137 research outputs found

    Ultrafast processes in N2 photoionization: implementacion of the XCHEM code

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    Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Química. Fecha de lectura: 27-09-2017Esta tesis tiene embargado el acceso al texto completo hasta el 27-03-2019This project studies the ultrafast photoionization (dissociative and non-dissociative) of molecular Nitrogen from a theoretical and computational point of view. Speci cally, we present the implementation of the XCHEM approach; a set of tools which allows the study, including electronic correlation, of photoionization going beyond simple benchmark systems. In particular we focussed on the successful description of autoionizing states; quasi-boundstates (coupled to continuum states) with a nite life time after which the system ionizes. Molecular Hydrogen was studied as a simple test case, allowing for comparison with results from well established methods. Subsequently the photoionization of molecular Nitrogen was studied in between the rst and third ionization threshold, i.e. where dissociation does not play a role. This is an important step establishing the XCHEM method's capability to handle complex multielectronic molecular systems. From a mathematical point of view the XCHEM method relies on a close coupling expansion of the electronic wavefunction, which is tted to the asymptotic behaviour seen in a Coulomb potential. The (ionic) scattering states are expanded in a novel hybrid Gaussian/B-Spline (GABS) basis set, whereas the (neutral) bound states are calculated using modi ed Quantum Chemistry Packages (QCP). As part of the development of this approach, this work focussed on the creation of original programs to be used in combination with modi ed and redesigned QCPs, allowing them to interface with the part of the XCHEM code dealing with scattering theory and involving GABS basis functions. The dissociatiove photoionization of molecular Nitrogen was also studied in collaboration with experimental e orts. In this experiment, Nitrogen was ionized using a single attosecond XUV pump pules and the subsequent dissociation dynamics was probed by a femtosecond IR pules (XUV pump/IR probe). The dependence of the kinetic energy of the fragments was recorded as a function of the delay between the pulses, giving rise to a clearly visible interference pattern. A theoretical description of this experiment, including large numbers of potential energy surfaces and taking into account non-adiabatic couplings, is proposed in this thesis and used to reproduce these results. From this model an interpretation of the observed experimental features is extracted.Este proyecto investiga desde punto de vista te orico y computacional la foto ionizaci on (disociativa y no disociativa) ultra r apida de la mol ecula de Nitr ogeno. Para simularla, presentamos la implementaci on del m etodo XCHEM : un conjunto de herramientas que permite estudiar la foto ionizaci on de sistemas complejos teniendo en cuenta correlaci on electr onica. En concreto, nos centramos en la descripci on de estados que experimentan autoionizaci on: estados casi ligados acoplados con estados del continuo. Dado que estos estados est an inmersos en el continuo, acaban decayendo a estados del cati on. Como primer sistema, hemos estudiado la mol ecula de Hidr ogeno lo que nos ha permitido comparar con resultados obtenidos con otros m etodos. Posteriormente hemos estudiado la foto ionizaci on de la mol ecula de Nitr ogeno entre el primer y el tercer umbral de ionizaci on. Eso constituye un paso importante, ya que establece la capacidad del m etodo XCHEM para describir sistemas complejos en mol eculas polielectr onicas. Desde el punto de vista matem atico el m etodo XCHEM esta basado en una expansi on close coupling, de la funci on de onda electr onica, que esta ajustada para describir el comportamiento asint otico observado en un potencial culombiano. Los estados del continuo (i onicos) se expanden en un nuevo conjunto de funciones de base que incluye una mezcla de funciones gausianas y B-Splines (GABS). Por otra parte, los estados ligados (neutros) se calculan con programas modi cadas de estructura electr onica (QCP). Como parte del desarrollo de este m etodo, en este trabajo se han programado una serie de programas originales que, en combinaci on con QCPs modi cados y redise~nados al efecto, permiten construir un interfaz entre estados ligados y estados del continuo. La foto ionizaci on disociativa de la mol ecula de Nitr ogeno se ha estudiado en colaboraci on con un grupo experimental. En el experimento, se emple o un pulso XUV de attosegundo para ionizar la mol ecula de Nitr ogeno y la din amica del proceso de disociaci on, provocada por este pulso, se sigui o con un pulso IR de femtosegundos a trav es de un esquema bombeo-sonda. Se midi o la dependencia de la energ a cin etica de los fragmentos molecular con el retraso entro los dos pulsos, dando lugar a un patr on de interferencia. En esta tesis, describimos este experimento usando un modelo te orico (que incluye cientos de super cies de energ a potencial con sus correspondientes acoplamientos no adiab aticos) que es capaz de reproducir los resultados experimentales. Adem as, profundizando en el modelo hemos sido capaces de entender la naturaleza del patr on de interferencia

