal-Molecular Interact
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Ultrafast Electron Transport across
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CPIMS Principal Investigator Abstra
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VUV LDPI image of a polymer photore
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Publications based on DOE support (
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entirely quenched, presumably due t
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Future Plans In addition to the TiO
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The relative path indexes of the ur
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We have recently carried out molecu
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have been obtained with γ-rays sug
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Two novel plasma devices have been
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(Figure 2) can be adsorbed exclusiv
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3. Future Plans Our planned work de
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Figure 1: Snap shots from molecular
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6. Varilly, P., A. J. Patel and D.
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hypothesis that delocalized charges
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Efforts will be made to observe ele
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Photochemistry and Solvation Dynami
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complementary to work we have carri
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The Co3O4 films for the optical pum
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chamber will allow us to heat the s
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calculations. Additional PMF calcul
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the gas solute first needs to cross
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laser system, which is an infrared
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Publications (10/1/2009-present) fo
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interest are: (i) What is the diffu
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and anilino groups attached to the
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had been formed. This coincides wit
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Figure 1. (A) Simplified bR photocy
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Figure 4. Photocurrent density of b
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single-file diffusion and cannot ca
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PUBLICATIONS SUPPORTED BY USDOE CHE
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arrangement. However, the hydrogen
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(5) “Water Dynamics in Salt Solut
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from high- energy x-ray diffraction
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concentrated aqueous solutions. The
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molecule’s center of mass, or alt
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Moreover, SHG studies by Saykally
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The effective fragment potential (E
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10. D.D. Kemp, J. Rintelman, M.S. G
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temperature dependent, equilibrium
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mimicking real organic photovoltaic
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100 kHz amplifier, SE-FSRS was succ
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photovoltaics will be explored via
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for laser surface reactions to thes
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This is a significant advantage of
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the STM, which might underlie the u
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Future Plans: Figure 3. One and two
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methane molecule and a lattice atom
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eactive than the Ni, experiment sug
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Fig. 1. PE spectra of M3Oy � (M =
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each of the intermediates obtained
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a regime where each solvent molecul
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observables that arise from subtle
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total potentials. One can see from
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Our calculations have shown that ch
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our understanding of structure and
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viscosity, η, are inversely relate
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photodissociation of the metal - NO
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from a Ru 2p3/2 orbital to an orbit
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Integrated O 2 PSD (ML) Integrated
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anisotropy in the structure of the
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XPS studies have been performed on
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Publications with BES support since
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MLCT (metal to ligand charge transf
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molecule-metal interface without a
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observe the predicted effects, whic
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(4*) Lymar, S. V.; Schwarz, H. A. J
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(τ .820 ns), but we have not been
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B. Photodissociation spectroscopy o
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oth BLYP and PBE. Our results have
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Publications from BES support (2010
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understanding of factors that contr
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Our talk will discussion our initia
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Page 167 and 168: method recently developed by Prende
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Page 173 and 174: "Probing the Hydration Structure of
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Page 177 and 178: 10) C. Zhang, M.E. Grass, A.H. McDa
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Page 187 and 188: 2. Optical Stark study of PtF (Acce
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Page 191 and 192: In marked contrast to the selective
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Page 197 and 198: approach on a model phenol-amine pr
Page 199 and 200: We performed pump-probe experiments
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Page 205 and 206: 4. Inelastic scattering of hydrogen
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Page 215 and 216: order of Li + > Na + > K + for comp
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Page 219 and 220: Technical Approach and Progress to
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Page 225 and 226: Studies of structure and reaction d
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Page 229 and 230: 9. S. Pandelov, J. C. Werhahn, B. M
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Page 233 and 234: Figure 1 (Left) The HOMO and LUMO e
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Page 237 and 238: Participant List Dr. Musahid Ahmed
Page 239 and 240: Professor Kenneth Eisenthal Columbi
Page 241 and 242: Dr. Ireneusz Janik Notre Dame Radia
Page 243 and 244: Professor Eric Potma University of
Page 245: Dr. James Wishart Brookhaven Nation
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Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

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