Frontiers of chemical dynamics

09:00-09:30 Classical molecular dynamics simulations of electronically non-adiabatic processes

Professor William H Miller ForMemRS, University of California, Berkeley, USA

Abstract

A recently described symmetrical quasi-classical (SQC) windowing methodology for classical trajectory simulations has been applied to the Meyer-Miller (MM) model for the electronic degrees of freedom in electronically non-adiabatic dynamics. The approach treats nuclear and electronic degrees of freedom (DOF) equivalently (i.e., by classical mechanics, thereby retaining the simplicity of standard molecular dynamics), providing “quantization” of the electronic states through the symmetrical quasi-classical (SQC) windowing model. The approach is seen to be capable of treating extreme regimes of strong and weak coupling between the electronic states, as well as accurately describing coherence effects in the electronic DOF (including the de-coherence of such effects caused by coupling to the nuclear DOF). It is able to provide the full electronic density matrix from the one ensemble of trajectories, and the SQC windowing methodology correctly describes detailed balance (unlike the traditional Ehrenfest approach). Calculations can be (equivalently) carried out in the adiabatic or a diabatic representation of the electronic states, and most recently it has been shown that a modification of the canonical equations of motion in the adiabatic representation eliminates (without approximation) the need for second-derivative coupling terms.

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09:45-10:15 When instantons get wet: path-integral rate theory for the condensed-phase

Dr Jeremy O Richardson, ETH Zurich, Switzerland

Abstract

Instanton theory results from a rigorous semiclassical derivation for the rate of a chemical reaction. However, due to a number of harmonic approximations, it is not applicable to study reactions in liquids, where many transition-states exist close to each other such that they cannot be treated independently.  Ring-polymer molecular dynamics avoids this problem by effectively sampling the instantons without making the harmonic approximation. A similar instanton theory can also be derived for the rate of the fundamental process of electron transfer, in which the electron dynamics are coupled strongly to the nuclear degrees of freedom such that the Born-Oppenheimer approximation cannot be made.  Again, however, this instanton theory cannot be applied to atomlistic models of liquids and a new ring-polymer sampling scheme is be required. It will be shown that starting from a physically motivated ansatz, it is possible to derive new ring-polymer sampling schemes which dominately sample the instanton configurations and thus give excellent approximations to the rate of electron transfer.

10:30-11:00 Coffee

11:00-11:30 Quantum statistics with classical dynamics: applications to liquid water and ice

Professor Stuart C Althorpe, University of Cambridge, UK

Abstract

In water and ice, the nuclear motion takes place on a single Born-Oppenheimer surface, under conditions of thermal equilibrium. The nuclei exhibit quantum properties, which a growing body of evidence suggests are caused almost entirely by the quantum Boltzmann statistics, with the dynamics of the nuclei being classical. Here we summarise a recently developed theory which explains how such a classical dynamics can arise as a result of certain properties of the quantum statistics. This dynamics involves the motion of smooth delocalised loops of the hydrogen atoms which, despite being classical, conserve the quantum Boltzmann distribution. Exact implementation of this dynamics is not possible because of a phase problem, but its approximate implementation can be done using a ‘planetary’ model (originally developed heuristically by others), in which each hydrogen nucleus is represented by two particles, one (the ‘centroid’) describing its position, the other (the ‘planet’) describing the extent of delocalisation. We report recent simulations of the infrared spectrum of liquid water and ice, obtained using the planetary model. Despite the approximations made, the model is capable of reproducing the line shapes of the bend and stretch peaks, which are found to be motionally narrowed by the dynamics of the centroid.

11:45-12:15 Quantum nonadiabatic dynamics from classical trajectories

Professor Nandini Ananth, Cornell University, USA

Abstract

Simulating energy transfer pathways in reactions at metal surfaces requires methods that describe electronically nonadiabatic processes, capture quantum coherence effects, and remain computationally feasible for high dimensional systems. Quantum-limit semiclassical methods meet almost all these criteria, but the computational costs scale poorly with system size limiting their applications. The recently derived Mixed Quantum-Classical Initial Value Representation (MQCIVR) provides a uniform semiclassical framework for the calculation of real-time correlation functions where a subset of system modes are treated in the quantum limit while the rest are treated in the classical limit. This is achieved by selectively filtering amplitude of the semiclassical integrand in regions of highly oscillatory phase, leading to improved numerical convergence without significant loss of accuracy. This method is applied to several model systems and its ability to systematically tune individual system modes between quantum-limit and classical-limit semiclassical behaviour clearly demonstrated. MQC-IVR is further extended to electronically nonadiabatic processes for the study of inelastic scattering at a metal surface.