    Photoionization of strongly correlated many-electron atoms and molecules: a hybrid-basis close-coupling interface to quantum chemistry packages

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    Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Química. Fecha de lectura: 21-09-2017Esta tesis tiene embargado el acceso al texto completo hasta el 21-03-201

    Studies of molecular photoionization of simple systems by advanced photon sources

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    This doctorate thesis reports on a variety of experimental investigations aiming to advance the understanding of fundamental processes in molecules and clusters by exploiting the properties of Synchrotron and FEL radiation: photoionization dynamics, double ionization, dissociation and molecular recognition were subject of investigation. The emphasis of the thesis lies on the application of advanced light sources in the study of photoionization processes in simple gas-phase molecules, with particular attention on chiro-optical properties of chiral systems. The valence photoionization dynamics of a chiral molecule, namely the epichlorohydrin molecule, was studied for the first time and a peculiar electron correlation effect was observed. The experimental data were supported by state-of-the-art theoretical calculations. VUV direct double ionization was studied for the methyl oxirane chiral molecule by the use of Photoelectron-Photoion-Photoion Coincidence spectroscopy using synchrotron radiation. The chiral recognition mechanism of 1-methoxy-2-propanol oligomers was studied by FEL based IRMPD-VUV vibrational spectroscopy, a technique that exploits the nature of the photoionization process in order to apply the IRMPD spectroscopy to systems of arbitrary structure. The collaboration between the Sapienza University of Rome, the CNRIOM institute, and the Elettra Instrumentation and Detector Laboratory, has resulted in the development of a position sensitive cross delay line electron detector integrated in an experimental apparatus with the flexibility to perform synchrotron radiation (SR) photoemission experiments on gasphase systems. The improvement of the apparatus detection system has stimulated the collaboration with the Theoretical Chemistry group of the University of Trieste, in a joint experimental and theoretical long-term research activity, whose first part was the study of the photoionization dynamics of the Osmium tetroxide molecule, a highly reactive tetraoxo complex

    Multiconfiguration methods for the numerical simulation of photoionization processes of many-electron atoms

