Simulation and control of electron and nuclear dynamics with strong and ultrashort laser pulses

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Químicas, Departamento de Química Física I, leída el 28/10/2016. Tesis formato europeo (compendio de artículos)Comprender la estructura y la dinámica de los procesos químicos a nivel molecular es un paso clave para el diseño de materiales con propiedades deseadas o para el control de las reacciones químicas. Desde los inicios de la mecánica cuática, el control de los fénomenos cuánticos ha sido uno de los principales objetivos en el campo de la física y la química. El desanollo de los láseres ultrarrápidos y ultraintensos ha permitido el uso de pulsos externos, no sólo para seguir el movimiento nuclear y electrónico [ 1-3], sino también para controlarlo de forma activa, es decir, manipular la dinámica molecular en la escala de tiempos en la que ocurren los procesos físicos y químicos, así como resolver las ecuaciones dinámicas que los gobiernan, de forma que pueda favorecerse un tipo de proceso en particular [4]. De esta forma, el campo de Control Cuántico (o coherente) se ha desarrollado conjuntamente con la Femtoquímica y la Attofísica. Las primeras propuestas de control surgieron independientemente con dos escenarios. Por un lado, Tannor y Rice propusieron un mecanismo de control en la variable temporal: el esquema pump-dump [5, 6], que es un precursor de lo que se llamaría control óptimo. Por otro lado, Brumer y Shapiro [7,8] propusieron un esquema de control coherente o resuelto en frecuencias. Sin embargo, estos esquemas sólo permiten el control de forma eficiente cuando se conocen el Hamiltoniano molecular y las superficies de energía potencial. Por ejemplo, en el esquema de Brumer y Shapiro, el mismo estado intermedio puede dar lugar a diferentes productos de reacción. En el esquema pump-dump, sólo es posible el control de transiciones verticales (ventana Frank-Condon) entre estados electrónicos...Understanding the structure and dynamics of chemical processes at the molecular level is a key step toward the design of materials with the desired properties, or the efficient control of chemical reactions. Many subtleties involving basic quantum properties, such as superposition of states and interfering pathways allow to highly increase the yield of a specific process, far beyond what the probability distribution would suggest, should it follow the classical rules of motion. The spectra of molecules is one of the strongest evidence of this phenomena. Rather than distributing its energy in a continuous way along the molecule, one can find resonances that relate to particular structures. The playground of quantum dynamics offers more spectacular predictions. Using the quantum correlations at our advantage, one can externally drive a molecule toward selecting specific states or chemical processes from the huge pool of competing processes that are energetically available. Much of the history of the probe and control of chemical processes has come side-byside with the development of lasers. As we will see, one can arguably relate this history as a process. The laser was first used as a tool to ignite and selectively probe specific states and processes given its fine-tunability and intensity. ·with ultrashort laser pulses came the first probe and control of the dynamics. Pulse shaping then allowed to promote the laser to the role of a chemical agent, using Rabitz's terminology [16]. Finally, the use of very strong non-resonant pulses is promoting the laser to the role of a catalyst. Obviously, all the different roles are still being enacted by the laser depending on the particular use we need. We will now review in more detail what particular features of lasers are mainly used and how they were developed in order to fullfil the different roles of igniting, probing, "reacting" with molecules, and "catalyzing" chemical processes...Depto. de Química FísicaFac. de Ciencias QuímicasTRUEunpu

Tunneling induced electron transfer between separated protons

We study electron transfer between two separated nuclei using local control theory. By conditioning the algorithm in a symmetric system formed by two protons, one can favored slow transfer processes, where tunneling is the main mechanism, achieving transfer efficiencies close to unity assuming fixed nuclei. The solution can be parametrized using sequences of pump and dump pi pulses, where the pump pulse is used to excite the electron to a highly excited state where the time for tunneling to the target nuclei is on the order of femtoseconds. The time delay must be chosen to allow for full population transfer via tunneling, and the dump pulse is chosen to remove energy from the state to avoid tunneling back to the original proton. Finally, we study the effect of the nuclear kinetic energy on the transfer efficiency. Even in the absence of relative motion between the protons, the spreading of the nuclear wave function is enough to reduce the yield of electronic transfer to less than one half.Comment: 7 pages, 8 figure

Chiral coherent control of electronic population transfer: towards all-optical and highly enantioselective photochemistry

