Within the scope of this thesis, the production of ultracold molecules at a temperature of a few µK with various isotopes of rubidium (Rb) and ytterbium (Yb) was examined by means of photoassociation spectroscopy and magnetic Feshbach resonances in combined conservative traps. The long-term goal of this experiment is the production of ultracold RbYb molecules in the rovibronic ground state.
It was possible to produce electronically excited ^{87}Rb ^{176}Yb molecules in a novel hybrid trap (HT) at a combined temperature of 1.7 µK by means of 1-photon photoassociation close to the Rb D1 line at 795 nm. This HT takes advantage of the different magnetic properties of Rb and Yb and allows for independent trapping and manipulation of the atomic species. It combines an Ioffe-Pritchard type magnetic trap for Rb and a near-resonant optical dipole trap for Yb. The excited molecular ^2 Pi_1/2 state could be characterized further extending previous works in a combined MOT and vibrational levels reaching binding energies up to E_b=-h x 2.2 THz could be assigned by trap-loss spectroscopy.
Almost every detected vibrational state consists of two resonances that could be assigned to the molecular analogue of the hyperfine structure of ^{87}Rb. An important experimental observation is a decrease in hyperfine splitting with increasing binding energy of a vibrational level. For the deepest found vibrational state the hyperfine splitting amounts only 70 % of the atomic value (817 MHz) which emphasizes a gradual passage from weakly to tightly bound molecules.
Furthermore, detailed attempts were undertaken to induce magnetic Feshbach resonances in ^{85}Rb and different Yb isotopes, especially ^{171}Yb in a crossed optical dipole trap at 1064 nm at temperatures of 10 µK. For this purpose, a homogeneous magnetic field was applied and scanned in small steps over the range of 495 G... 640 G. Unfortunately, our efforts were without success. Additionally, well known as well as not yet reported Rb_2 Feshbach resonances in ^{85}Rb and ^{87}Rb were detected. These resonances allowed for a detailed analysis of the typical behaviour of Feshbach resonances.
The first experimental realization of photoassociation spectroscopy in a conservative is also a major step towards the ultimate project goal of producing ultracold RbYb molecules in the electronic ground state. A spatial confinement, which is in principle possible in the HT, is a prerequisite for transferring the molecules to the rovibronic ground state, e.g. with the help of a STIRAP process.
We report on the creation of ultracold heteronuclear and electronically excited Rb*Yb molecules in a hybrid conservative trap by photoassociation of ultracold
87Rb and
176Yb. The molecules are formed below the
Rb5p1(2P1/2)+Yb6s2(1S0) dissociation limit and the resonances are detected using trap-loss spectroscopy. By addressing vibrational levels with binding energies down to ${E}_{B}=\ensuremath{-}h\ifmmode\times\else\texttimes\fi{}2.2\phantom{\rule{0.16em}{0ex}}\mathrm{THz}$, we study the change in hyperfine coupling of the diatomic molecule as a function of internuclear separation. We observe a decreasing hyperfine splitting for more tightly bound excited molecular states where the hyperfine splitting is reduced by more than
30 compared to the atomic value for
87Rb.
This thesis describes the formation of electronically excited but translationally cold molecules formed from rubidium atoms and two isotopes of ytterbium (176Yb and 174Yb) by means of photoassociation. The experiments were performed in a combined MOT with 1e9 rubidium atoms and 2e6 ytterbium atoms at temperatures of less than 1mK.
Photoassociation lines were found by trap loss spectroscopy throughout a wavelength range of 2nm near the 795nm D1 transition in rubidium. The majority of lines belong to two vibrational series in the excited YbRb* molecule, converging on a system of a ground state ytterbium atom and an excited rubidium atom. The strong variation of line strength between different vibrational lines is explained through the Franck-Condon principle. An improved version of the Leroy-Bernstein equation was used to extract the leading dispersion coeffcient of the potential from the vibrational progression.
Most of the observed lines show a resolved rotational structure as expected from a basic quantum mechanical model. The series
terminates with the third or forth rotational component due to the ground state centrifugal barrier.The measured rotational constants agree very well with calculations based on the C6 coefficient.
The discovery of a splitting of the rotational components into subcomponents indicates an uncommon angular momentum coupling described by Hund's case (e). Variations in the depth of the subcomponents indicates a similar splitting in the ground state, with the energies of the substates based on the alignment of the rubidium atom's magnetic dipole moment relative to the angular momentum carried by an approaching ytterbium
atom. This creates an additional ground state barrier, partially suppressing some of the subcomponents.