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    Numerical simulations present an indispensable way to the understanding of physical processes. In quantum mechanics, where the theoretical description is given in terms of the time-dependent Schrödinger equation (TDSE), the road is, however, difficult for any but the simplest systems. This is particularly true if one considers photoionization processes of atoms and mole\-cules, which at the same time require an accurate description of bound and continuum states, and therefore an extensive region of space to be sampled during the calculation. As a consequence, direct simulations of photoionization processes are currently only feasible for systems composed of up to three particles. Despite this fundamental restriction, many physical effects can be essentially described by single- and two-electron models, among them high-order harmonic generation and non-sequential double-ionization of atoms and mole\-cules. A plethora of numerical investigations have been performed on atomic and molecular hydrogen and helium in the last two decades, and these have had a strong impact on the current understanding of photoionization. On the other hand, there are processes which are characterized by the interplay of a larger number of electrons, such as tunnel ionization, the Auger effect, and, to give a more recent example, the temporal delay between the photo-emission of electrons from different shells of neon and krypton. The many-electron character of these effects complicates the accurate, time-resolved simulation, and hence, no universally applicable method exists. The present work develops two theoretical methods which are promising candidates for closing this gap, the multiconfigurational time-dependent Hartree-Fock (MCTDHF) method and the time-dependent restricted active space configuration interaction (TD-RASCI) method. Both represent the wavefunction in a linear subspace of the many-body Hilbert space and follow particular strategies to avoid the exponential problem. This makes it possible to treat a much larger number of electrons than with the direct techniques mentioned previously. The MCTDHF method is already well established in the scientific community, but has been applied only rarely to photoionization processes so far. On the other hand, the TD-RASCI method is an original contribution, and is applied for the first time to solutions of the time-dependent Schrödinger equation. Further, through the invention of appropriate, grid-like single-particle basis sets, we adjust these general approaches to efficiently treat photoionization processes in many-electron atoms and molecules. After their thorough introduction, the MCTDHF and the TD-RASCI method are applied to several topics of photoionization physics. Among them is, first, the problem of calculating cross sections of atoms, for which we particularly consider helium, beryllium and neon. In most parts, this is accomplished for the first time in the framework of the developed methods. Next, we consider the two-photon double-ionization of helium, which has attracted considerable interest in recent years, and perform simulations with the MCTDHF method. We further apply the TD-RASCI method to study two-color pump-probe process in beryllium, the simulation of which requires an explicitly time-dependent treatment. We find that both methods are highly appropriate for accurately describing correlated single-ionization processes. Moreover, the TD-RASCI method is able to model relevant doubly-excited states, which are of central importance for a variety of physical processes.Trotz dieser fundamentalen Einschränkung lassen sich viele physikalische Effekte bereits durch Ein- und Zweiteilchenmodelle beschreiben, darunter zum Beispiel die Erzeugung höherer Harmonischer oder die nicht-sequentielle Doppelionisation. In diesem Sinne wurde in den letzten zwei Jahrzehnten eine Vielzahl numerischer Untersuchungen an atomarem und molekularem Wasserstoff sowie Helium unternommen, welche einen starken Einfluss auf das momentane Verständnis von Photoionisations-Prozessen nahmen. Andererseits gibt es jedoch physikalische Effekte, die durch das Zusammenwirken einer größeren Anzahl von Elektronen gekennzeichnet sind, etwa die Tunnel-Ionisation, der Auger-Effekt oder die kürzlich entdeckte zeitliche Verzögerung in der Emission von Elektronen aus verschiedenen atomaren Schalen von Neon und Krypton. Der immanente Vielteilchencharakter macht die zeitaufgelöste Simulation dieser Prozesse zu einer schwierigen Aufgabe, für die es bisher keine universell einsetzbare und gleichzeitig akkurate Methode gibt. In dieser Arbeit werden zwei theoretische Methoden zur Simulation von Photoionisations-Prozessen von Vielteilchenatomen und -molekülen vor\-gestellt, die vielversprechende Kandidaten zur Schließung dieser vorhandenen Lücke darstellen, nämlich die zeitabhängige Multikonfigurations-Hartree-Fock (MCTDHF) Methode sowie die zeitabhängige restricted-active-space Konfigurations-Wechselwirkungsmethode (TD-RASCI). Beide stellen die quantenmechanische Wellenfunktion in einem linearen Unterraum des Vielteilchen-Hilbertraumes dar und folgen dabei speziellen Ansätzen um das Problem des exponentiellen Wachstums zu vermeiden. Dadurch kann eine weitaus größere Teilchenzahl als mit der zuvor erwähnten direkten Technik simuliert werden. Weiterhin werden diese zunächst sehr allgemeinen Methoden durch den Gebrauch geeigneter Basissätze auf die effiziente Beschreibung von Photoionisations-Prozessen optimiert. Nach ihrer Einführung werden die MCTDHF und TD-RASCI Methode auf aktuelle Themen der Photoionisations-Physik angewandt. Zunächst wenden wir uns der Berechnung von Ionisations-Streuquerschnitten der Atome Helium, Beryllium und Neon zu, welche weitgehend zum ersten Male mithilfe der eingeführten Methoden untersucht wird. Des Weiteren studieren wir die Zwei-Photonen-Ionisation von Helium, der in jüngerer Zeit großes theoretisches Interesse zukam, mithilfe von Simulationen mit der MCTDHF Methode. Als grundlegendes Beispiel eines explizit zeitabhängigen Prozesses wird darüberhinaus die Pump-Probe Ionisation von Beryllium betrachtet. Unsere Untersuchungen zeigen, dass sowohl die MCTDHF Methode als auch die TD-RASCI Methode die Einelektronen-Photoionisation akkurat zu beschreiben vermag. Mithilfe der TD-RASCI Methode ist es zudem möglich, selektierte doppelt-angeregte Zustände in die Rechnung zu integrieren, welche eine zentrale Rolle bei einer Vielzahl physikalischer Prozesse spielen

    APPLICATION AND DEVELOPMENT OF THE LINEAR COMBINATION OF ATOMIC ORBITALS B-SPLINE DENSITY FUNCTIONAL THEORY METHOD FOR THE MOLECULAR ELECTRONIC CONTINUUM