Recent experiments have demonstrated highly enantioselective population transfer among rotational states using microwave fields with non-coplanar polarizations, a milestone in our ability to control chiral molecules. Extending this ability to the optical domain would open the door to all-optical enantioselective photochemistry, the next milestone in this direction. Here we show that the spatial variation of the field presents a major challenge along this route, limiting the interaction region to a thin layer with a thickness of the order of a wavelength. We provide a solution to this restriction by carefully designing the quantum pathways leading to the excited state. Our simulations reveal that the proposed scheme can result in differences of up to 19% in the excited state populations of R and S carvone. This is a major improvement over what is possible with circularly polarized light (~0.01%) and brings all-optical enantioselective photochemistry within reach for practical applications.Comment: 10 pages, 4 figure

Different Flavors of Exact-Factorization-Based Mixed Quantum-Classical Methods for Multistate Dynamics

The exact factorization approach has led to the development of new mixed quantum-classical methods for simulating coupled electron-ion dynamics. We compare their performance for dynamics when more than two electronic states are occupied at a given time, and analyze: (1) the use of coupled versus auxiliary trajectories in evaluating the electron-nuclear correlation terms, (2) the approximation of using these terms within surface-hopping and Ehrenfest frameworks, and (3) the relevance of the exact conditions of zero population transfer away from nonadiabatic coupling regions and total energy conservation. Dynamics through the three-state conical intersection in the uracil radical cation as well as polaritonic models in one dimension are studied

Impulsive alignment of 4He-CH3I: a theoretical study

We simulate the non-adiabatic laser alignment of the weakly bound 4He-CH3I complex based on a quantum mechanical wave packet calculation for a model He-CH3I interaction potential. Two different regimes are found depending on the laser intensity. At intensities typical of non-adiabatic alignment experiments, the rotational dynamics resembles that of the isolated molecule. This is attributed to the fact that after the initial prompt alignment peak the complex rapidly dissociates. The subsequent revival pattern is due to the free rotation of the molecule detached from the helium atom. It is superimposed to a flat background corresponding to ∼20% of the wave packet which remains bound, containing lower rotational excitation. At lower intensities, dissociation is avoided but the rotational excitation is not high enough to provide an efficient alignment and a broad non-regular structure is observed. Besides, the interaction of the He atom with the molecule quenches any possible alignment. These interpretations are based on the calculation of different observables related to the rotational motion. We compare our findings with recent experimental and theoretical results of non-adiabatic alignment of linear molecules solvated in helium nanodroplets or weakly interacting with one helium atom

HF trimer: A new full-dimensional potential energy surface and rigorous 12D quantum calculations of vibrational states

HF trimer, as the lightest cyclic hydrogen-bonded (HB) trimer, has long been a favorite prototype system for spectroscopic and theoretical investigations of the structure, energetics, and dynamics of hydrogen-bond networks, and the role that nonadditive, three-body interactions play in shaping these properties. Recently, rigorous quantum 12D calculations of the coupled intra- and intermolecular vibrations of this fundamental HB trimer [P.M. Felker and Z. Bačić, J. Chem. Phys. 2023, 158, 234109] were performed, employing an older ab initio-based many-body potential energy surface (PES). While the theoretical results were found to be in reasonably good agreement with the available spectroscopic data, it was also evident that it is highly desirable to develop a more accurate 12D PES of HF trimer. Motivated by this, here we report a new 12D PES of this paradigmatic system. Approximately 42,540 geometries were sampled and calculated at the level of CCSD(T)-F12a/AVTZ. The permutationally invariant polynomial-neural network based Δ-machine learning approach [Y. Liu and J. Li, J. Phys. Chem. Lett. 2022, 13, 4729] was employed to perform cost-efficient calculations of the basis-set-superposition error (BSSE) correction. By strategically selecting data points, this approach facilitated the construction of a high-precision PES with BSSE correction, while requiring only a minimal number of BSSE value computations. The fitting error of the final PES is only 0.035 kcal/mol. To assess its performance, the 12D fully coupled quantum calculations of excited intra- and intermolecular vibrational states of HF trimer are carried out using the rigorous methodology developed by us earlier. The results are found to be in a significantly better agreement with the available spectroscopic data than those obtained with the previously existing 12D PES

Helium-induced electronic transitions in photo-excited BA+ –HEn exciplexes

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Study of the Decoherence Correction Derived from the Exact Factorization Approach for Nonadiabatic Dynamics

We present a detailed study of the decoherence correction to surface hopping that was recently derived from the exact factorization approach. Ab initio multiple spawning calculations that use the same initial conditions and the same electronic structure method are used as a reference for three molecules: ethylene, the methaniminium cation, and fulvene, for which nonadiabatic dynamics follows a photoexcitation. A comparison with the Granucci−Persico energy-based decoherence correction and the augmented fewest-switches surface-hopping scheme shows that the three decoherence-corrected methods operate on individual trajectories in a qualitatively different way, but the results averaged over trajectories are similar for these systems