[en] The study of superfluid helium has been carried out mainly by physicists. In recent years, taking advantage of the potentialities presented by superfluid helium nanodroplets (HeNDs) as inert matrices at very low temperatures (0.37 K), the chemical community became involved in its application to high-resolution spectroscopy. More recently (early 2000s), this community began to be involved in research using HeNDs to investigate chemical reactivity in this quantum solvent. As for the theoretical studies on the dynamics of physicochemical processes in HeNDs, they have been possible about five years ago and the number of theoretical dynamics studies, despite their interest, is very scarce. The main objective of this thesis is to contribute to the development of the research in this area. To introduce the reader into the topic, Chapter 1 is divided into four sections: the first one describes the properties of helium, the second one considers the history of the discovery and research carried out on the superfluidity phenomenon, the third one outlines the properties of superfluid helium nanodroplets, and the last one gives an overview of the applications and fields of study implying HeNDs. The theoretical and numerical methods used to describe superfluid liquid helium are detailed in Chapter 2. In the first section attention has been paid on the density functional theory (DFT) and its time dependent extension for real-time simulations (TDDFT). The second section describes the main density functionals used and the third section is aimed to present the numerical methods employed to perform the TDDFT calculations. The following four chapters contain the original studies carried out in this thesis. The investigation of the capture process of a Ne atom by a HeND can be found in Chapter 3. Here, the atom is treated using classical mechanics and the influence of energy and angular momentum is examined for a wide set of initial conditions. The microscopic mechanism, energy and angular momentum exchanges and vortex formation have been extensively analysed. The present contribution corresponds to the first systematic analysis of the influence of angular momentum in the capture process and vortex formation. Chapter 4 represents a natural evolution from Chapter 3 and describes the formation of a neon dimer or neon adduct inside a superfluid helium nanodroplet, treating both atoms classically. Analogously as in the previous chapter, angular momentum has also been taken into consideration and the mechanism, energy an angular momentum exchanges and vortex formation are analysed. These two chapters complement and extend two previous investigations of our group where the Ne atoms were treated using standard quantum mechanics at zero angular momentum. The contents of Chapter 4 correspond to the second theoretical investigation on bimolecular reaction dynamics in HeNDs. The following two chapters use a full quantum hybrid approach to explore rotational and vibrational energy relaxation dynamics. Chapter 5 corresponds to the first theoretical study reported so far on the rotational energy relaxation dynamics of molecules in HeNDs. This process has been studied using several isotopes of the H2 molecule (fast rotors) and considering a set of initial excitations and nanodroplet sizes. The last investigation (Chapter 6) is centred on the study of the vibrational energy relaxation in HeNDs. Thus, the influence of the energy gap between the vibrational levels, molecule-helium interaction energy and nanodroplet size on the vibrational relaxation dynamics has been analysed, taking as a reference the I2@(4He)100 doped nanodroplet which was recently studied in our group. To the best of our knowledge it is the first time that the influence of these key factors has been examined. Finally, in Chapters 7 and 8 the main conclusions and a summary in Catalan are presented.

[es] Las nanogotas de helio superfluido (HeNDs) son matrices inertes y nanoreactores ideales a baja temperatura (0.37 K). Esto ha atraído a los químicos pues posibilitan realizar espectroscopia de alta resolución, así como estudiar de la reactividad y síntesis en condiciones especiales. La dinámica teórica de procesos en HeND ha sido posible tan sólo hace cinco años y, a pesar de su interés, todavía hay muy pocos estudios. Esta tesis pretende contribuir a la investigación en esta área. El Capítulo 1 describe las propiedades del helio, la superfluidez y las propiedades y aplicaciones de las HeNDs. La teoría del funcional de la densidad (DFT) y su extensión dependiente del tiempo (TDDFT), los principales funcionales para HeNDs y los métodos numéricos se presentan en el Capítulo 2. Los siguientes capítulos contienen los estudios originales de esta tesis. En el Capítulo 3 se investiga la captura de Ne en una HeND donde el átomo se trata clásicamente. El mecanismo microscópico, intercambios de energía y momento angular y formación de vórtices se han analizado ampliamente. Este es el primer análisis detallado de la influencia del momento angular en la captura y la formación de vórtices. El Capítulo 4 describe la formación de Ne2/Ne-Ne en HeND tratando ambos átomos clásicamente. El mecanismo, intercambios de energía y momento angular y formación de vórtices también se han estudiado. Los Capítulos 3 y 4 complementan y amplían dos investigaciones de nuestro grupo donde los átomos se trataron cuánticamente con momento angular cero. En los dos capítulos siguientes se estudian las relajaciones rotacional y vibracional utilizando enfoques cuánticos híbridos. El Capítulo 5 corresponde al primer estudio teórico de la relajación rotacional de moléculas en HeNDs, y se han considerando varios isótopos de H2, excitaciones iniciales y tamaños de nanogota. El Capítulo 6 detalla la influencia de la separación energética, interacción molécula-helio y tamaño de nanogota en la relajación vibracional en HeND, habiéndose tomando como referencia el sistema I2@(4He)100. Es la primera vez que se examina el efecto de estas propiedades clave en la dinámica. Los Capítulos 7 y 8 presentan las principales conclusiones y un resumen en catalán, respectivamente.