This is the manuscript of my "Habilitation à diriger des recherches", where I present the research work that I have done after my PhD, defended in 2009. The manuscript is divided in two parts. The first one is dedicated to atomic-structure calculations with neutral and trivalent lanthanides, in the contexts of ultracold gases and rare-earth doped solids. The second part deals with long-range interactions in ultracold gases of alkali-metal atoms and diatomic molecules, as well as lanthanide atoms. The detailed description of long-range interactions serves to characterize ultralow-temperature phenomena, like photoassociation and collisional shielding.
This is the manuscript of my "Habilitation à diriger des recherches", where I present the research work that I have done after my PhD, defended in 2009. The manuscript is divided in two parts. The first one is dedicated to atomic-structure calculations with neutral and trivalent lanthanides, in the contexts of ultracold gases and rare-earth doped solids. The second part deals with long-range interactions in ultracold gases of alkali-metal atoms and diatomic molecules, as well as lanthanide atoms. The detailed description of long-range interactions serves to characterize ultralow-temperature phenomena, like photoassociation and collisional shielding.
The term "long-range interactions" refers to electrostatic and magnetostatic potential energies between atoms and molecules with mutual distances ranging from a few tens to a few hundreds Bohr radii. The involved energies are much smaller than the usual chemical bond energies. However, they are comparable with the typical kinetic energies of particles in an ultracold gas (T≪1K), so that the long-range interactions play a central role in its dynamics. The progress of research devoted to ultracold gases shed a new light on the well-established topic of long-range interactions, because: (i) the interacting atoms and molecules can be prepared in a well-defined quantum (electronic, vibrational, rotational, fine or hyper-fine), ground or excited level; and (ii) long-range interactions can be tailored at will using external electromagnetic fields. In this chapter, we present the essential concepts and mathematical relations to calculate long-range potential energies. We start with deriving the multipolar expansion of the electrostatic interaction energy between classical charge distributions, both in Cartesian coordinates for pedagogical purpose, and in spherical coordinates for practical use. Then we combine multipolar expansion and quantum perturbation theory, to obtain the general first- and second-order energy corrections, including the well-known van der Waals energy. We consider two central examples in the current context of ultracold gases: (i) a pair of alkali-metal atoms and (ii) a pair of alkali-metal heteronuclear diatomic molecules submitted to an electric field. We highlight the key role of the total angular momenta of the interacting particles and of the complex, irrespective of their electronic or nuclear, orbital or spin nature.
The term long-range interactions refers to electrostatic and magnetostatic potential energies between atoms and molecules with mutual distances ranging from a few tens to a few hundreds of Bohr radii. The energies involved are much smaller than the usual chemical bond energies. However, they are comparable with the typical kinetic energies of particles in an ultracold gas (T ≪ 1 K), so that the long-range interactions play a central role in its dynamics. The progress of research devoted to ultracold gases sheds a new light on the well-established topic of long-range interactions, because (i) the interacting atoms and molecules can be prepared in a well-defined quantum (electronic, vibrational, rotational, fine or hyperfine), ground or excited level; and (ii) long-range interactions can be tailored at will using external electromagnetic fields. In this chapter, we present the essential concepts and mathematical relations to calculate long-range potential energies. We start by deriving the multipolar expansion of the electrostatic interaction energy between classical charge distributions, both in Cartesian coordinates for pedagogical purpose, and in spherical coordinates for practical use. Then we combine multipolar expansion and quantum perturbation theory, to obtain the general first- and second-order energy corrections, including the well-known van der Waals energy. We consider two central examples in the current context of ultracold gases: (i) a pair of alkali metal atoms and (ii) a pair of alkali metal heteronuclear diatomic molecules in the presence of an electric field. We highlight the key role of the total angular momenta of the interacting particles and of the complex, irrespective of their electronic or nuclear, orbital or spin nature.
The dynamics of ultracold quantum gases composed of atoms or molecules with extremely low translational energy E t /k B << 1 millikelvin is dominated by the long-range mutual interactions between particles. Such studies require a detailed modelling of the atom-molecule or molecule-molecule long-range interactions inside the quantum gas with or without the presence of external fields. This paper focuses on two particular aspects relevant to ultracold chemistry: (i) The calculation of rates for photoassociation of a cold atom and a cold molecule to create ultracold excited trimers, based on the knowledge of the long-range interaction between the two species, showing that such rates are encouraging under the density and temperature conditions of a nearly degenerate quantum gas; (ii) The determination of the long-range interactions between two identical polar bialkali molecules in their rovibronic ground level in the presence of an electric field, showing the possibility for the formation of ultracold polar tetramers by stimulated emission.
