Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Y43: Spectroscopy, Photochemistry, and Electrochemistry |
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Sponsoring Units: DCP Chair: Jeff Owrutsky, Naval Research Laboratory Room: Hilton Baltimore Holiday Ballroom 2 |
Friday, March 22, 2013 8:00AM - 8:12AM |
Y43.00001: Quasiparticle Representation of Coherent Nonlinear Optical Signals of Multiexcitons Benjamin Fingerhut, Kochise Bennet, Oleksiy Roslyak, Shaul Mukamel Elementary excitations of many-Fermion systems can be described within the quasiparticle approach which is widely used in the calculation of transport and optical properties of metals, semiconductors, molecular aggregates and strongly correlated quantum materials. The excitations are then viewed as independent harmonic oscillators where the many-body interactions between the oscillators are mapped into anharmonicities. We present a Green's function approach based on coboson algebra for calculating nonlinear optical signals and apply it onwards the study of two and three exciton states. The method only requires the diagonalization of the single exciton manifold and avoids equations of motion of multi-exciton manifolds. Using coboson algebra many body effects are recast in terms of tetradic exciton-exciton interactions: Coulomb scattering and Pauli exchange. The physical space of Fermions is recovered by singular-value decomposition of the over-complete coboson basis set. The approach is used to calculate third and fifth order quantum coherence optical signals that directly probe correlations in two- and three exciton states and their projections on the two and single exciton manifold. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y43.00002: Surface induced selective deposition of Dysprosium Polyoxometalate on HOPG surface studied by STM and STS David Costa Milan, Elena Pinilla Cienfuegos, Salvador Cardona Serra, Eugenio Coronado Miralles, Carlos Untiedt Lecuona Scanning Tunneling Microscope (STM) and scanning Tunnelling spectroscopy (STS) techniques have been used to study the Preyssler type Polyoxometalate K$_{12}$[DyP$_{5}$W$_{30}$O$_{110}$] molecules deposited on Highly Oriented Pyrolytic Graphite surface (HOPG). Chainlike arrangements of clusters containing two or three molecules, as well as different cluster sizes are observed. As many structural artifacts are present on the graphite surface, like Moir\'{e} patterns, that could look like the molecular deposits, we have studied their STS and size to ensure the presence of the POM molecules on the surface. This article shows the possibility of addressing POMs on a flat surface to obtain their electronic properties through STS. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y43.00003: Detecting excited-state vibrational dynamics by broadband infrared or Raman probes; A unified picture based on loop diagrams Konstantin E. Dorfman, Benjamin Fingerhut, Shaul Mukamel Vibrational motions in electronically excited states can be probed either by time and frequency resolved infrared or by off resonant stimulated Raman techniques. Using loop diagrams, which represent forward and backward propagation of the wavefunction we derive similar multipoint correlation function expressions for both signals which are suitable for quantum microscopic simulations. The effective temporal ($\Delta $t) and spectral ($\Delta \omega )$ resolution of the techniques is not solely controlled by experimental knobs since it also depends on the system dynamics being probed. The Fourier uncertainty $\Delta \omega \Delta $t \textgreater\ 1 is never violated. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y43.00004: Interfacial Matrix Stabilization Spectroscopy (IMSS) studies of CO and O2 interactions with thin films of oxide-supported Au nanoparticles Nina K. Jarrah, David T. Moore Interfacial Matrix Stabilization Spectroscopy (IMSS) employs energy-dissipating cryogenic matrix isolation techniques combined with FTIR to enable stabilization and detection of pre-reactive complexes of CO and O2 formed on oxide-supported gold nanoparticles (AuNPs). Following deposition of CO and O2 in an argon matrix at 10-20K, annealing to warmer temperatures (28-32K) promotes diffusion of isolated dopant molecules through the matrix to binding sites on a thin film of catalyst. Matrix-solvated pre-reactive complexes form at the surface and are characterized spectroscopically. Comparison of observed complexes in IMSS experiments with results from direct adsorption studies, in absence of a matrix, can provide a measure for the stabilizing effects of matrix solvation. Subsequent surface warming following stabilization of the pre-reactive complexes reveals qualitative information about relative binding energies of formed intermediates of CO, O2, and the supported AuNPs. A series of FTIR spectra mapping the evolution of vibrational bands during the annealing process and tracking the various surface-bound species will be presented and comparisons to direct adsorption experiments will be discussed. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y43.00005: Rotational Spectroscopy at Sub-Angstrom Level: Rotationaland Vibrational Excitations of Molecular Hydrogen measured by the Scanning Tunneling Microscope Shaowei Li, Arthur Yu, Hui Wang, Freddy Toledo, Zhumim Han, Ruqian Wu, Wilson Ho The power of rotational spectroscopy has long been demonstrated in the frequency domain by microwave spectroscopy, but its application in real space has been limited. Using a scanning tunneling microscope (STM) and inelastic electron tunneling spectroscopy (IETS), we are able to conduct real-space measurements of rotational transitions of gaseous hydrogen molecules physisorbed on Au(110) surface at 10 K. The j=0 to j=2 rotational transition for para-H$_{2}$ and HD as well as the v=0 to v=1 vibrational transitions for H$_{2}$, D$_{2}$ and HD were observed by STM-IETS. By varying the tip-substrate distance, we could precisely investigate how the environmental coupling modifies the structure, including the bond length, of a single molecule with sub-Angstrom resolution. Rotational spectroscopy at the single molecule level provides a powerful tool for chemical identification as well as bond length measurement in both the frequency and space domains. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y43.00006: Two-Photon Ionization of a Low IP Molecule (TDAE) Byron Smith, Robert Compton Very low ionization potential molecules (\textless\ 6 eV) have been studied thoroughly for their use in low temperature plasmas, charge-transfer salts, and as an alternative to liquid scintillation in photomultiplier tubes. One such molecule is tetrakis(dimethylamino)ethylene (TDAE) with a previously measured IP of 5.2 $\pm$ 0.05 eV using electron and photon impact time of flight mass spectrometry.\footnote{N. Mirsaleh-Kohan, et al., \textit{Int. J. Mass Spectrom.} \textbf{304}, 56-65(2011).} Two-photon ionization photoelectron spectroscopy of TDAE at 441 nm and 355 nm results in an IP of 5.22 $\pm$ 0.14 eV. In addition to the photoelectron peak associated with direct ionization, a peak was observed corresponding to thermal energy electrons ($\sim$ 0 eV). This has been previously assigned to an intense short-lived auto-ionizing state\footnote{B. Soep, et al., \textit{J. Eur. Phys. D} \textbf{14}, 203 (2001).} which quickly cools to a zwitterionic intermediate. We assign this state to a collective excitation as an alternative explanation of the source of the slow electrons. The collective state involves electron correlation within the parent molecule as well as the degeneracy of the auto-ionizing state. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y43.00007: Modeling the color of natural dyes Xiaochuan Ge, Arrigo Calzolari, Simon Binnie, Stefano Baroni We report on a theoretical study, based on time-dependent density-functional theory, of various factors affecting the optical properties of a few representative anthocyanins, a class of molecules responsible for the color of many fruits, flowers, and leaves, which have also aroused some interest for photovoltaic applications. We first address the influence of substituting different side groups in the phenyl ring of flavylium dyes. We find that these dyes can be classified into three broad classes, according to the number of peaks (1, 2, or 3) featured in the visible range, and give a rationale to this finding. We then examine the effects of solvent-induced thermal fluctuations and dielectric screening, by calculating the spectrum of a representative molecule in solution, for each one these classes. This is achieved by first running an ab initio molecular dynamics simulation of an explicit model for the water-solvated molecule, and then accumulating time averages of the optical spectra calculated on the fly. The effects of thermal fluctuations are shown to overshadow those of dielectric screening, and more dramatic the larger the number of peaks in the gas phase. The effects of different functionals (GGA vs. hybrids) on the calculated spectra are also addressed. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y43.00008: Terahertz Time-Domain Spectroscopy of Nitrogen Ice Bagvanth R. Sangala, Perry A. Gerakines, David J. Hilton We have used terahertz time-domain spectroscopy from 0.1-1.6 THz to study thin films of solid N$_2$ from 10-25 K. A temperature dependent absorption line shift was observed near 1.46 THz as the temperature increased from 10 to 25 K, where the center frequency of the absorption line decreased with temperature. We can fit these data to a model assuming a standard Lennard-Jones potential with the addition of a quadrupole-quadrupole interaction. We modeled the shift in the resonant absorption with a lattice expansion that includes previously published thermal expansion coefficients in N$_2$ ice, the gas-phase Lennard-Jones parameters, and the gas-phase quadrupole moments. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y43.00009: Temperature-dependent and time-resolved emission studies of \textit{cis}-Ru(bpy)$_2$(N$_3$)$_2$ Helen K. Gerardi, Douglas J. Brown, Ryan Compton, Walter J. Dressick, Edwin J. Heilweil, Jeffrey C. Owrutsky The electronic properties of a Ru$^{II}$ cyclometalated dye complex, \textit{cis}-Ru(bpy)$_{2}$(N$_{3})_{2}$, were examined with time-resolved and temperature-dependent visible emission measurements. Compared to two related solar cell dye-sensitizer species, \textit{cis}-Ru(bpy)$_{2}$(NCS)$_{2}$ and \textit{cis}-Ru(bpy)$_{2}$(CN)$_{2}$, the azide (N$_{3})$ pseudohalide ligand dramatically changes the electronic properties of the dye. The uv-vis absorption spectra of \textit{cis}-Ru(bpy)$_{2}$(N$_{3})_{2}$ in various solvents reveal that its metal-to-ligand charge transfer band (MLCT) is located more than 50 nm to the red of the MLCT bands found for the other two complexes. Furthermore, while room temperature emission is readily observed for \textit{cis}-Ru(bpy)$_{2}$(NCS)$_{2}$ and \textit{cis}-Ru(bpy)$_{2}$(CN)$_{2,}$ the emission is much weaker for \textit{cis}-Ru(bpy)$_{2}$(N$_{3})_{2}$. We report the first observation of luminescence from \textit{cis}-Ru(bpy)$_{2}$(N$_{3})_{2}$ by measuring it in 4:1 EtOH:MeOH matrices at temperatures below 140~K. Emission bands are observed at 665 nm and 620 nm (514 nm excitation). The quantum yield of this species was estimated by comparing the integrated emission signal of \textit{cis}-Ru(bpy)$_{2}$(N$_{3})_{2}$ to that of \textit{cis}-Ru(bpy)$_{2}$(CN)$_{2}$ at 77~K and was determined to be exceptionally low (6~x~10$^{-4}$~). The luminescence lifetime of \textit{cis}-Ru(bpy)$_{2}$(N$_{3})_{2}$ at 77 K was measured to be approx. 800~ns, implying an extremely slow radiative rate of 780 s$^{-1}$. The long radiative rate and low quantum yield led us to further investigate the photolability and electrochemical behavior of the azide complex. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y43.00010: How Fissors works: tracking vibrationally adiabatic conformational change with femtosecond stimulated Raman spectroscopy Jeffrey Cina, Phil Kovac With the help of a two-dimensional model system comprising a slow conformational degree of freedom and a higher-frequency vibration, we investigate the optical generation and dynamical information content of femtosecond stimulated Raman spectroscopy (FSRS or fissors). Our treatment makes use of an assumption that the motion of a wave packet describing the relatively slow---but still ultrafast---conformational change is vibrationally adiabatic. We present calculated fissors signals for regimes in which the conformational change is or is not sufficiently slow to result in an evolving fissors lineshape whose center frequency tracks the ``instantaneous'' vibrational frequency. