[ca] Les nanogotes d'heli superfluid (HeNDs) són matrius inerts i nanoreactors ideals a baixa temperatura (0.37 K). Això ha atret l'atenció de químics doncs permeten realitzar espectroscopia d'altra resolució, estudiar la reactivitat i sintetitzar en condicions especials. L'estudi teòric de la dinàmica de processos en HeND ha estat possible tan sols fa cinc anys i, tot i el seu interès, n'hi ha molt pocs estudis. L'objectiu d'aquesta tesi és contribuir a la recerca en aquesta àrea. El Capítol 1 descriu les propietats de l'heli, la història de la superfluïdesa i les propietats i aplicacions de les HeNDs. La teoria del funcional de la densitat (DFT) i l'extensió de la mateixa depenent del temps (TDDFT), els principals funcionals per HeNDs i els mètodes numèrics es presenten al Capítol 2. Els següents capítols contenen els estudis originals d'aquesta tesi. En el Capítol 3 s'investiga la captura de Ne en una HeND on l'àtom es tracta clàssicament. El mecanisme, els intercanvis d'energia i moment angular i la formació de vòrtexs s'han analitzat àmpliament. Aquest és el primer anàlisi rigorós de la influència del moment angular en la captura i formació de vòrtexs. El Capítol 4 descriu la formació de Ne2/Ne-Ne en HeND tractant ambdós àtoms clàssicament. El mecanisme, bescanvis d'energia i moment angular i formació de vòrtexs també s'han estudiat. És el segon estudi sobre reaccions bimoleculars en HeNDs. Els Capítols 3 i 4 complementen i amplien dues investigacions del nostre grup on els àtoms es van tractar quànticament amb moment angular zero. En els propers dos capítols es consideren les relaxacions rotacional i vibracional utilitzant enfocs quàntics híbrids. El Capítol 5 correspon al primer estudi teòric de la relaxació rotacional de molècules en HeNDs, i s'han considerat varis isòtops de H2 i excitacions inicials i mides de nanogota. El Capítol 6 detalla la influència de la separació energètica vibracional, interacció molècula-heli i mida de nanogota en la relaxació vibracional en HeNDs, agafant com a referència el sistema I2@(4He)100. És el primer cop que s'examina l'efecte d'aquestes propietats clau. Els Capítols 7 i 8 presenten les principals conclusions i un resum en català, respectivament.

[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.