Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session Y02: Structure and Spectroscopy |
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Sponsoring Units: DCP Chair: Stephen Bradforth, Univ of Southern California Room: LACC 150B |
Friday, March 9, 2018 11:15AM - 11:27AM |
Y02.00001: Quantum Mechanical Study of Lithium Fluoride Clusters Ajit Hira, Amanda Trujillo, Sadia Ahmad, Chloe Robinson, Naomi Lehman The materials of interest for this research are Lin and Fn clusters (n = 2 thru 15), and the hybrid clusters formed from these mono-atomic clusters. The study of clusters can reveal the effects of substrate geometry on the behavior of adsorbates. Metal clusters and non-metal clusters are especially suited for the study of quantum size effects and for formation of metallic states, and are ideal candidates for catalytic processes. Hybrid ab initio methods of quantum chemistry (particularly the DFT-B3LYP model) will be used to derive optimal geometries for the clusters of interest. We compare calculated binding energies, bond-lengths, ionization potentials, electron affinities and HOMO-LUMO gaps for the clusters. Effects of crystal symmetries corresponding to the bulk structures will be investigated. Of particular interest is the class of lithium fluoride clusters of the type LinF, including Li2F, Li3F and Li4F. Atomic and molecular clusters, generated by supersonic expansions, are a favorite medium for study of reactions and spectra. We will also study the rotational spectra of lithium clusters, fluorine clusters, and hybrid clusters. |
Friday, March 9, 2018 11:27AM - 11:39AM |
Y02.00002: Magic-number effect on the interfacial transport of hydrated ion clusters Duanyun Cao, Jinbo Peng, Zhili He, Yiqin Gao, Limei Xu, Ying Jiang, Enge Wang Ion hydration at interfaces is important for an extremely wide range of fields and processes, one of the key issues is to uncover the microscopic factors that govern the transport of the interfacial hydrated ions. Using a combined scanning tunneling microscopy and noncontact atomic force microscopy system, we can construct individual Na+ hydrates (1-5 water molecules) on a NaCl(001) surface and investigate their mobility in a controlled manner. We found that Na+●3H2O diffuses orders of magnitude faster than other ion hydrates. Ab initio calculations revealed that such high mobility arises from the existence of a peculiar metastable state, in which the three water molecules around the Na+ can rotate collectively with an exceptionally small barrier. The above picture applies even at room temperature according to the classical molecular dynamics simulations. Our work suggests that there may exist a magic-number effect on the transport of interfacial hydrated ions, which is determined by the degree of symmetry match between the hydrates and the interface. |
Friday, March 9, 2018 11:39AM - 11:51AM |
Y02.00003: Convection-reaction-diffusion study for non-linear reaction systems using Stochastic Rotation Dynamics Aditya Sengar, Hans Kuipers, Rutger Santen, Johan Padding In this study, we aim to study a coupled convection-reaction-diffusion system involving reactions that respond to the local concentration of reactant over a reactive surface non-linearly. Stochastic rotation dynamics (SRD), a mesoscale coarse grained technique has been used to study the particle-particle interaction in the bulk and connection to the surface reactive system is achieved by obtaining rate mechanism information from micro-kinetic data which contains the heat of each participating reaction step. Evolution of these reactions leads to a multi-component mixture inside the bulk of the system where the hydrodynamic and thermal interactions of participating species lead to Maxwell-Stefan-like diffusion. We look at the spatial and temporal concentration profiles of reactant across the model reactor developed and study them as a function of Peclet number and Damkohler number. Both these numbers give a qualitative and quantitative comparison between the convective, diffusive and reactive time scales, with which we construct model systems with different physical parameters sharing the same dimensionless numbers. |
Friday, March 9, 2018 11:51AM - 12:03PM |
Y02.00004: Angular distributions in negative-ion photodetachment via photoelectron imaging spectroscopy Andrei Sanov Photoelectron imaging has transformed into a versatile tool for probing the electronic structures of atoms, molecules, and clusters. It also allows observing the transformations of molecular electronic structures in chemical reactions. We will discuss recent advances in the characterization of exotic molecular and cluster anions, as well as neutral reactive intermediates using photoelectron imaging spectroscopy. The experimental results in these studies fuel the development of practical (experiment-friendly) theoretical models enabling the analysis of photoelectron angular distributions and their interpretation as signatures of molecular electronic structures. |