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y43.00011: A photoelectrochemical (PEC) study on graphene oxide based hematite thin films heterojunction (R-GO/Fe$_2$O$_3)$ Poonam Sharma, Michael Zachariah, Sheryl Ehrman, Rohit Shrivastava, Sahab Dass, Vibha R. Satsangi Graphene has an excellent electronic conductivity, a high theoretical surface area of 2630 m$^2$/g and excellent mechanical properties and, thus, is a promising component for high-performance electrode materials. Following this, GO has been used to modify the PEC response of photoactive material hematite thin films in PEC cell. A reduced graphene oxide/iron oxide (R-GO/Fe$_2$O$_3)$ thin film structure has been successfully prepared on ITO by directly growing iron oxide particles on the thermally reduced graphene oxide sheets prepared from suspension of exfoliated graphene oxide. R-GO/Fe$_2$O$_3$ thin films were tested in PEC cell and offered ten times higher photocurrent density than pristine Fe$_2$O$_3$ thin film sample. XRD, SEM, EDS, UV-Vis, Mott-Schottky and Raman studies were carried out to study spectro-electrochemical properties. Enhanced PEC performance of these photoelectrodes was attributed to its porous morphology, improved conductivity upon favorable carrier transfer across the oxides interface. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y43.00012: Site-Specific Photosimulated Reactions of O$_{2}$ on TiO$_{2}$(110) Zhi-Tao Wang, N. Aaron Deskins, Igor Lyubinetsky We report the direct observation at an atomic level with high-resolution scanning tunneling microscopy of photostimulated reactions of single O$_{2}$ molecules on reduced TiO$_{2}$(110) surfaces at 50 K. Two distinct reactions of O$_{2}$ desorption and dissociation occur at different active sites of terminal Ti atoms and bridging O vacancies, respectively demonstrating the critical relation between photoreactivity and adsorption sites on TiO$_{2}$. These two reaction channels follow very different kinetics. Hole-mediated O$_{2}$ desorption is promptly and fully completed, while electron-mediated O$_{2}$ dissociation is much slower and is quenched above some critical O$_{2}$ coverage. Density functional theory calculations indicate that both coordination and charge state of an O$_{2}$ molecule chemisorbed at specific site largely determine a particular reaction pathway. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y43.00013: Cobalt (hydro)oxide electrodes under electrochemical conditions: a first principle study Jia Chen, Annabella Selloni There is currently much interest in photoelectrochemical water splitting as a promising pathway towards sustainable energy production. A major issue of such photoelectrochemical devices is the limited efficiency of the anode, where the oxygen evolution reaction (OER) takes place. Cobalt (hydro)oxides, particularly Co3O4 and Co(OH)2, have emerged as promising candidates for use as OER anode materials. Interestingly, recent in-situ Raman spectroscopy studies have shown that Co3O4 electrodes undergo progressive oxidation and transform into oxyhydroxide, CoO(OH), under electrochemical working conditions. (Journal of the American Chemical Society \textbf{133}, 5587 (2011))Using first principle electronic structure calculations, we provide insight into these findings by presenting results on the structural, thermodynamic, and electronic properties of cobalt oxide, hydroxide and oxydroxide CoO(OH), and on their relative stabilities when in contact with water under external voltage. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y43.00014: Near-field Nanoscopy and Spectroscopy of Phase Coexistence in LiFePO$_4$ Electrode Microcrystals Ivan T. Lucas, Alexander McLeod, Jaroslaw S. Syzdek, Derek S. Middlemiss, Robert Kostecki, D.N. Basov Due to instrumental limitations, the microscopic description of lithiation and delithiation processes in low-cost LiFePO$_4$ electrodes has remained uncertain and subject to controversy. Using infrared near-field imaging, we present evidence for a novel coexistence of phases within single LiFePO$_4$ microcrystals. First-principles calculations of the phonon response of lithiated and delithiated end-phases are compared with broadband nano-FTIR (Fourier transform infrared) spectroscopy data to reveal the mid-infrared vibrational signature of lithiation. By resolving this signature at the nano-scale, we observe a propagation of phase boundaries within these crystals over the course of chemical delithiation. In addition, by comparing theoretical modeling with spatially resolved nano-FTIR spectra measured across a single crystal at partial delithiation, we assemble a tomographic view of phases distributed hundreds nanometers beneath the crystal surface. These experiments set the stage for quantitative nano-spectroscopy of new composite electrode materials, assisting in the rational design of next-generation electrical energy storage systems. [Preview Abstract] |
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