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    2004/2005Lo scopo della presente tesi è l'applicazione e lo sviluppo del metodo LCAO B-spline DFT, basato sulla Combinazione Lineare degli Orbitali Atomici (LCAO), sulla Teoria del Funzionale Densità (DFT) e sull'impiego delle funzioni di base Bspline, per lo studio della fotoionizzazione molecolare. Nel Capitolo 1 viene considerata una breve introduzione al processo di fotoionizzazione e alla teoria della fotoionizzazione molecolare. Nel Capitolo 2 vengono presentati i metodi computazionali utilizzati durante il presente lavoro di tesi. I risultati ottenuti sono presentati nei successivi Capitoli della tesi. Il lavoro è suddiviso in tre parti. Nella prima parte (Capitoli 3-6) il metodo LCAO B-spline DFT è applicato allo studio del Dicroismo Circolare nella Distribuzione Angolare (CDAD) dei fotoelettroni emessi da molecole chirali, ionizzate da luce circolarmente polarizzata di definita elicità. Un primo studio computazionale sull'effetto CDAD è focalizzato su una serie di derivati chirali dell'ossirano, con lo scopo di identificare tendenze lungo la serie molecolare. I risultati evidenziano una inaspettata sensibilità da parte del dicroismo circolare verso cambiamenti nella struttura elettronica molecolare. Inoltre i valori calcolati sottolineano come lintensità dell'effetto CD AD è da attribuire non tanto alla chiralità dello stato iniziale, ma bensì alla capacità della funzione d'onda del fotoelettrone (completamente delocalizzata su tutta la molecola) di sondare l'asimmetria nel potenziale molecolare effettivo. Il metodo LCAO B-spline DFT è quindi applicato allo studio del dicroismo circolare nella distribuzione angolare dei fotoelettroni emessi dai livelli di core e di valenza degli enantiomeri della canfora. I risultati sono confrontati con i valori calcolati attraverso l'approccio Continuum Multiple Scattering: i due metodi teorici mostrano un sostanziale accordo nei risultati. Inoltre, un confronto dei risultati calcolati con i dati sperimentali disponibili mostra un accordo sostanziale se non addirittura quantitativo. E' inoltre presentato uno studio teorico e sperimentale sul dicroismo circolare nella distribuzione angolare dei fotoelettroni emessi dai livelli di valenza del 3- idrossitetraidrofurano, il quale è una molecola relativamente flessibile. Questo lavoro mette luce su nuove caratteristiche del dicroismo in fotoemissione, il quale appare molto sensibile a fattori di tipo conformazionale. L'influenza degli effetti conformazionali sui parametri dinamici di fotoionizzazione, con particolare attenzione all'effetto CDAD, è quindi investigata in dettaglio. Il metodo LCAO E-spline DFf è applicato ai conformeri della molecola (1R,2R)-1,2-dibromo-1,2-dicloro-1,2-difluoroetano. Il parametro dicroico mostra una significativa sensibilità verso il conformero della molecola. Ciò suggerisce che il profilo energetico del parametro dicroico potrebbe essere utilizzato come un fingerprint dei conformeri di una molecola chirale. Il metodo computazionale è anche applicato alla rotazione del metile nella molecola (S)-ossirano, in modo da verificare l'assunzione che la rotazione del metile non influisca in maniera significativa sui valori calcolati dell'effetto CDAD. Si verifica invece come la rotazione del gruppo metile provochi variazioni inaspettate e drammatiche sul profilo del parametro dicroico. La seconda parte del lavoro (Capitolo 7) riguarda l'investigazione teorica sulla correttezza dell'assunzione che il Eranching Ratio nella ionizzazione dei livelli di core di siti atomici chimicamente diversi dello stesso elemento, segua il rapporto statistico. Il metodo LCAO E-spline DFT è stato impiegato per calcolare in maniera accurata le sezioni d'urto associate alla ionizzazione degli orbitali ls dei carboni per un set di molecole organiche scelte. I risultati mostrano che deviazioni nella sezione d'urto inducono forti andamenti non statistici da parte dei Eranching Ratios relativi alla ionizzazione dei livelli core, fino a diversi e V sopra la soglia di ionizzazione. La parte finale della tesi (Capitolo 8) riguarda il problema delle shape resonances. La razionalizzazione delle strutture risonanti, spesso presenti nello spettro molecolare del continuo, è un problema ampiamente discusso. Nel presente lavoro una nuova metodologia per localizzare e caratterizzare in termini molecolari le shape resonances viene proposta. Partendo da un sistema modello, è stato sviluppato un metodo per caratterizzare le risonanze in termini di contributo da parte degli orbitali virtuali di valenza. Il metodo sviluppato è quindi esteso al caso molecolare ed applicato alla localizzazione e caratterizzazione delle shape resonances che appaiono nella fotoionizzazione dei livelli crg di core e di valenza di N1.XVIII Ciclo1977Versione digitalizzata della tesi di dottorato cartacea

    Investigating the Dynamics and Fragmentation of Nitroaromatic Radical Cations Through Femtosecond Time-Resolved Mass Spectrometry and Computational Chemistry

