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Register a new userTime- and Site-Resolved Kinetic NMR: Real-Time Monitoring of Off-Equilibrium Chemical Dynamics by 2D Spectrotemporal Correlations
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Added by
Michael Jaroszewicz
Publication date
2021
fields
Physics
and research's language is
English
Authors
Michael J. Jaroszewicz
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Nuclear magnetic resonance (NMR) spectroscopy provides detailed information pertaining to dynamic processes through line-shape changes, which have been traditionally limited to equilibrium conditions. However, there is a wealth of information to be gained by studying chemical reactions under off-equilibrium conditions -- e.g., in states that arise upon mixing reactants that subsequently undergo chemical changes -- and in monitoring the formation of reaction products in real time. Herein, we propose and demonstrate a time-resolved kinetic NMR experiment that combines rapid mixing techniques, continuous flow, and single-scan spectroscopic imaging methods, leading in unison to a new 2D spectro-temporal NMR correlation which provides high-quality kinetic information of off-equilibrium dynamics. These kinetic 2D NMR spectra possess a spectral dimension conveying with high resolution the individual chemical sites, correlated with a time-independent, steady-state spatial axis that delivers unique information concerning temporal changes along the chemical reaction coordinate. A comprehensive description of the kinetic and spectroscopic features associated to these spectro-temporal NMR analyses is presented, factoring in the rapid-mixing, the flow and the spectroscopic NMR imaging. An experimental demonstration of this methods novel aspects was carried out using an enzymatically catalyzed reaction, leading to site- and time-resolved kinetic NMR data that are in excellent agreement with control experiments and literature values.
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This review article discusses advances in the use of time-resolved photoelectron spectroscopy for the study of non-adiabatic processes in molecules. A theoretical treatment of the experiments is presented together with a number of experimental examples.
The electronic and nuclear dynamics in methanol, following 156~nm photoexcitation, are investigated by combining a detailed analysis of time-resolved photoelectron spectroscopy experiments with electronic structure calculations. The photoexcitation pump pulse is followed by a delayed 260~nm photoionization probe pulse, to produce photoelectrons that are analyzed by velocity map imaging. The yield of mass-resolved ions, measured with similar experimental conditions, are found to exhibit the same time-dependence as specific photoelectron spectral features. Energy-resolved signal onset and decay times are extracted from the measured photoelectron spectra to achieve high temporal resolution, beyond the 20~fs pump and probe pulse durations. When combined with {it ab initio} calculations of selected cuts through the excited state potential energy surfaces, this information allows the dynamics of the transient excited molecule, which exhibits multiple nuclear and electronic degrees of freedom, to be tracked on its intrinsic few-femtosecond timescale. Within 15~fs of photoexcitation, we observe nuclear motion on the initially bound photoexcited 2$^{1}$A$$ (S$_2$) electronic state, through a conical intersection with the 1$^{1}$A$$ (S$_3$) state, which reveals paths to photodissociation following C--O stretch and C--O--H angle opening.
Supercomputers are complex systems producing vast quantities of performance data from multiple sources and of varying types. Performance data from each of the thousands of nodes in a supercomputer tracks multiple forms of storage, memory, networks, processors, and accelerators. Optimization of application performance is critical for cost effective usage of a supercomputer and requires efficient methods for effectively viewing performance data. The combination of supercomputing analytics and 3D gaming visualization enables real-time processing and visual data display of massive amounts of information that humans can process quickly with little training. Our system fully utilizes the capabilities of modern 3D gaming environments to create novel representations of computing hardware which intuitively represent the physical attributes of the supercomputer while displaying real-time alerts and component utilization. This system allows operators to quickly assess how the supercomputer is being used, gives users visibility into the resources they are consuming, and provides instructors new ways to interactively teach the computing architecture concepts necessary for efficient computing
Unravelling the main initial dynamics responsible for chiral recognition is a key stepin the understanding of many biological processes. However this challenging task requires a sensitive enantiospecic probe to investigate molecular dynamics on their natural femtosecond timescale. Here we show that, in the gas phase, the ultrafast relaxationdynamics of photoexcited chiral molecules can be tracked by recording Time-ResolvedPhotoElectron Circular Dichroism (TR-PECD) resulting from the photoionisation bya circularly polarized probe pulse. A large forward/backward asymmetry along theprobe propagation axis is observed in the photoelectron angular distribution. Its evolution with pump-probe delay reveals ultrafast dynamics that are inaccessible in theangle-integrated photoelectron spectrum nor via the usual electron emission anisotropyparameter ($beta$). PECD, which originates from the electron scattering in the chiral molecular potential, appears as a new sensitive observable for ultrafast molecular dynamicsin chiral systems.
Optical pumping of solids creates a non-equilibrium electronic structure where electrons and photons combine to form quasiparticles of dressed electronic states. The resulting shift of electronic levels is known as the optical Stark effect, visible as a red shift in the optical spectrum. Here we show that in a pump-probe setup we can uniquely define a non-equilibrium quasiparticle bandstructure that can be directly measurable with photo-electron spectroscopy. The dynamical photon-dressing (and undressing) of the many-body electronic states can be monitored by pump-probe time and angular resolved photoelectron spectroscopy (tr-ARPES) as the photon-dressed bandstructure evolves in time depending on the pump-probe pulse overlap. The computed tr-ARPES spectrum agrees perfectly with the quasi-energy spectrum of Floquet theory at maximum overlap and goes to the the equilibrium bandstructure as the pump-probe overlap goes to zero. Additionally, we show how this time-dependent non-equilibrium quasiparticle structure can be understood to be the bandstructure underlying the optical Stark effect. The extension to spin-resolved PES can be used to predict asymmetric dichroic response linked to the valley selective optical excitations in monolayer transition metal dichalcogenides (TMDs).
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