13:30-14:00 On the mechanism of selective adsorption of ions to aqueous interfaces: graphene/water vs air/water

Professor Richard J Saykally, University of California, Berkeley, USA

Abstract

The behaviour of ions at aqueous interfaces has been a subject of much controversy for over a century. By exploiting the strong charge-transfer-to-solvent (CTTS) resonances of selected anions in aqueous electrolytes, their adsorption properties have measured by deep UV-SHG spectroscopy methods for both air/water and graphene/water interfaces. Temperature and concentration dependences determined by both experiment and computer simulations for the air/water case reveal that the strong interfacial adsorption observed for weakly hydrated ions is enthalpically driven by hydration forces and impeded by a novel entropy effect (capillary wave suppression). Extension of this approach to the water-graphene interface reveals a surprising similarity to the air-water case, albeit with different mechanistic details. The recent development of a broadband deep UV SFG spectroscopy technique has produced detailed CTTS spectra of interfacial ions, for which comparisons with bulk CTTS spectra provide additional new insights.

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14:15-14:45 Photoelectron spectroscopy in the gas-phase and in liquid-jets

Professor Helen H Fielding, University College London, UK

Abstract

Much of our detailed understanding of the intrinsic electronic relaxation dynamics of photoexcited molecules has come from gas-phase experiments and calculations involving isolated molecules, free from interactions with solvent or protein environments. However, electronically excited states are sensitive to their microenvironment, particularly in polar solvents such as water, the most important medium in chemistry and biology, and in proteins. Photoelectron spectroscopy is the ideal tool for probing the electronic structure of molecules through the measurement of electron binding energies. This presentation will describe recent work from our group employing photoelectron spectroscopy in molecular and anion beams and in a liquid-microjet to compare the electronic structure and relaxation dynamics of biologically relevant chromophores following photoexcitation in the gas-phase and in aqueous solution.

15:00-15:30 Tea

15:30-16:00 Ultrafast chemical dynamics in solution

Professor Andrew J Orr-Ewing FRS, University of Bristol, UK

Abstract

The study of reactions in solution represents a new frontier in the field of chemical reaction dynamics. Interactions with the surrounding solvent modify the energy landscape controlling the atomic motions during the chemical reaction, and influence the flow of energy released as reactants transform to products. The timescale for reaction is comparable to that required for solvent reorganisation to accommodate transition states or reaction products, and the microscopic structure of the solvent must therefore be considered when developing an understanding of a reaction mechanism in solution. Ultrafast transient absorption spectroscopy of photochemical and reaction dynamics makes possible detailed studies of both the reaction dynamics in solution and the solvent response. Examples will be presented of the mechanisms of reactions in both weakly and strongly interacting solvents, and comparisons will be drawn with the corresponding dynamics in the gas phase to highlight the consequences of the surrounding liquid environment.

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16:15-16:45 Chemical physics birthday stories for David: from ultrafast plasmonic photoelectron imaging studies to thermodynamics/kinetics of RNA folding at the single molecule level

Professor David Nesbitt, JILA - University of Colorado, Boulder

Abstract

Chemists have been amazingly successful with the manipulation of complicated molecular systems, whereas to a physicist, even simple molecules can seem overwhelmingly complex. Physical chemists are blessed (or cursed) with a dual scientific personality. We are attracted to real world chemical systems and yet are often not satisfied posing questions without the requisite rigour to hope for fundamental answers. This talk will provide an overview of work in Professor Nesbitt’s labs that attempts to address complex molecular systems but with a physical chemist’s eye toward finding the underlying simplicity. In particular, Professor Nesbitt will focus on recent results from his labs in the following two areas: 1) electron oscillation in Au, Ag, and Cu nanostructures (i.e.,  “plasmons”) leads to intense absorption strengths and offers enormous potential for real-world applications in solar energy. He will address how the novel combination of ultrafast OPO lasers, in-vacuo microscopy, and velocity map imaging of the electron photoemission can provide a novel, powerful and remarkably sensitive experimental platform for exploring the fundamental chemical physics and spectroscopy of nanoplasmonic materials. 2) The second topic will be the use of confocal microscopy, fluorescence resonance energy transfer (FRET), and time correlated single photon counting methods to explore the kinetics and thermodynamics of RNA folding at the single molecule level. One issue in particular will be probing the effects of microscopic viscosity and molecular “crowding” on tertiary and secondary structure motifs responsible for “docking” single stranded RNA oligomers into biochemically competent 3D structures. In each area, the focus will be on simple physical pictures that help explain and interpret the underlying chemical physics.

