Above-threshold ionization and photoelectron spectra in atomic systems driven by strong laser fields

Above-threshold ionization (ATI) results from strong field laser-matter interaction and it is one of the fundamental processes that may be used to extract electron structural and dynamical information about the atomic or molecular target. Moreover, it can also be used to characterize the laser field itself. Here, we develop an analytical description of ATI, which extends the theoretical Strong Field Approximation (SFA), for both the direct and re-scattering transition amplitudes in atoms. From a non-local, but separable potential, the bound-free dipole and the re-scattering transition matrix elements are analytically computed. In comparison with the standard approaches to the ATI process, our analytical derivation of the re-scattering matrix elements allows us to study directly how the re-scattering process depends on the atomic target and laser pulse features -we can turn on and off contributions having different physical origins or corresponding to different physical mechanisms. We compare SFA results with the full numerical solutions of the time-dependent Schroedinger equation (TDSE) within the few-cycle pulse regime. Good agreement between our SFA and TDSE model is found for the ATI spectrum. Our model captures also the strong dependence of the photoelectron spectra on the carrier envelope phase of the laser field.Comment: 29 pages, 6 figures, submitted to PR

Strong-field processes in atoms and polyatomic molecules

In this thesis, we develop a general theory to describe the dynamics of electrons that are ionized when an atom or molecule is exposed to a strong low frequency laser field. Our approach extends and improves the well-established theoretical strong-field approximation (SFA). Additionally, our modified strong field approximation (MSFA) can be extended in a natural way from atomic systems to a more complex molecules and multielectron systems. Our scheme involves two innovative aspects: (i) First, the bound-continuum and rescattering matrix elements can be analytically computed for both atomic and multicenter molecular systems, using a nonlocal short range (SR), but separable, potential. When compared with the standard models, these analytical derivations make possible to directly examine how the ATI and HHG spectra depend on the driven media and laser-pulse features. Furthermore, our model allows us to disentangle the different processes contributing to the total spectra, amongst other capabilities, and it allows us to adjust both the internuclear separation and atomic or molecular potential in a direct and simple way. Furthermore, we can turn on and off contributions having distinct physical origins or corresponding to different mechanisms that correspond to (1) direct tunneling ionization; (2) electron escattering/recombining on the center of origin; and, finally, (3) electron rescattering/recombining on a different center. (ii) Second, the multicenter matrix elements in our theory are free from nonphysical coordinate-systemdependent terms; this is accomplished by adapting the coordinate system to the center from which the corresponding time-dependent wave function originates. Having established the basic formalism, we then study the HHG and ATI processes for a variety of atomic and molecular systems. We compare the SFA results with the full numerical solutions of the timedependent Schrödinger equation (TDSE), when available, within the few-cycle pulse regime. We show how our MSFA can be used to look inside the underlying physics of those phenomena. With our tool it is possible to investigate the interference features, ubiquitously present in every strong-field phenomenon involving a multicenter target, or to describe laser-induced electron diffraction (LIED) measurements retrieving molecular structural information from the photoelectron spectra. Our approach paves the way to study the HHG and ATI processes in much more complex molecular targets. Additionally, it potentially can be extended to study these kind of recombination and rescattering scenarios in solid targets.En esta tesis, desarrollamos una teoría general para describir la dinámica de ionización de electrones cuando un átomo o molécula está expuesto a un campo externo fuerte y de longitud de onda larga. Nuestra teoría: la aproximación de campo fuerte modificada (MSFA), es capaz de describir la interacción de un pulso de luz, no sólo con átomos sino también con moléculas y sólidos. La MSFA está construida como una extensión natural y consecuente del modelo atómico, describiendo desde los sistemas más simples hasta las moléculas más complejas, incluyendo sistemas de muchos electrones. Nuestro enfoque abarca dos aspectos innovadores: (i) En primer lugar, los elementos de matriz que describen la dispersión e interacciones de electrones en el continuo se calculan analíticamente, tanto para sistemas atómicos como moleculares. Esto se logra utilizando un tipo de potencial de corto alcance (SR), no local y separable. En comparación con los modelos estándares, estas derivaciones analíticas permiten examinar directamente cómo los espectros ATI y HHG dependen de las características del pulso láser. Nuestra derivación analítica permite diferenciar los diferentes procesos que contribuyen al espectro total, además de que nos permite fijar la distancia internuclear y el potencial atómico o molecular de una manera directa y sencilla. También es posible activar y desactivar las contribuciones que tienen diferentes orígenes físicos o que corresponden a diferentes mecanismos como, (1) ionización directa por túnel; (2) dispersión/recombinación de electrones en el átomo de ionización; y, por último, (3) dispersión/recombinación de electrones en un átomo distinto al de ionización. (ii) En segundo lugar, en nuestra teoría los elementos matriciales de los sistemas multi-atómicos se encuentran libres de calibraciones no físicas y son independientes del sistema de coordenadas. Esto se consigue adaptando el sistema de coordenadas al átomo del que se origina la correspondiente función de onda dependiente del tiempo. Una vez establecido el formalismo básico del MSFA, estudiamos los procesos de HHG y ATI para una gran variedad de sistemas atómicos y moleculares. Comparamos los resultados del MSFA con las soluciones numéricas de la ecuación de Schrödinger dependiente del tiempo (TDSE), cuando sea posible. Demostramos que nuestro modelo de MSFA puede ser utilizado para estudiar la física de los procesos fundamentales que están detrás de HHG y ATI. Con esta herramienta es posible investigar los procesos de interferencia, inherentes a todos los fenómenos de campo fuerte, en sistemas multi-céntricos. También es posible describir mediciones experimentales de difracción de electrones inducida por láser (LIED), permitiendo recuperar información estructural mediante el análisis de los espectros de fotoelectrones. Nuestro modelo abre el camino para estudiar los procesos de HHG y ATI en sistemas de moléculas complejas. Además tiene la potencialidad de poder ser fácilmente extendido para estudiar procesos de recombinación y dispersión, no sólo en moléculas grandes, sino también en sólidos

