Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session W21: Dynamics and Spectroscopy |
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Sponsoring Units: DCP Chair: Scott Sayres, Arizona State Univ Room: 302 |
Friday, March 6, 2020 8:00AM - 8:12AM |
W21.00001: Broadband Microwave Spectroscopy of Lignin, Biofuels, and Their Pyrolysis Intermediates Alicia Hernandez-Castillo, Timothy Scott Zwier, John F Stanton The chemical complexity of the fast-expanding list of potential plant derived biofuels pose a challenge to the scientific community seeking to provide a molecular understanding of their combustion. The direct relationship between molecular structure and rotational frequencies makes rotational spectroscopy highly structure specific; therefore, it is an ideal tool to characterize resonance-stabilized radicals. A high-temperature flash pyrolysis micro-reactor coupled with a supersonic expansion were used to generate the radicals, and a protocol called “strong-field coherence breaking (SFCB)” was used to analyze and speed up the line assignments. The 2-furanyloxy radical, a primary, resonance-stabilized radical formed by loss of a methyl group in the pyrolysis of 2-methoxy furan, was detected and its molecular parameters were determined. Furthermore, the phenoxy radical was investigated and structurally characterized by doing mass-correlated microwave studies. The new structural insights derived from analysis of the observed radicals and the new broadband rotational techniques will be discussed. |
Friday, March 6, 2020 8:12AM - 8:24AM |
W21.00002: Making a pure beam of cold, state-selected particles with tuneable velocity Jutta Toscano, Christopher J. Rennick, Michal Hejduk, Timothy P. Softley, Brianna R. Heazlewood Generating a controllable and pure source of molecular free-radicals or open-shell atoms has been one of the primary barriers hindering the detailed study of radical processes in the laboratory. Radicals are intrinsically highly reactive species that are challenging to isolate as a pure sample. As a result of this, detailed information on the kinetics and dynamics of elementary reactions involving radicals is extremely scarce. |
Friday, March 6, 2020 8:24AM - 8:36AM |
W21.00003: Production of O2 from Ultrafast Pump-Probe CO2 Photodissociation Jacob Garcia, Ryan Shaffer, Scott Sayres Carbon dioxide (CO2) is ubiquitous in our atmosphere and plays important roles in the life cycle, the greenhouse effect, and has potential applications in industrial processes as a chemical feedstock. Concern over the ever-increasing concentration of CO2 in our environment has sparked a renewed interest in its photoabsorption dynamics. Photodissociation of CO2 proceeds via multiple pathways depending on available energy, but commonly into CO and O. Here, ultrafast pump-probe spectroscopy is applied to study the excited state dynamics of CO2. Multiphoton excitation from a 35 fs pump laser impulsively transfers CO2 from the X → A state, preparing a bending vibrational wavepacket that influences its dissociation.Using mass spectrometry, we report changes in the fragmentation pattern as a function of time delay between the pump and probe laser pulses that reflect the vibrational motion. At well-defined time delays the dissociation oscillates between observable CO+and O2+ fragments. The state relaxes to the ground state through a conical intersection. |
Friday, March 6, 2020 8:36AM - 8:48AM |
W21.00004: Non-adiabatic dynamics of the highly excited uracil cation Patricia Vindel Zandbergen, Spiridoula C Matsika, Neepa Maitra Conical intersections facilitating non-adiabatic transitions have been found to play a key role in many photo-physical pathways. Radical cations of nuclear acid bases involved in charge transfer processes in DNA exhibit ultrafast dynamics governed by the existence of non-adiabatic couplings between excited states, where electron-nuclei coupled dynamics become important. We investigate the dynamics of the uracil cation, using highly-accurate coupled electron-ion dynamics methods and compare the results with more commonly used and less computationally intensive methods. In particular, trajectory-based surface hopping dynamics has proven to be a powerful tool to study coupled nuclear-electronic dynamics, but it does not properly account for quantum decoherence. Instead, a new coupled-trajectory approach has been proposed based on the exact electron-nuclear correlation from the exact factorization of a full molecular wave function. Numerical simulations with model systems have shown that the electron−nuclear coupling beyond the non adiabatic coupling terms can describe the quantum coherence properly. Within this method the quantum (de)-coherence in large molecules can be treated correctly at the same computational cost as the original surface hopping dynamics. |
Friday, March 6, 2020 8:48AM - 9:00AM |
W21.00005: Feshbach-Dyson method for calculations of Auger decay rates Wojciech Skomorowski, Anna Krylov Photon absorption in the X-ray domain generally leads to creation of highly excited species with core-electron vacancies. One of the major relaxation channels for such states is Auger decay - a process accompanied by spontaneous emission of free electron(s). While Auger decay in atoms and molecules has been known for long now, its predictive modeling still poses a significant challenge for theorists, as it intrinsically involves electronic continuum and many-body electronic states which are formally unbound (metastable). |
Friday, March 6, 2020 9:00AM - 9:12AM |
W21.00006: Sub-20-fs broadband transient transmission spectroscopy in the UV range used to track primary photoinduced processes in thiobases Danielle Cristina Ferreira, Irene Conti, Rocio Borrego-Varillas, Artur Nenov, Lucia Ganzer, Cristian Manzoni, Marco Garavelli, Giulio Cerullo, Ana Maria de Paula Thiobases are DNA/RNA nucleobases where an exocyclic carbonyl oxygen is replaced by a sulfur atom. Thiation induces a red-shift in the absorption spectrum and causes also a dramatic change in the photophysics with respect to the canonical nucleobases: while in nucleobases internal conversion from the photoexcited singlet state to the ground state mediated by a conical intersection is the main deactivation channel, the major relaxation pathway in thiobases is via the population of triplet states through an ultrafast intersystem crossing. |
