Publication: Estudio de reacciones fotoquímicas con metodología semiclásica incluyendo acoplamientos no adiabáticos, espín-órbita y con campos láser
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2018-09-06
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Universidad Complutense de Madrid
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Abstract
Durante el transcurso de esta tesis se ha llevado a cabo la puesta a punto de un método de dinámica semiclásica denominado SHARC (Surface Hopping including arbitrary Couplings). Este método permite estudiar la dinámica de moléculas de tamaño medio en sus estados excitados incluyendo simultáneamente todos los acoplamientos intrínsecos que existen entre ellos más allá de la aproximación de Born Oppenheimer (espín órbita, no adiabáticos) y los acoplamientos externos creados por pulsos láser. Se trata por tanto de una nueva herramienta de gran utilidad para los estudios teóricos de cualquier proceso fotoquímico involucrando tránsitos radiativos, cruces intersistema y conversión interna en intersecciones cónicas. Los métodos semiclásicos se basan en un tratamiento mixto cuántico y clásico. Los estados electrónicos y sus acoplamientos se calculan cuánticamente, y a partir de ellos se calculan las probabilidades que determinan el estado en el que se desarrolla la dinámica y las fuerzas que se ejercen sobre los núcleos, cuya dinámica se sigue integrando las ecuaciones del movimiento de Newton. Tanto los potenciales (fuerzas) como las probabilidades de salto dependen de forma crucial de la representación en la que se describe la función de ondas electrónica. En SHARC se emplea la representación adiabática de forma consistente y para estimar el cálculo de las probabilidades se emplean algoritmos tipo surface hopping. En la tesis aplicamos el método SHARC en dos situaciones extremas, que suponen todo un desafío para cualquier método no cuántico. En primer lugar se sigue la dinámica bajo pulsos láser muy intensos. Comparando los resultados de SHARC con los resultados cuánticos para distintos modelos, mostramos que SHARC permite simular de forma eficaz y con resultados cuantitativamente válidos un proceso de transferencia selectiva de población entre dos estados electrónicos por absorción de dos fotones en Na2. El proceso, denominado APLIP (Adiabatic Passage by Light Induced Potentials) implica la conservación del número cuántico vibracional. Pese a ser un método semiclásico, SHARC representa de forma correcta no sólo la población electrónica, sino también, cualitativamente, la energía vibracional en el estado final, lo que constrasta con los resultados obtenidos usando otros métodos semiclásicos. En segundo lugar, aplicamos SHARC al estudio de la fotodisociación del ICH3, IClCH2 e IBrCH2, moléculas que presentan un acoplamiento espín órbita fuerte, así como una intersección cónica a lo largo de su coordenada de reacción, lo que implica la presencia de acoplamientos no adiabáticos fuertes durante la dinámica. Se obtuvieron resultados con SHARC tanto usando modelos de potenciales (incluyendo o no el efecto de la absorción láser) como a través de dinámica directa (dinámica on the fly), mostrando buena concordancia con los resultados bibliográficos obtenidos tanto experimentalmente como con otras metodologías.
During this thesis it has been done a fine tunning of a semiclassical method of dynamics called SHARC Surface Hopping including Arbitrary Couplings. This method allows to study the dynamics of middle size molecules in their excited states including simultaneously all the couplings that intrinsecally exist between them beyond the Born-Oppenheimer aproximation (spin-orbit, non adiabatic) and external couplings created by laser pulses. Therefore it is a new tool of great usefulness for theoretical studies in any photochemical process involving radiative transitions, intersystem crossings and internal conversion in conical intersections. Semiclassical methods are based on a mixed quantum and classical treatment. Electronic states and their couplings are calculated in a quantum way, and using them the probabilities that determine the states where the dynamics evolves are calculated, and also forces that they apply over nuclei, whose dynamics is followed integrating Newton’s equations of motion. Both, potentials (forces) and hopping probabilities depend in an essential way on the representation in which the electronic wavefunction is described. In the thesis we apply SHARC method in two critical situations, that involve a challenge for other non-quantum method. In first place, dynamics under very strong laser fields is followed. Comparing SHARC results with quantum methods for several different models, we show that SHARC allows us to simulate in an efficient way and with quantitative results a selective population transfer process between two electronic states by two photon absortion in Na2. The process, called APLIP (Adiabatic Passage by Light-Induced Potentials) implies the conservation of the vibrational number. Despite being a semiclassical method, SHARC performs in a correct way not only the electronic population, but also qualitatively, the vibrational energy in the final state, what contrasts with results obtained using other semiclassical methods. Secondly, we aplied SHARC to the ICH3 photodissociation study, it is a molecule that has a strong spin-orbit coupling, in addition to a conical intersection along its reaction coordinate, which implies the presence of strong non-adiabatic couplings during the dynamics. Results was obtained with SHARC both, using potential models (including or not the effect of lase absortion) and through a direct dynamics (“on the fly”), showing good agreement with bibliographic results obtained both experimentally and with other methodologies.
During this thesis it has been done a fine tunning of a semiclassical method of dynamics called SHARC Surface Hopping including Arbitrary Couplings. This method allows to study the dynamics of middle size molecules in their excited states including simultaneously all the couplings that intrinsecally exist between them beyond the Born-Oppenheimer aproximation (spin-orbit, non adiabatic) and external couplings created by laser pulses. Therefore it is a new tool of great usefulness for theoretical studies in any photochemical process involving radiative transitions, intersystem crossings and internal conversion in conical intersections. Semiclassical methods are based on a mixed quantum and classical treatment. Electronic states and their couplings are calculated in a quantum way, and using them the probabilities that determine the states where the dynamics evolves are calculated, and also forces that they apply over nuclei, whose dynamics is followed integrating Newton’s equations of motion. Both, potentials (forces) and hopping probabilities depend in an essential way on the representation in which the electronic wavefunction is described. In the thesis we apply SHARC method in two critical situations, that involve a challenge for other non-quantum method. In first place, dynamics under very strong laser fields is followed. Comparing SHARC results with quantum methods for several different models, we show that SHARC allows us to simulate in an efficient way and with quantitative results a selective population transfer process between two electronic states by two photon absortion in Na2. The process, called APLIP (Adiabatic Passage by Light-Induced Potentials) implies the conservation of the vibrational number. Despite being a semiclassical method, SHARC performs in a correct way not only the electronic population, but also qualitatively, the vibrational energy in the final state, what contrasts with results obtained using other semiclassical methods. Secondly, we aplied SHARC to the ICH3 photodissociation study, it is a molecule that has a strong spin-orbit coupling, in addition to a conical intersection along its reaction coordinate, which implies the presence of strong non-adiabatic couplings during the dynamics. Results was obtained with SHARC both, using potential models (including or not the effect of lase absortion) and through a direct dynamics (“on the fly”), showing good agreement with bibliographic results obtained both experimentally and with other methodologies.
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Tesis de la Universidad Complutense de Madrid, Facultad de Ciencias Químicas, Departamento de Química-Física I, leída el 22/09/2017