09:00-09:30 Chemical dynamics and heterogeneous catalysis

Professor George C Schatz, Northwestern University, USA

Abstract

CO₂ reduction to syngas and N₂ reduction to ammonia are processes of fundamental interest to energy science. In this talk, theory is used to provide new directions to this research in two areas. The first part of the talk discusses plasma enhanced dry reforming, and photocatalytic conversion of N₂ to ammonia. Dry reforming is a process wherein CH₄ and CO₂ react to give syngas and/or liquid fuels. Dry reforming is normally done under high temperature and pressure conditions, with a Ni catalyst, however it has recently been discovered that if a plasma is also present near the catalyst, then it is possible to get this reaction to go under modest conditions close to room temperature and atmospheric pressure. The role of the plasma in this process is poorly understood. The talk focuses on the use of electronic structure studies and Born-Oppenheimer molecular dynamics to describe gas-surface reactions that arise from plasma species. The plasma is known to fragment the reacting gases, especially CH₄ into CH₃ + H and CH₂ + 2H, so a highlight of this work concerns reaction of atomic hydrogen and CH₂ with adsorbed CO₂ and CO to give CO, water, formaldehyde, methanol and other products. The results include comparisons with experiments from the Koel group at Princeton, and with other groups, and it is found that hot-atom and Eley-Rideal mechanisms play an important role. The second part of this talk considers the reaction mechanism underlying recent work from the Kanatzides group at Northwestern in which it was discovered that iron-sulphur clusters present in gel materials can participate in the photocatalytic conversion of N₂ to ammonia under ambient conditions. The theoretical studies use broken symmetry density functional theory to reveal a mechanism for this process that is related to what happens with the nitrogenase enzyme, but with important differences that arise from photon-induced delivery of electrons to the iron-sulphur clusters.

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09:45-10:15 Towards a chemically accurate description of reactions of molecules with metal surfaces

Professor Geert-Jan Kroes , Leiden University, The Netherlands

Abstract

Heterogeneously catalyzed processes are of large importance: the production of the majority of chemicals involves catalysis at some stage. Heterogeneously catalyzed processes consist of several elementary reactions. Accurately calculating their rates requires the availability of accurate barriers for the rate controlling steps. Unfortunately, currently no first principles methods can be relied upon to deliver the required accuracy. To solve this problem, in 2009 a novel implementation of the specific reaction parameter approach to density functional theory (SRP-DFT) was formulated. This allowed reproducing experiments for H₂ reacting on copper surfaces, and to determine barrier heights for H₂-Cu systems, with chemical accuracy. The original procedure used was not extendable to reactions of molecules heavier than H₂ with surfaces, because the metal surface was treated as static. This problem has now been solved through a combination of SRP-DFT with Ab Initio Molecular Dynamics (AIMD). This method was applied to the dissociative chemisorption of methane on a Ni surface, a rate-limiting step in the steam reforming reaction. Experiments on CHD₃ + Ni(111) were reproduced with chemical accuracy, and a value of the reaction barrier height was derived that is claimed to be chemically accurate. Even better results were obtained for CHD₃ + Pt(111), suggesting that the SRP density functional for methane interacting with Ni(111) is transferable to methane interacting with other group X metal surfaces. Even more interestingly for applications to catalysis, the SRP functional derived for methane reacting with Ni(111) also gives a very accurate description of molecular beam sticking experiments on CHD₃ + Pt(211).

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10:30-11:00 Coffee

11:00-11:30 Quantum dynamics studies of the Cl + CH4 reaction

Professor Dong Hui Zhang, Dalian Institute of Chemical Physics, China

Abstract

The Cl+CH₄→HCl+CH₃ reaction has been the subject of extensive experimental and theoretical investigations due to its crucial role in the Cl/O₃ destruction chain mechanism in the stratosphere, and has also become a prototype for studying mode specificity and bond selectivity in polyatomic reactions with a late barrier. Earlier quantum dynamics studies on a high-quality potential energy surface (PES) constructed by Czako and Bowman (CB) revealed that there is a distinctive peak in the total reaction probabilities for the total angular momentum J=0 at collision energy of about 3 kcal/mol, which was inferred to be related to a dynamics resonance in the reaction. In this talk, Professor Zhang will present a quantum dynamics study of the reaction on a new PES by using the reduced dimensionality model of Palma and Clary by restricting the non-reacting CH₃ group in a C3V symmetry. The calculated total reaction probabilities for the total angular momentum J=0 on the new PES, which is of a quantitative level of accuracy, exhibit a clear peak structure as on the CB PES. Detailed dynamics analysis uncovered that the peak structure does not originate from a dynamics resonance, it is a very reaction probability oscillation associated with the heavy-light-heavy (HLH) nature of the reaction. State-to-state quantum dynamics calculations revealed that the HLH oscillation in the reaction has important influence to product rotation distributions, and also leave a clear peak in the backward scattering direction which can be detected by a cross-molecular beam experiment.