Wannier-Bloch approach to localization in high harmonics generation in solids

Emission of high-order harmonics from solids provides a new avenue in attosecond science. On one hand, it allows to investigate fundamental processes of the non-linear response of electrons driven by a strong laser pulse in a periodic crystal lattice. On the other hand, it opens new paths toward efficient attosecond pulse generation, novel imaging of electronic wave functions, and enhancement of high-order harmonic generation (HHG) intensity. A key feature of HHG in a solid (as compared to the well-understood phenomena of HHG in an atomic gas) is the delocalization of the process, whereby an electron ionized from one site in the periodic lattice may recombine with any other. Here, we develop an analytic model, based on the localized Wannier wave functions in the valence band and delocalized Bloch functions in the conduction band. This Wannier-Bloch approach assesses the contributions of individual lattice sites to the HHG process, and hence addresses precisely the question of localization of harmonic emission in solids. We apply this model to investigate HHG in a ZnO crystal for two different orientations, corresponding to wider and narrower valence and conduction bands, respectively. Interestingly, for narrower bands, the HHG process shows significant localization, similar to harmonic generation in atoms. For all cases, the delocalized contributions to HHG emission are highest near the band-gap energy. Our results pave the way to controlling localized contributions to HHG in a solid crystal, with hard to overestimate implications for the emerging area of atto-nanoscience

Social determinants affecting under-five mortality

Introducción: los determinantes sociales de la salud experimentan significación e importancia en el último medio siglo en la medida en que la Organización Mundial de la Salud ha identificado que la salud está determinada por las condiciones en las que la gente nace, crece, vive y trabaja.Objetivo: caracterizar el estado de los determinantes sociales de mayor incidencia en la mortalidad del menor de cinco años en pacientes fallecidos en esta institución.Métodos: se realizó un estudio cuanti-cualitativo en fallecidos menores de cinco años en el Hospital “José Luis Miranda” en el período comprendido de enero de 2016 a diciembre de 2018. La variable de estudio se operacionalizó en indicadores estructurales (nivel socioeconómico, escolaridad y lugar de residencia) individuales (color de la piel) y del sistema de salud (accesibilidad, terapia oportuna y adecuada y causa de defunción). Por su interés fueron incluidos como descriptores poblacionales la edad y el sexo.Resultados: la mayor incidencia (41,5%) se encontró en el grupo etario entre uno y cuatro años, con un predominio de los varones (58,4%); un nivel socioeconómico medio (66,1%) y un origen rural (64,15%). La accesibilidad al sistema de salud que sobresale es la regular (52,8%) y la principal causa de defunción fueron las enfermedades infecciosas (30,1%).Conclusiones: existen importantes reservas para un mejoramiento de los indicadores de salud de la población infantil menor de cinco años en la Provincia de Villa Clara cuando en la gestión institucional se toma en cuenta el estado de los determinantes sociales de la salud.Introduction: the social determinants of health have gained significance and importance over the past half century as the World Health Organization has identified health as determined by the conditions in which people are born, grow up, live and work.Objective: to characterize the state of the social determinants that have the greatest impact on under-five mortality in patients who have died in this institution.Methods: a qualitative and quantitative study was carried out on the deaths of children under five years of age at the José Luis Miranda Hospital during the period January 2016 to December 2018. The study variable was operationalized in structural indicators (socioeconomic level, schooling and place of residence), individual (skin color) and health system (accessibility, timely and adequate therapy and cause of death). Age and sex were included as population descriptors due to their interest.Results: the highest incidence (41.5%) was found in the age group between one and four years old, with a predominance of males (58.4%); a medium socioeconomic level (66.1%) and a rural origin (64.15%). Accessibility to the health system was regular (52.8%) and the main cause of death was infectious diseases (30.1%).Conclusions: there are important reservations about improving health indicators for the under-five population in Villa Clara Province when institutional management takes into account the state of the social determinants of health

