This thesis was established in the field of ultracold atoms where the interaction of highly excited D-state electrons with rubidium atoms was examined. This work is divided into two main parts: In the first part we study D-state Rydberg molecules resulting from the binding of a D-state Rydberg electron to a ground state rubidium atom. We show that we can address specific rovibrational molecular states by changing our laser detuning and thus create perfectly aligned axial or antialigned toroidal molecules, in good agreement with our theoretical calculations. Furthermore the influence of the electric field on the Rydberg molecules was investigated, creating novel states which show a different angular dependence and alignment. In the second part of this thesis we excite single D-state Rydberg electrons in a Bose-Einstein condensate. We study the lifetime of these Rydberg electrons, the change of the shape of our condensate and the atom losses in the condensate due to this process. Moreover, we observe quadrupolar shape oscillations of the whole condensate created by the consecutive excitation of Rydberg atoms and compare all results to previous S-state measurements. In the outlook we propose a wide range of further experiments including the proposal of imaging a single electron wavefunction by the imprint of its orbit into the Bose-Einstein condensate.
This thesis was established in the field of ultracold atoms where the interaction of highly excited D-state electrons with rubidium atoms was examined. This work is divided into two main parts: In the first part we study D-state Rydberg molecules resulting from the binding of a D-state Rydberg electron to a ground state rubidium atom. We show that we can address specific rovibrational molecular states by changing our laser detuning and thus create perfectly aligned axial or antialigned toroidal molecules, in good agreement with our theoretical calculations. Furthermore the influence of the electric field on the Rydberg molecules was investigated, creating novel states which show a different angular dependence and alignment. In the second part of this thesis we excite single D-state Rydberg electrons in a Bose-Einstein condensate. We study the lifetime of these Rydberg electrons, the change of the shape of our condensate and the atom losses in the condensate due to this process. Moreover, we observe quadrupolar shape oscillations of the whole condensate created by the consecutive excitation of Rydberg atoms and compare all results to previous S-state measurements. In the outlook we propose a wide range of further experiments including the proposal of imaging a single electron wavefunction by the imprint of its orbit into the Bose-Einstein condensate.
Combining ultracold atomic gases with the peculiar properties of Rydberg excited atoms gained a lot of theoretical and experimental attention in recent years. Embedded in the ultracold gas, an interaction between the Rydberg atom and the surrounding ground state atoms arises through the scattering of the Rydberg electron from an intruding perturber atom. This peculiar interaction gives rise to a plenitude of previously unobserved effects. Within the framework of the present thesis, this interaction is studied in detail for Rydberg P-states in rubidium. Due to their long lifetime, atoms in Rydberg states are subject to scattering with the surrounding ground state atoms in the ultracold cloud. By measuring their lifetime as a function of the ground state atom flux, we are able to obtain the total inelastic scattering cross section as well as the partial cross section for associative ionisation. The fact that the latter is three orders of magnitude larger than the size of the formed molecular ion indicates the presence of an efficient mass transport mechanism that is mediated by the Rydberg–ground state interaction. The immense acceleration of the collisional process shows a close analogy to a catalytic process. The increase of the scattering cross section renders associative ionisation an important process that has to be considered for experiments in dense ultracold systems. The interaction of the Rydberg atom with a ground state perturber gives rise to a highly oscillatory potential that supports molecular bound states. These so-called ultralong-range Rydberg molecules are studied with high resolution time-of-flight spectroscopy, where we are able to determine the binding energies and lifetimes of the molecular states between the two fine structure split 25P-states. Inside an electric field, we observe a broadening of the molecular lines that indicates the presence of a permanent electric dipole moment, induced by the mixing with high angular momentum states. Due to the mixing of the ground state atom’s hyperfine states by the molecular interaction, we are able to observe a spin-flip of the perturber upon creation of a Rydberg molecule. Furthermore, an incidental near-degeneracy in the underlying level scheme of the 25P-state gives rise to highly entangled states between the Rydberg fine structure state and the perturber’s hyperfine structure. These mechanisms can be used to manipulate the quantum state of a remote particle over distances that exceed by far the typical contact interaction range. Apart from the ultralong-range Rydberg molecules that predominantly consist of only one low angular momentum state, a class of Rydberg molecules is predicted to exist that strongly mixes the high angular momentum states of the degenerate hydrogenic manifolds. These states, the so-called trilobite- and butterfly Rydberg molecules, show very peculiar properties that cannot be observed for conventional molecules. Here we present the first experimental observation of butterfly Rydberg molecules. In addition to an extensive spectroscopy that reveals the binding energy, we are also able to observe the rotational structure of these exotic molecules. The arising pendular states inside an electric field allow us, in comparison to the model of a dipolar rotor, to extract the precise bond length and dipole moment of the molecule. With the information obtained in the present study, it is possible to photoassociate butterfly molecules with a selectable bond length, vibrational state, rotational state, and orientation inside an electric field. By shedding light on various previously unrevealed aspects, the experiments presented in this thesis significantly deepen our knowledge on the Rydberg–ground state interaction and the peculiar effects arising from it. The obtained spectroscopic information on Rydberg molecules and the changed reaction dynamics for molecular ion creation will surely provide valuable data for quantum chemical simulations and provide necessary data to plan future experiments. Beyond that, our study reveals that the hyperfine interaction in Rydberg molecules and the peculiar properties of butterfly states provide very promising new ways to alter the short- and long-range interactions in ultracold many-body systems. In this sense the investigated Rydberg–ground state interaction not only lies right at the interface between quantum chemistry, quantum many-body systems, and Rydberg physics, but also creates many new and fascinating possibilities by combining these fields.