Friday, March 9, 2018 12:03PM - 12:15PM |
Y02.00005: Ice-Ih revisited: no proton tunneling observed in a quasielastic neutron scattering experiment Alexander Kolesnikov, G. Ehlers, Eugene Mamontov, Andrei Podlesnyak Significance of quantum effects in water has been well recognized. In particular, observation of a broad (~0.4 meV) quasielastic neutron scattering (QENS) signal in H2O ice-Ih at T=5K [1] has attracted much attention. The QENS signal was explained by a model of concerted proton tunneling. There were a few attempts of theoretical understanding of the effect. Using path integral simulations, it was shown that quantum fluctuations in ice-Ih allow for collective tunneling of six protons in proton-ordered six rings [2]. However in [3] it was noted that concerted tunneling in ice-Ih should have very low frequency. In this study we performed QENS measurements of H2O ice-Ih by using CNCS (T=1.7K) and BASIS (T=4K) spectrometers at SNS, with respective energy resolution similar to and 5 times better than the energy resolution in the original experiment [1]. We did not observe any QENS broadening, and the measured spectra for the ice-Ih and the reference vanadium sample were almost identical. Therefore, we conclude that there is no proton tunneling in ice-Ih at temperatures down to 1.7K measurable on energy scale 3 μeV and above. |
Friday, March 9, 2018 12:15PM - 12:27PM |
Y02.00006: Defects Cause Sub-gap Luminescence in a Crystalline Tetracene Derivative R. Eric McAnally, Jon Bender, Laura Estergreen, Ralf Haiges, Stephen Bradforth, Jahan Dawlaty, Sean Roberts, Aaron Rury We use steady-state and ultrafast nonlinear spectroscopies in combination with ab initio electronic structure calculations to explain light emission below the optical gap of crystalline samples of 5,12-diphenyl tetracene (DPT). In particular, the properties of vibrational coherences imprinted on a probe pulse transmitted through a DPT single crystal indicate discrete electronic transitions below the band-gap energy of this organic semiconductor. Analysis of coherence spectra and vibrational assignments derived from calculations motivate us to conclude that electronic states due to structural defects give rise to these discrete transitions and sub-gap light emission. These results provide fundamental insights into the assignment and properties of mid-gap states in organic materials important for their application in next generation photonics and opto-electronics technologies. |
Friday, March 9, 2018 12:27PM - 12:39PM |
Y02.00007: Neutron Powder Diffraction and Infrared Spectroscopy Study of the Ammonium Sulfate at Low Temperature Amal Al-Wahish, Krishna Kharel, Ozge Gunaydin-Sen Our study reports the results of the temperature-dependent Fourier-transform infrared (FTIR) spectroscopy, a differential scanning calorimetry (DSC) and a neutron powder diffraction (NPD) on the ammonium sulfate at RT (paraelectric) and 30 K (ferroelectric). Heat capacity measurements with DSC show a clear anomaly at 223 K at the phase transition.Paraelectric-ferroelectric phase transition in ammonium sulfate associated with the hydrogen bonding presents an interesting and challenging research area. We determine the hydrogen bonds at different temperature and compare our NPD at RT with previously reported study1. We assign FTIR spectral features of the sample and reveal the nature of their displacement patterns. |
Friday, March 9, 2018 12:39PM - 12:51PM |
Y02.00008: Probing Molecular Crystals by Combined Inelastic Neutrons Scattering (INS) Experiments and First-principle Techniques Anup Pandey, Ada Sedova, Yongqiang Cheng, Daemen Luke, Anibal Ramirez-Cuesta Fundamental properties of molecular crystals depend on both the arrangement of molecules, and the molecular interactions within the solid. We have combined state-of-art density functional theory calculations with inelastic neutron scattering (INS) experiments on the nucleic acid nitrogenous bases, simple pharmaceuticals and pharmaceuticals cocrystals (PC) using the VISION vibrational spectrometer at SNS, revealing details about intermolecular interactions as well as the performance of various computational methods. VISION provides an invaluable probe into molecular motions at atomic-level details; the physical formalism describing INS is rather simple, thus a near-exact estimate of the experimental data is possible by DFT-based normal mode calculations. This synergistic relationship is exploited to probe the vibrational signatures of molecular motions and interactions in these crystals, and changes induced due to the co-crystallization, to develop a predictive computational method for use in applications. The crucial thermal effects are investigated by employing the quasi-harmonic approximation. This comparative study is important to a deeper understanding of biomolecular systems as well as drug design in the field of pharmaceuticals. |
