Theoretical Investigations of H2 Formation on Interstellar Silicates Surfaces Javier Navarro Ruiz Tesi Doctoral Estudi de Doctorat en Química Directors: Mariona Sodupe Roure Albert Rimola Gibert Departament de Química Facultat de Ciències 2015 Universitat Autònoma de Barcelona Departament de Química Unitat de Química Física Memòria presentada per aspirar al Grau de Doctor per Javier Navarro Ruiz, Javier Navarro Ruiz Vist i plau, Mariona Sodupe Roure Albert Rimola Gibert Bellaterra, 29 d’octubre de 2015 Als meus pares i germà Preface Among the molecules in space, H2 is one of the most relevant of the universe. It is the most abundant one in the interstellar medium and is a key intermediate for the formation of bigger molecules. Its formation is complex, but due to its inherent relevance understanding its interaction and its formation can be considered as a paradigm of the astrophysical process. The present thesis is structured in four chapters. Chapter 1 introduces the astrochemical framework in which the thesis is located, pointing out the presence of interstellar hydrogen in the interstellar medium and where it takes place, in the interstellar dust grains around. After presenting the goals this thesis aims, Chapter 2 overviews the general theoretical aspects behind it, such as electronic structure, density functional methods, solids modelling and tunnelling effects, providing finally the computational details entailed. Chapter 3 corresponds to results and discussion and is divided into different sections. Section 3.1 presents some physicochemical properties of the crystalline bulk structure and the corresponding surfaces of Mg2SiO4 forsterite and of the Fe-containing Mg1.5Fe0.5SiO4 olivine systems. Section 3.2 reports the adsorption of H atoms and their recombination to form a H2 molecule on the crystalline Mg2SiO4 forsterite (010) surface and Section 3.3 analyses the relevance of surface morphology by considering the H2 formation on the crystalline Mg2SiO4 forsterite (001) and (110) surfaces. Finally, Section 3.4 investigates the influence of Fe2+ atoms by modelling the physisorption/chemisorption of H atom on the Fe-containing (010) surface, subsequently taking place the formation of H2. Chapter 4 addresses the general conclusions of the present thesis and possible future perspectives, Chapter 5 includes the references cited and Appendix A and B supports the information given in Chapter 3. Additionally, at the beginning of this work, a list of abbreviations of some terms used throughout the text is appended. Acknowledgements Primer de tot m'agradaria donar les gràcies als meus directors, sense ells aquesta tesi doctoral no hauria estat possible. He tingut el privilegi d'haver estat tutelat per persones tan extraordinàries com la Prof. Mariona Sodupe i el Dr. Albert Rimola, sota els quals he après profundament i perfeccionat moltes de les tècniques computacionals que ara conec i que s'analitzen detalladament en aquesta tesis. Expressar el meu agraïment a l’Agència de Gestió d’Ajuts Universitaris i de Recerca (AGAUR) de la Generalitat de Catalunya i a la Universitat Autònoma de Barcelona (UAB) pel suport financer concedit gràcies al Programa d’Ajuts per a la Contractació de Personal Investigador Novell (FI-DGR, NÚM. EXPEDIENT: 2015FI_B 00953 i 2015FI_B2 00083). Gràcies també al meu grup de recerca “Computational Studies of (Bio)inorganic Systems” (GETAB Group) i a les subvencions que ha rebut a través dels projectes del Ministeri de Ciència i Innovació (CTQ2011-24847/BQU, CTQ2013-40347-ERC, CTQ2014-59544-P i CTQ2014-60119-P) i de la Generalitat de Catalunya (2009SGR-638 i 2014SGR-482), molts dels congressos als quals he pogut assistir com a doctorand van ser possibles. També estic agraït al Consorci de Serveis Universitaris de Catalunya (CSUC) pels recursos computacionals proporcionats per l'assoliment d'aquesta tesi. Agraeixo especialment a Angelos Michaelides el haver-me donat la oportunitat de pertànyer al seu grup de recerca “Interfaces: Catalytic and Environmental” (ICE Group) i el seu suport durant la meva estada predoctoral al London Centre for Nanotechnology de la University College London (UCL) i al Thomas Young Centre a Londres, Regne Unit. També vull agrair al nostre estimat col·laborador des de fa temps i coautor de diversos treballs plegats, el Prof. Piero Ugliengo de la Università degli Studi di Torino, les seves valuoses contribucions als diversos treballs presentats en aquesta tesi. Estic en deute amb els excel·lents doctorands i professors amb els què he compartit moltes estones entranyables i he treballat en els darrers anys, per les nombroses aportacions que aquí es presenten i per tot el què m’han ensenyat i, en general, a tot el personal de la Unitat de Química Física. Finalment, però no per