In this dissertation I investigate cold Rydberg atoms and molecules in which the angular-momentum character of the quantum states involved strongly influences their properties and dynamics. In the first part I focus on long-range diatomic Rydberg molecules formed by a rubidium D-state Rydberg atom and a second rubidium atom in its ground state. Spectroscopic measurements of molecular binding energies are presented showing the effect of the Rydberg atom size and fine-structure coupling on the molecular potentials. A theoretical model is introduced that takes into account all relevant angular-momentum couplings between the molecular constituents, successfully reproducing experimental observations. Calculations of adiabatic potentials and binding energies, molecular-state lifetimes, electric and magnetic dipole moments are also presented. In the second part, I describe the production and magnetic trapping of cold circular Rydberg atoms. The circular Rydberg atoms are generated out of a cold gas of rubidium using the crossed-fields method and magnetically trapped. The trapping force is employed to induce center-of-mass oscillations of the trapped atom sample. Trap parameters and observed oscillation frequencies are used to measure the magnetic moments of the circular Rydberg atoms. Trap losses and the atomic internal-state evolution in the 300 Kelvin thermal background are also investigated.
We report on the formation of ultralong-range Rydberg D-state molecules via photoassociation in an ultracold cloud of rubidium atoms. By applying a magnetic offset field on the order of 10 G and high resolution spectroscopy, we are able to resolve individual rovibrational molecular states. A full theory, using a Fermi pseudopotential approach including s- and p-wave scattering terms, reproduces the measured binding energies. The calculated molecular wave functions show that in the experiment we can selectively excite stationary molecular states with an extraordinary degree of alignment or antialignment with respect to the magnetic field axis.
In this dissertation, I explore several aspects of Rydberg physics with laser spectroscopy. The experiments center around the use of a very deep, cavity-generated, one-dimensional optical lattice in a cold atom sample. The optical lattice produces elongated clouds of dense, cold atoms via dipole trapping and is able to generate extreme light shifts in the atoms due to its high intensity. I describe experiments which may be divided into two main classes. In the first, I use the dipole trap to prepare a dense sample of atoms for photoassociation of Rb atom pairs into diatomic Rydberg molecules. The Rydberg molecules are formed by low-energy scattering of the Rydberg electron of one atom from the other atom, which is a ground-state atom. This novel binding mechanism also has relevance for chemical physics. I present binding energy measurements for eight different cases of Rydberg molecules. The binding energies reveal information about electron-atom collisions, such as their scattering lengths, and about the spin couplings present within the molecular system. In the second class of experiments, I introduce two proposed measurements in which the optical lattice is ramped to very high intensity during the laser excitation, creating AC Stark shifts in the transition frequencies. In one measurement, low-lying atomic states are probed. Measurements of the light shifts are expected to yield data on the dynamic polarizability and the photoionization cross section of the 5D3/2-state in rubidium in a 1064-nm light field. I show corresponding preliminary experimental data. The polarizability and photo-ionization cross-section measurements will provide tests of atomic structure theory, which is important both at a fundamental level and for applications such as atomic clocks. In another proposed measurement, Rydberg atoms in the lattice experience ponderomotive shifts and strong mixing of their angular-momentum states, allowing direct optical excitation of the lattice-mixed high-angular-momentum states. I describe the theoretical basis and experimental preparation for spectroscopically characterizing the lattice-mixed states. The ability to optically excite selected high-angular-momentum states without the use of static or RF fields may be useful for quantum information processing and simulation.