Friday, March 9, 2018 12:51PM - 1:03PM |
Y02.00009: Calculations of vibrational spectra with rectangular collocation: suitability for catalytic systems, tests of PES representations and selection of important regions of space Sergei Manzhos, Tucker Carrington We present the latest developments in the rectangular collocation method to compute vibrational spectra. Rectangular collocation allows using a small number of fixed-shape or tuned basis functions of various forms to compute anharmonic spectra from a relatively small number of potential energy surface (PES) samples. It is well suited for calculations of anharmonic spectra when an analytic PES is unavailable and when underlying ab initio calculations are costly, such as molecules on surfaces. We show that collocation points can be placed in important regions of space further reducing computational cost. Accurate anharmonic spectra for a four-atomic molecules are computable on a desktop. We compare spectra on PESs fitted with neural networks and Gaussian process regression and show that the method also permits to get a clearer understanding of how errors due to PES representation translate into errors in the spectrum. |
Friday, March 9, 2018 1:03PM - 1:15PM |
Y02.00010: Photophysical properties of the Triplet and the "bound" Triplet-Triplet state
in Pentacene Dimers Souratosh Khan, Sumitendra Mazumdar Covalently linked dimers can serve as ideal prototypes for intramolecular singlet fission where |
Friday, March 9, 2018 1:15PM - 1:27PM |
Y02.00011: Theory of Transient Excited State Absorptions in Pentacene and Derivatives:
The Triplet-Triplet Biexciton Versus Free Tripletsa Sumitendra Mazumdar, Souratosh Khan Recent experiments in several singlet-fission materials have found that the triplet-triplet |
Friday, March 9, 2018 1:27PM - 1:39PM |
Y02.00012: Abstract Withdrawn
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Friday, March 9, 2018 1:39PM - 1:51PM |
Y02.00013: Fabrication of an Artificial Aromatic Molecule Constructed from Silicon Dangling Bonds and Measurement of its Molecular Orbitals Using Scanning Tunneling Microscopy and Spectroscopy Jonathan Wyrick, Xiqiao Wang, Pradeep Namboodiri, Scott Schmucker, Richard Silver Hydrogen atoms that terminate a silicon (100) surface can be selectively removed with atomic-precision by means of feedback controlled lithography with a scanning tunneling microscope tip. Atomic sites on the silicon surface that have been stripped of their hydrogen atoms present a dangling bond above the bare silicon atom that remains. Studies on linear chains of dangling bonds have led to and provided preliminary evidence for the hypothesis that dangling bonds on the Si (100) surface can be thought of as artificial atoms that, when arranged together in an appropriate manor, will form molecular analogues. For the first time, we demonstrate a 2-dimensional arrangement of dangling bonds that forms an artificial silicon-based aromatic molecule surrounded by a hydrogen terminated surface. This is confirmed by scanning tunneling spectroscopy measurements of the resulting molecular orbitals which are found to be in good agreement with those predicted by density functional theory calculations. |
Friday, March 9, 2018 1:51PM - 2:03PM |
Y02.00014: 2D THz-THz-Raman Simulations in Liquid Bromoform Ioan-Bogdan Magdau, Griffin Mead, Ralph Welsch, Ian Finneran, Geoffrey Blake, Thomas Miller Low frequency excitations in the condensed phase play a crucial role in processes such as solvation, dynamics of bio-molecules and liquid phase chemistry. Two dimensional Terahertz-Raman methods measure the 3rd order non-linear response of mater to a sequence of three laser pulses (Raman-THz-THz, THz-Raman-THz or THz-THz-Raman) and provide means to study the origin of anharmonicity and coupling between these thermally excited, low frequency modes. We have developed theoretical techniques to model the non-linear responses using a range of methods that involve different levels of approximations, including methods based on all-atom simulation and reduced density matrix modeling. We employ these methods to analyze recently measured THz-THz-Raman response in liquid bromoform. |
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