Friday, March 6, 2020 9:12AM - 9:24AM |
W21.00007: Effect of polar molecules in spin crossover transition of Fe(H2B(pz)2)2(bipy) Thilini Ekanayaka, Paulo S Costa, Guanhua Hao, Alpha T. N'Diaye, Lucie Routaboul, Pierre Braunstein, Xin Zhang, Jian Zhang, Qin-Yin Shi, Vicki Schlegel, Bernard Doudin, Axel Enders, Peter A Dowben The spin crossover phenomenona is a temperature-induced transition of diamagnetic low spin (LS) state to paramagnetic high spin (HS) state. This transition can also be triggered by pressure, light or magnetic field. The Fe(II) spin crossover molecules are known to exhibit spin-crossover transition and we have shown that [Fe{H2B(pz)2}2(bipy)] exhibits a temperature dependent spin-crossover transition from its LS state to the HS state, if the temperature is increased above approximately 160 K. The temperature dependence of the spin state occupancy Fe(II) complex can be affected by addition of polar molecules. The highly polar p-benzoquinonemonoimine zwitterion mixed with [Fe{H2B(pz)2}2(bipy)] resulted in locking [Fe{H2B(pz)2}2(bipy)] largely into a low spin state while addition of the di-ethyl derivative of p-benzoquinonemonoimine zwitterion mixed with [Fe{H2B(pz)2}2(bipy)] did not appear to perturb the spin crossover transition. Addition of different polar molecule, benzimidazole with small dipole moment results in a re-entrant spin crossover transition. |
Friday, March 6, 2020 9:24AM - 9:36AM |
W21.00008: Chemical reaction pathways estimated by a transformation between manifolds Mark Palenik Chemical reaction pathways are often estimated by linearly interpolating a set of internal coordinates, such as bond lengths, between reactant and product structures. However, these internal coordinates are not generally isomorphic to atomic positions. For example, the number of bond lengths between N atoms is N(N−1)/2, which is usually greater than the 3N -6 coordinates that specify unique molecular geometries. If arbitrarily chosen, the bond lengths may not be physically realizable because they violate the triangle inequality. The solution used in the method known as linear synchronous transit is to employ a least squares minimization to approximate the desired set of generalized coordinates. However, this method is prone to generating discontinuous reaction pathways and outright failure at generating transition states. As an alternative, we treat the spaces of generalized coordinates and Cartesian coordinates as separate manifolds and locally transform a velocity vector from the former into the latter. This results in a first-order differential equation for the reaction path which is numerically integrable even when linear synchronous transit fails. |
Friday, March 6, 2020 9:36AM - 9:48AM |
W21.00009: Harvesting force and energy from the thermal motion of gas molecules Tom zhu This presentation will describe a few methods for producing force and energy from the thermal motion of gas molecules. The force that exerts to a surface by the gas molecules in collision with the surface is due to the change of the momentum of the gas molecules. For an elastic collision, there is no lost in the kinetic energy of the gas molecule. The rebound speed is the same as the initial speed. However, for a real system, there is always lost or gain in the kinetic energy in the collision. The force exerted to the surface by the gas molecule in collision with the surface will be smaller if there is kinetic energy lost in the collision because lower rebound speed. We may design a high-lost surface and a low-lost surface to produce a net force, to do work with this force, and at the same time to lower the gas temperature which could be maintained by absorbing the heat from external source. The net result is transfer the heat to work which can be used to, for example, generate electricity, without producing the waste heat. The force can also be used as propulsion means, or to lift weight against the gravity, for example elevating the vehicle of the hyper-loop, without consuming any energy. |
Friday, March 6, 2020 9:48AM - 10:00AM |
W21.00010: Theory of Diffusion-Limited Reactions between Proteins in a Confined Environment Chen Lin, Joseph Rudnick, Robijn F Bruinsma Smoluchowski theory is extensively used to compute diffusion-limited reaction rates. It is based on the assumption that the reactants diffuse in from infinity. However, in many cases, this assumption fails for association reactions between biological macromolecules because they initially are confined inside a certain volume. Examples are association reactions between the linked subdomains of polyproteins, such as the Gag protein of HIV-1 and the assembly reactions of the molecular components of a virus in a "virus factories". Confinement greatly increases reaction rates. The talk will discuss how Smoluchowski's theory is generalized to include a confined environment by introducing particles at the finite range. |
Friday, March 6, 2020 10:00AM - 10:12AM |
W21.00011: Chemical Cloaking Francesco Avanzini, Gianmaria Falasco, Massimiliano Esposito Any object embedded in a chemical gradient will typically distort it. We propose a strategy to hide an object in a gradient by coating it with a chemical reaction-diffusion network which can act as an active cloaking device. By controlling the concentration of some species in its immediate surroundings, the chemical reactions redirect the gradient as if the object was not there. We also show that the energy required to cloak can be extracted from the chemical gradient itself. |
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