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    Chemists have sought to control molecular dissociation with lasers for decades. Effective control of unimolecular dissociation was only achieved with the development of high-intensity ultrashort pulsed lasers and coherent control techniques that operate on timescales faster than vibrational energy redistribution. In view of this, our lab has specialized in the study of polyatomic radical cations using femtosecond time-resolved mass spectrometry (FTRMS). The interest in radical cations stems from the fact that they are highly reactive species that contribute to many physical, chemical, and biological processes. For instance, radical cations participate in shock initiation of detonated energetic materials used as explosives and propellants. In this regard, we have studied some nitroaromatic radical cations commonly used as models for energetic materials. We discuss some results involving the dynamics of vibrational wave packets and rearrangement/fragmentation pathways. Concerning vibrational wave packet dynamics, we employed computational chemistry to predict the most efficient probe wavelength for our experimental measurements on nitrobenzene cation demonstrating the feasibility and convenience of this approach. We also investigated pump-probe control schemes to manipulate fragmentation product yields in p-nitrotoluene (PNT) cation. Finally, we investigated the dissociation dynamics and fragmentation pathways of o-nitroaniline, a model compound for the military explosive 2,4,6-triamino-1,3,5-trinitrobenzene (TATB). This model seems to capture the hydrogen bonding features that lead to energetically unfavorable rearrangement/fragmentation pathways in TATB. We expect that our experimental and computational results provide insights into the inherent stability of this molecule that explains the low sensitivity (and therefore relatively high safety) of TATB as an explosive

    Electron, Photon, and Positron Scattering Dynamics of Complex Molecular Targets

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    Electron scattering cross sections have been computed for pyridine and pyrimidine using the static-exchange approximation with model potential to account for dynamic electron correlation. To obtain well-converged orbitals, we have expanded all partial waves to a maximum angular momentum of l = 60 for both targets. We have obtained total cross sections for electron scattering energies to 20 eV. Both targets display similar features, namely a dipole-induced increase in the integrated cross section at scattering energies below 5 eV, and peaks corresponding to resonances in b1, a2, and b1 symmetries. These resonances were investigated through a Siegert eigenstate analysis and Breit-Wigner fit of the SECP eigenphase sums. They were also compared to the virtual orbitals obtained from a minimum basis set Hartree-Fock calculation on both targets. We consider electron scattering resonances from cis-diamminedichloroplatinum, [Pt(NH3)2Cl2], the ligand molecular species Cl2 (1Sigma+g ), and the isolated transition metal center Pt in a nondegenerate atomic state (1S) at the SECP level of theory. As a rigorous comparison to the single-state, single-configuration SECP level results of these smaller, yet electron dense targets, we have also considered scattering from ground state Cl2 and Pt in the 1S and 3D states in the multichannel configuration-interaction (MCCI) approximation originally developed for photoionization for scattering up to 10 eV. Photoionization cross sections and angular distributions in the recoil frame (RFPAD) and molecular frame (MFPAD) have been computed for inner-shell C 1s and Cl 2p ionization from the chloroalkanes chloromethane and chloroethane, with ionization leading to a variety of ionic fragment states. We have also computed valence level ionization from the nitro molecule nitromethane CH3NO2 leading to the dissociation of the CN bond. All of these calculations were performed in the frozen-core Hartree-Fock approximation. Even at this level of theory, we obtain computed results that compare well to the photoelectronphotoion coincidence measurements. The fullerene C20 is the smallest fullerene predicted to exist, with most relevant structural calculations suggesting the reduction of the icosahedral symmetry into one in which the target species possesses at maximum only a dihedral axis. We have computed positron scattering cross sections for the molecule in two low-symmetry structural isomers Ci and C2, within the HF approximation. Density functional expressions were used to incorporate important positron-electron interactions within the calculation. We have found similar cross sections and resonance features for both isomers, including a positron scattering resonance whose density is found within the framework of the fullerene cluster

    Theoretical study of vibrationally resolved photoionization of diatomic and polyatomic molecules

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    Tesis doctoral inédita, leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Química. Fecha de lectura: 17-12-201

    Quenching of Molecular Photodissociation by Interatomic Coulombic Decay

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    The impact of Interatomic Coulombic Decay (ICD) on the photoinduced dynamics in the first four vibrational levels of the first Rydberg state of CO in CO-Mg clusters is investigated. A method for calculating the required ICD widths is introduced. The method is tested by calculations of ICD widths of outer-valence excited states in NeAr, HCN-Mg and inner-valence excited states in NeMg
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