això menys important, paraules de gratitud vers la meva família i amics, un pilar fonamental durant tots aquests anys, el seu recolzament ha estat molt gratificant. Javier Navarro Ruiz Bellaterra, Octubre del 2015 List of Abbreviations Abbreviation Meaning AGB Asymptotic Giant Branch AGN Active Galactic Nuclei AIMD Ab Initio Molecular Dynamics BSSE Basis Set Superposition Error CC Coupled Cluster method CI Configuration Interaction method CO Crystalline Orbital cr Cosmic Ray DFT Density Functional Theory DRC Distinguished Reaction Coordinate ECP Effective Core Potential EPM Electrostatic Potential Map ER Eley–Rideal FUSE Far Ultraviolet Spectroscopic Explorer GGA Generalized Gradient Approximation GPW Gaussian and Plane Waves method GTBF Gaussian-type Bloch Functions GTF Gaussian-type Functions HA Hot Atom HF Hartree–Fock HK Hohenberg–Kohn IR Infrared light ISM Interstellar Medium KS Kohn–Sham LCAO Linear Combination of Atomic Orbitals LDA Local Density Approximation LSDA Local Spin Density Approximation LH Langmuir–Hinshelwood MBPT Many Body Perturbation Theory PAH Polycyclic Aromatic Hydrocarbon List of Abbreviations PAW Plane Augmented Wave PDR Photon–Dominated Region PP Pseudopotential PW Plane Wave SCF Self–Consistent Field TPD Temperature–Programmed Desorption TST Transition State Theory UV Ultraviolet light VIS Visible light YSO Young Stellar Object ZPE Zero-Point Energy xii Contents 1. INTRODUCTION ........................................................................................................ 1 1.1. INTERSTELLAR MEDIUM ........................................................................................... 1 1.2. INTERSTELLAR HYDROGEN .................................................................................... 10 1.2.1. Relevance ....................................................................................................... 10 1.2.2. Spectroscopic Detection ................................................................................. 11 1.2.3. Formation ...................................................................................................... 14 1.2.4. Previous Astrochemical Modelling and Experimental Studies ...................... 17 1.2.5. Chemical Role of H2 ....................................................................................... 18 1.3. INTERSTELLAR DUST GRAINS ................................................................................ 20 1.3.1. Composition ................................................................................................... 21 1.3.2. Formation ...................................................................................................... 22 1.3.3. Olivines .......................................................................................................... 24 1.4. PREVIOUS THEORETICAL WORKS ........................................................................... 26 1.4.1. Formation, Structure and Modelling of Olivines ........................................... 26 1.4.2. Surface Molecular Hydrogen Formation ....................................................... 28 1.5. OBJECTIVES ............................................................................................................ 29 2. METHODOLOGY ..................................................................................................... 31 2.1. GENERAL CONCEPTS ON ELECTRONIC STRUCTURE CALCULATION ........................ 31 2.2. WAVE FUNCTION-BASED METHODS ...................................................................... 33 2.2.1. Hartree-Fock Approximation ......................................................................... 33 2.2.2. Post Hartree-Fock Methods ........................................................................... 33 2.3. DENSITY FUNCTIONAL METHODS .......................................................................... 36 2.3.1. The Foundations of DFT ................................................................................ 36 2.3.2. The Kohn–Sham Method ................................................................................ 37 2.3.3. Approximations to the Exchange–Correlation Potential ............................... 39 2.3.4. Some Difficult Cases for DFT ........................................................................ 42 2.4. MODELLING SOLIDS AND SURFACES ...................................................................... 43 2.4.1. Cluster Approach ........................................................................................... 43 2.4.2. Periodic Approach ......................................................................................... 44 2.4.2.1. The Bloch Theorem ...................................................................................