This thesis is a theoretical study of the behavior of electrons in atoms and molecules subjected to external electromagnetic fields. Three electronic systems have been central in this work, the atomic and quasimolecular Rydberg states and the carbon allotrope graphene. The exaggerated properties of the highly excited Rydberg atoms, such as long lifetime, large spatial extension and low binding energy make them excellent candidates for both theoretical and experimental considerations. The analytical solutions of the hydrogen atom in quantum mechanics provide a good description for most Rydberg atoms. This is exploited in the work presented in this thesis and the accompanying papers, where we show results of numerical computations on interacting Rydberg atoms, with and without the presence of radiating fields, as well as dynamic Rydberg wave packets influenced by a train of femtosecond laser pulses. In the final part of the thesis the optical process of high-order harmonic generation in an extended molecular structure, represented by graphene, is considered. Rydberg atoms have a large dipole moment due to the large average electronic radius in the atom, which leads to a very strong dipole-dipole coupling between interacting particles in a gas of cold Rydberg atoms. This interaction, which is dependent on the internuclear separation between the atoms, induces a shift in the energy levels, which again prohibits the excitation of more than one atom in the gas, an effect commonly known as the Rydberg blockade. A correlated behavior is observed for the intrashell transitions in mutually excited Rydberg atoms. The microwave field driven dynamics is significantly supressed for internuclear distances below a given conditional radius which scales linearly with the principal quantum number n. Such an entangled electronic behavior may open for the realization of quantum gates applied in quantum informatics. The long-range interaction of Rydberg atoms is dominated by the dipole-dipole terms. Nevertheless, the investigation of two interacting Rydberg atoms, without the influence of electromagnetic radiation, has shown that the lower order multipolar terms play a vital role in the energy shifts as well as for the electronic probability distributions. We found that the energy curves, when including all multipole orders, correspond to both repulsive and attractive states with a stronger repulsion than attraction. We also propose a laser assisted approach for stabilization of Rydberg gases by controlled transitions between such attractive and repulsive energy states. Single-electron ionization of a dynamic Rydberg wave packet confined within a single energy level has been investigated. The ionization probability of an intrashell Rydberg wave packet depends strongly on the degree of polarization. For a wave packet driven repeatedly between the circular (least polarized) state and the linear (most polarized) state by a rotating microwave field and subjected to a train of femtosecond laser pulses, the ionization probability will depend crucially on the laser repetition rate and number of pulses. The angular-resolved ionization probability and energy distribution in the continuum are also influenced by these parameters. The last part of the thesis is devoted to the study of high-order harmonic generation (HHG) in graphene. In the simple man’s model HHG can be described by an electron that is ionized and driven in an external field, after which it returns to the parent atom and recombines, emitting a high-energy photon with frequency that is a multiple of the incident laser frequency. We show that the harmonic spectra obtained from the interaction with linearly as well as circularly polarized femtosecond lasers yield harmonics up the classical limit for extended molecules. In contrast to diatomic molecules, the stable grid structure of graphene ensures that the harmonic generation cutoff remains constant with increasing signal power as the graphene diameter extends beyond the classical limit. Additionally, for circularly polarized lasers harmonics exceeding the classical predictions have been observed.
In this thesis, I report on experiments on the detection and the production of molecules in an ultracold atomic gas. These experiments can be divided into two main parts. The first part is about state-to-state experiments where chemical reaction products are detected in a quantum state resolved way. The second part is about Rydberg molecules. In the first part, I investigate three-body recombination of neutral Rb, which is a chemical reaction leading to the formation of Rb2 dimers. The quantum states of the three-body recombination product molecules are investigated for different initial atomic samples. This includes different atomic hyperfine states and different atomic magnetic sub states that are prepared. The molecular reaction product is state-selectively ionized and detected afterwards. The detection scheme resolves total electron spin, the vibrational, the rotational, the hyperfine and even the magnetic sub state of the product molecules. We determine the population distribution of the product molecules covering the majority of all produced molecules. The insights gained enable us to formulate various propensity rules for the three-body recombination of 87Rb. The second part is furthermore divided into two parts: the investigations of charged and uncharged Rydberg molecules. The investigations of charged Rydberg molecules are the first step towards one of our goals, the photoassociation of BaRb+ or Rb2+ Rydberg molecules. For this work, we have developed a novel method to measure Stark spectra within an ion trap. In experiments with neutral Rb2 Rydberg molecules we observe a complex spin structure of the molecular states. The substructure emerges partially due to a novel, recently predicted spin-orbit interaction of the Rydberg electron and the perturber atom, which we prove for the first time.