Étude théorique des spectres atomiques de lanthanides neutres et trivalents : Application aux gaz ultra-froids et aux solides dopés Le refroidissement laser et le piégeage des lanthanides ont ouvert la possibilité de réaliser de nouvelles expériences avec des gaz dipolaires ultra-froids, qui peuvent être utilisés, par exemple, pour la simulation quantique de la physique du solide. Pour identifier de nouveaux candidats au refroidissement laser, il est crucial de disposer d’une connaissance spectroscopique précise de l’atome considéré. La première grande orientation de ma thèse est la modélisation du spectre des lanthanides neutres, d'une part de l'erbium (Er), un élément de la partie droite de la rangée des lanthanides, particulièrement attractif en raison d'une transition très fine, reliant l'état fondamental et un état excité métastable, et d'autre part du néodyme (Nd), un élément de la partie gauche de la rangée, qui n'a pas encore été explorée pour le refroidissement laser. En utilisant la méthode semi-empirique implémentée dans les codes de Cowan, nous avons amélioré la description des niveaux de configurations impaires de Er.Nous avons aussi calculé des quantités telles que le facteur de Landé, la durée de vie et la polarisabilité de l'état excité cité plus haut, pour les comparer aux résultats expérimentaux obtenus par un groupe d'Innsbruck en Autriche.Ensuite, pour Nd, nous avons pu interpréter plus de 200 niveaux expérimentaux de la base de données du NIST appartenant aux deux parités, puis nous avons proposé de nouvelles transitions de refroidissement laser. La suite logique de ces travaux est le calcul des coefficients d'Einstein, nécessaires pour caractériser l'efficacité du refroidissement laser et du piégeage des atomes.La plupart des lanthanides existent généralement sous forme de cations trivalents. En raison de la présence d’électrons non appariés, les lanthanides présentent d'intéressantes propriétés électromagnétiques et lumineuses. Sous leur forme trivalente, ils sont largement utilisés à des fins industrielles, dans les lasers et comme (co-)dopants dans les amplificateurs optiques à fibres, par exemple dopée à l'erbium.La deuxième grande orientation de ma thèse est consacrée aux lanthanides trivalents, utilisés comme dopants dans un matériau hôte. Toutes les transitions intéressantes pour ces applications sont induites par le champ cristallin. La théorie de Judd-Ofelt est utilisée depuis des décennies pour décrire ces transitions, mais il existe certaines restrictions liées à la manière dont elle est formulée, en particulier pour l'europium (Eu3+). Dans cette thèse, nous avons effectué des calculs spectroscopiques, en utilisant la même méthodologie que pour les lanthanides neutres, pour trois éléments : Eu3+, Nd3+ et Er3+.Nous avons proposé une extension de la théorie de Judd-Ofelt dans laquelle nous incluons une description précise du spectre de l'ion dopant, et nous tenons compte de la variation de l'indice de réfraction du matériau hôte avec la longueur d'onde. Les résultats de notre modèle sont satisfaisants, car il nous donne un aperçu physique de toutes les transitions observées, y compris celles qui sont interdites par la version standard de la théorie. Par ailleurs, notre modèle reproduit quantitativement les forces d'oscillateurs d'absorption expérimentales. Le code utilisé pour effectuer les calculs est disponible sur GitLab.Comme perspective, nous prévoyons de traiter les transitions en lumière polarisée ou entre des sous-niveaux individuels des ions. Cela sera possible dans notre modèle, car les seuls paramètres ajustés sont ceux du champ cristallin. Cela ouvre la possibilité de modéliser les propriétés spectroscopiques de matériaux hôtes à l'échelle nanométrique.
Due to their low temperature of less than a milliKelvin and their long-range anisotropic interactions, ultracold polar molecules offer promising applications for quantum computing, the measurement of fundamental constants and ultracold chemistry and may allow the realization of new forms of quantum matter. The diatomic molecule 6Li40K is an excellent candidate for their implementation due to its large electric dipole moment. In this book, the author presents the first steps towards the creation of ultracold polar 6Li40K molecules. He gives a detailed description of the construction of an experimental apparatus which prepares, at ultracold temperatures, a gas mixture of the species 6Li and 40K. He demonstrates the association of these fermionic atoms to ultracold bosonic excited 6Li40K* molecules by photoassociation. The molecules will have to be transferred to their rovibrational and electronic ground state in order to become stable and polar. Photoassociation spectroscopy has been performed to gain information about convenient transfer schemes. Finally the author presents a novel method for the manipulation of ultracold particles, which can be applied to polar 6Li40K molecules.