Symphony on strong field approximation

This paper has been prepared by the Symphony collaboration (University of Warsaw, Uniwersytet Jagielloński, DESY/CNR and ICFO) on the occasion of the 25th anniversary of the 'simple man's models' which underlie most of the phenomena that occur when intense ultrashort laser pulses interact with matter. The phenomena in question include high-harmonic generation (HHG), above-threshold ionization (ATI), and non-sequential multielectron ionization (NSMI). 'Simple man's models' provide both an intuitive basis for understanding the numerical solutions of the time-dependent Schrödinger equation and the motivation for the powerful analytic approximations generally known as the strong field approximation (SFA). In this paper we first review the SFA in the form developed by us in the last 25 years. In this approach the SFA is a method to solve the TDSE, in which the non-perturbative interactions are described by including continuum–continuum interactions in a systematic perturbation-like theory. In this review we focus on recent applications of the SFA to HHG, ATI and NSMI from multi-electron atoms and from multi-atom molecules. The main novel part of the presented theory concerns generalizations of the SFA to: (i) time-dependent treatment of two-electron atoms, allowing for studies of an interplay between electron impact ionization and resonant excitation with subsequent ionization; (ii) time-dependent treatment in the single active electron approximation of 'large' molecules and targets which are themselves undergoing dynamics during the HHG or ATI processes. In particular, we formulate the general expressions for the case of arbitrary molecules, combining input from quantum chemistry and quantum dynamics. We formulate also theory of time-dependent separable molecular potentials to model analytically the dynamics of realistic electronic wave packets for molecules in strong laser fields. We dedicate this work to the memory of Bertrand Carré, who passed away in March 2018 at the age of 60

Symphony on strong field approximation

This paper has been prepared by the Symphony collaboration (University of Warsaw, Uniwersytet Jagielloński, DESY/CNR and ICFO) on the occasion of the 25th anniversary of the ‘simple man’s models’ which underlie most of the phenomena that occur when intense ultrashort laser pulses interact with matter. The phenomena in question include high-harmonic generation (HHG), above-threshold ionization (ATI), and non-sequential multielectron ionization (NSMI). ‘Simple man’s models’ provide both an intuitive basis for understanding the numerical solutions of the time-dependent Schrödinger equation and the motivation for the powerful analytic approximations generally known as the strong field approximation (SFA). In this paper we first review the SFA in the form developed by us in the last 25 years. In this approach the SFA is a method to solve the TDSE, in which the non-perturbative interactions are described by including continuum–continuum interactions in a systematic perturbation-like theory. In this review we focus on recent applications of the SFA to HHG, ATI and NSMI from multi-electron atoms and from multi-atom molecules. The main novel part of the presented theory concerns generalizations of the SFA to: (i) time-dependent treatment of two-electron atoms, allowing for studies of an interplay between electron impact ionization and resonant excitation with subsequent ionization; (ii) time-dependent treatment in the single active electron approximation of ‘large’ molecules and targets which are themselves undergoing dynamics during the HHG or ATI processes. In particular, we formulate the general expressions for the case of arbitrary molecules, combining input from quantum chemistry and quantum dynamics. We formulate also theory of time-dependent separable molecular potentials to model analytically the dynamics of realistic electronic wave packets for molecules in strong laser fields.Peer Reviewe