Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session E32: Time-dependent and Time-independent Approaches (B)Focus
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Sponsoring Units: DCP Chair: Tucker Carrington, Queen's University Room: BCEC 204A |
Tuesday, March 5, 2019 8:00AM - 8:36AM |
E32.00001: Towards the calculation of multidimensional vibrational spectra Invited Speaker: Kenneth Ruud Great advances are being made in the experimental realization of multiphoton spectroscopy, such as the experimental confirmation of four- and five-photon absorption in the visible energy range as well as two-dimensional Raman spectra. |
Tuesday, March 5, 2019 8:36AM - 9:12AM |
E32.00002: Anharmonicity and vibrational coupling from ab-initio molecular dynamics Invited Speaker: Nadia Rega We discuss on going advances in developing methods based on ab-initio molecular dynamics to simulate and understand vibrational dynamics, with focus on anharmonicity and vibrational coupling. This achievement is essential to understand at molecular level photoinduced processes on fast and ultrafast scale (femtoseconds to picoseconds). To this aim, we adopt time resolved vibrational analysis designed to follow equilibrium and transient vibrational dynamics extracted from ground and excited state trajectories.[1-5] We present results obtained for charge transfer complexes and photochromic reactions. We also discuss perspectives, limits and future challenges of these methods. |
Tuesday, March 5, 2019 9:12AM - 9:24AM |
E32.00003: UV Photodissociation Dynamics of Bromoform Studied by Ultrafast Inner-Shell Transient Absorption Spectroscopy Han Wang, Benjamin W Toulson, Mario Borgwardt, Oliver Gessner, David Prendergast Small bromoalkanes have been the focus of multiple studies owing to the importance of bromine chemistry in the atmosphere as bromine is ~60 times more destructive towards stratospheric ozone than chlorine. Here we study the photochemistry of the low-lying electronically excited states of bromoform (CHBr3). To understand the physical mechanism of UV-induced C-Br bond breaking, fewest switches surface hopping (FSSH) calculations are performed to simulate the dynamics of CHBr3 after photon excitation. Using intermediate geometries and electronic structures from the FSSH simulation, XUV absorption spectra are calculated with the linear-response time-dependent DFT method for comparison with femtosecond time-resolved inner-shell absorption measurements. The combined theoretical-experimental study indicates that C-Br bond scission proceeds on a timescale of ~30-40 fs, followed by continued electronic interaction between the departing Br atom and the remaining CHBr2 fragment on an ~80-90 fs timescale. The study demonstrates how photochemical processes may be probed through a combination of ultrafast XUV/X-ray transient absorption spectroscopy, excited state molecular dynamics simulations, and core-level near-edge XUV/X-ray absorption calculations. |
Tuesday, March 5, 2019 9:24AM - 9:36AM |
E32.00004: Probing Ultrafast Dissociation Dynamics of Pentafluorobenzene Cation (C6F5H+) with Electrons Ming-Fu Lin, Xiaozhe Shen, Pedro Nunes, Jie Yang, Renkai Li, Stephen Weathersby, Rob Parrish, Todd Martinez, Martin Centurion, Thomas Jacob Arcangelo Wolf, Xijie Wang, Michael Minitti Real-time observation of a structural change of photoexcited molecular ion is important to understand the fundamental reaction mechanism. Here, we use mega-electronvolt ultrafast electron diffraction technique (MeV-UED) to directly construct a molecular movie of photogenerated pentafluorobenznen cation. The molecular cation was produced by multiphoton ionization of C6F5H at 269 nm. This cation dissociates into fragment ions within ~10 ps time scale through statistical internal energy redistribution prior to the following bond dissociaiton and ring opening. Parir distribution function (PDF) analysis allows us to pin down the specific chemical bond dissociaiton process in this unimolecular dynamics. |
Tuesday, March 5, 2019 9:36AM - 9:48AM |
E32.00005: State-resolved thermal reaction rate from ring-polymer surface hopping Xuecheng Tao, Philip Shushkov, Thomas Miller Employing the recently developed isomorphic Hamiltonian framework for including nuclear quantum effects in mixed quantum-classical non-adiabatic dynamics, [J. Chem. Phys., 148, 102327 (2018)] we present a flux-side formulation of state-resolved thermal reaction rates for ring-polymer surface hopping. The method is shown to be robust and straightforwardly implemented, and numerical results reveal that RPSH in the isomorphic Hamiltonian framework leads to excellent dividing-surface independence, due to improved preservation of the path-integral statistics. The method is further applied to inverstigate F+H2 reactive scattering with an ab initio multi-level potential energy surface and its effectiveness is demonstrated with preliminary results. |
Tuesday, March 5, 2019 9:48AM - 10:00AM |
E32.00006: Mapping of the Excited State Potential Energy Surface during Molecular Photo-isomerization to Control Chemical Reactions Rachel Glenn We present a new perspective of light matter interaction with molecules. When using shaped pulses, we explain fundamentally how our understanding of absorption and dispersion changes. We show how the phase of the pulse can change an absorption line-shape to a dispersion-like line-shape. Contrary to conventional belief, we show that the first-order polarization is sensitive to the phase of the electric field. We show that by performing single pulse experiments that it is possible to "map" out the propagation of the wave packet in the excited state potential energy surface. In addition, we describe how to measure the momentum dependent lifetime of the wave packet through a conical intersection. We use these fundamental ideas to answer the question of finding an optimum coherent pulse(s) that will maximize the formation of a desired chemical species. |
Tuesday, March 5, 2019 10:00AM - 10:12AM |
E32.00007: Modeling Molecular Spectra with Interpretable Atomistic Neural Networks Michael Gastegger, Kristof T Schütt, Huziel Sauceda, Klaus-Robert Müller, Alexandre Tkatchenko Deep neural networks are emerging as a powerful tool in quantum chemistry, combining the benefits of high-level electronic structure methods with excellent computational efficiency. The recently developed SchNet model provides an accurate description of molecules and materials across chemical compound space, as well as easy access to energy conserving force fields [1]. Here, we demonstrate that the modular nature of deep models can also be exploited to enhance their versatility and offer insights beyond the basic relationships learned by the network. First, we adapt existing architectures to model different spectroscopic quantities, such as molecular infrared spectra [2]. Going beyond the simple prediction of properties, we then explore modifications of SchNet in the form of latent features. Although these variables are inferred, they correspond to readily interpretable physical concepts, such as molecular charge distributions [3]. |
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