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11:45-12:15 Quantum interferences in inelastic molecular collisions

Professor Millard H Alexander, University of Maryland, USA

Abstract

In the textbook experiment of Young, quantum interference between two distinct trajectories leads to oscillations in the observed scattering of an atomic beam through two slits. Similar interferences patterns can be observed as oscillations in the intensities of scattering of a molecular beam from a given initial rotational state into various final states. This quantum interference structure becomes especially rich when the initial and final states are coupled by two electronic potential energy surfaces. This field will be reviewed with particular attention to work done in collaboration with Professor Clary.

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13:30-14:00 Illuminating atmospheric reaction pathways via chemical dynamics

Professor Marsha I Lester, University of Pennsylvania, USA

Abstract

Alkene ozonolysis is a primary oxidation pathway for alkenes emitted into the troposphere and also an important source of atmospheric hydroxyl radicals. Alkene ozonolysis takes place on a reaction path with multiple minima and barriers along the way to OH products. In particular, a key reaction intermediate, known as the Criegee intermediate, R₁R₂COO, had eluded detection until very recently. In this laboratory, the simplest Criegee intermediate CH₂OO and methyl-, dimethyl-, ethyl-, and vinyl-substituted Criegee intermediates have been generated by an alternative synthetic route, detected by VUV photoionization, and characterized on a strong π*←π transition. Recent studies have focused on vibrational activation of Criegee intermediates in the vicinity of and at energies much below the barrier associated with hydrogen transfer that leads to OH radical products. Infrared action spectra of the Criegee intermediates are obtained, along with time-resolves rates for appearance of OH radical products following vibrational activation under collision-free conditions. Complementary theoretical calculations are carried out to predict the energy-dependent unimolecular decay rates of the Criegee intermediates. Quantum mechanical tunneling through the barrier is shown to make a significant contribution to the decay rates. The results are extended to thermally averaged unimolecular decay of stabilized Criegee intermediates under atmospheric conditions.

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14:15-14:45 Chemical reactions and energy transfer in cold supersonic flows – a meeting ground for experiment and theory?

Professor Ian Sims, University of Rennes 1, France

Abstract

David Clary’s pioneering adiabatic capture calculations on low temperature barrierless reactions provided a strong motivation for experiments in low temperature gas kinetics, which lead to the discovery of whole classes of neutral-neutral reactions that remain rapid or even become faster as the temperature is decreased down to 10 K or even below. An overview will be given of the importance of determining temperature dependent rate constants and product branching ratios for elementary chemical reactions and collisional energy exchange for understanding the creation and destruction of molecules in space. In particular attention will be focused on studies of the reactivity of carbon-containing radical species with organic co-reagents leading to efficient molecular growth even at the low temperatures of dense interstellar clouds (10—20 K) and circumstellar envelopes or of the atmospheres of planets and their moons, such as that of Titan (70—180 K). We employ both pulsed and continuous flow CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme, or Reaction Kinetics in Uniform Supersonic Flow) apparatuses, combined with a variety of laser photochemical techniques, as well as, most recently, synchrotron photo-ionization mass spectrometry and chirped pulse mm-wave rotational spectroscopy.

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15:00-15:30 Tea

15:30-16:00 Probing organic photochemistry by multi-mass velocity-map imaging

Professor Claire Vallance, University of Oxford, UK

Abstract

The domain of velocity-map imaging has expanded rapidly from highly detailed quantum-state resolved studies of small molecules to investigations into the photofragmentation dynamics of much larger molecules, including model systems of direct relevance to atmospheric chemistry, astrochemistry, photobiology, and organic photochemistry. Such studies present a number of challenges. The numerous fragmentation channels available to a polyatomic molecule generally make it unfeasible to attempt to record state-resolved images for each and every fragment. Instead, universal (or near-universal) ionization schemes are employed to ionize and therefore detect all products simultaneously. This allows the scattering distribution for each fragment to be recorded, though at the expense of quantum-state resolution. New imaging sensors capable of detecting individual particles with nanosecond time resolution allow images for all fragments to be acquired simultaneously within a single experiment, with covariance analysis of the data set revealing the correlated scattering distributions of pairs of photofragments. This talk will review recent technical advances in the arena of multi-mass imaging, as well as exploring examples of their application to a number of chemical systems.

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16:15-16:45 Geometric phase effect in chemical reaction

Professor Xueming Yang, Dalian Institute of Chemical Physics, CAS, China

Abstract

It is long known theoretically that geometric phase effect is important in chemical reactions with conical intersections. Even though there has a number of theoretical studies for this effect since 1970s, experimental observation of such effect proves to be extremely difficult and fruitless. Recently, we have constructed a new crossed beams imaging machine and observed the geometric phase effect in the H+HD→H₂+D reaction at collision energy near the conical intersection of this reaction using high resolution threshold ion imaging technique. In addition, we have also observed experimental evidence of geometric phase effect at collision energy significantly lower than conical intersection energy using high resolution H-atom Rydberg tagging technique. These new experiments allows us to probe this important effect in chemical reaction at the most fundamental level.

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