Using a rate equation model developed for this purpose, a maximum formation rate of 2.5e6 molecules per second was calculated over the volume of the entire trap.
The work presented here is an important step on the way to the formation of molecules that are not only translationally ultracold, but also in the electronic and rovibrational ground state.
We report the production of ultracold heteronuclear
Cs∗Yb molecules through one-photon photoassociation applied to an ultracold atomic mixture of Cs and Yb confined in an optical dipole trap. We use trap-loss spectroscopy to detect molecular states below the
Cs(2P1/2)+Yb(1S0) asymptote. For
133Cs174Yb, we observe 13 rovibrational states with binding energies up to $\ensuremath{\sim}500\phantom{\rule{0.28em}{0ex}}\mathrm{GHz}$. For each rovibrational state we observe two resonances associated with the Cs hyperfine structure and show that the hyperfine splitting in the diatomic molecule decreases for more deeply bound states. In addition, we produce ultracold fermionic
133Cs173Yb and bosonic
133Cs172Yb and
133Cs170Yb molecules. From mass scaling, we determine the number of vibrational levels supported by the 2(1/2) excited-state potential to be 154 or 155.
This thesis reports the investigation of the scattering properties of a mixture of Rb and Cs and the formation of ultracold Feshbach molecules. The production of Feshbach molecules is a crucial step towards the production of ultracold polar molecules, which is of significant interest for a wide range of potential applications. We have investigated the scattering properties of a mixture of
85Rb and
133Cs in their lowest spin channel in a magnetic field range from 0 to 700\,G. Furthermore, we explored the Feshbach spectrum of
85Rb alone in both, the
(f=2,mf=−2) and
(2,+2) states up to a magnetic field of 1000\,G. Additionally a Feshbach resonance in a
(2,+2)+(3,+3) spin mixture was experimentally confirmed. We associated Cs
2 Feshbach molecules using a Feshbach resonance at 19.9\,G. \mbox{
2.1(1)×104} molecules with a temperature of
∼60\,nK were produced from a sample of
3.28(2)×105 Cs atoms with a PSD of 0.20(1). Due to a magnetic field gradient, the molecules `bounce' at an avoided crossing between two states at 13.5\,G. This gradient field was also used to produce multiple molecular clouds from one atomic sample. A combination of both techniques led to a `collision' between two Cs
2 clouds. Furthermore, we associated up to
∼5000 heteronuclear
87RbCs Feshbach molecules using an interspecies resonance at 197.1\,G. Confined in the dipole trap the molecules have a lifetime of 0.21(1)\,s. We have measured the magnetic moment of the molecules in different internal states in a magnetic field range from 181 to 185\,G. Molecular loss spectroscopy on electronically excited states was performed to identify candidates for the intermediate state of a STIRAP transfer of the molecules into their rovibrational ground state. Subsequently, the binding energy of the rovibrational ground state was measured to be 3811.574(1)\,cm
−1, using two-photon spectroscopy.
This thesis presents the development of an experimental apparatus designed to investigate the ultracold collisional properties for mixtures of Cs and Yb, with a long-term view to the creation of ultracold CsYb molecules via indirect cooling methods. The unpaired electron spin that is inherent to molecules of this form gives rise to a magnetic dipole moment in addition to a ground state electric dipole moment. This enables extra control over molecular interactions and should enable the experimental simulation of spin lattice models. We focus on the implementation of a system designed to controllably laser cool and optically trap Yb. The first step in this system is the production of a magneto-optical trap (MOT) on the triplet 1S0 to 3P1 transition of Yb. With careful control over the cooling beam detunings and power, gravitational-assisted Doppler cooling allows samples of Yb to be prepared at 22 uK. This regime of enhanced Doppler cooling is investigated and proves to be a crucial step to ensuring good transfer of cold Yb to optical traps. The construction and characterisation of single and crossed beam optical dipole traps for Yb are discussed. The single beam optical trap has been used to verify a model for the optical trapping of Yb in its ground state. This trap has also been utilised as a tool for the measurement of the light shift on the 1S0 to 3P1 transition at a wavelength of 1070~nm. In the main experimental sequence, Yb atoms are loaded from the magneto-optical trap into the crossed optical dipole trap, allowing evaporative cooling ramps to quantum degeneracy to be performed. This highly-reproducible system typically forms Bose-Einstein condensates with 2 x 10^5 174Yb atoms. This thesis additionally reports on the progress made towards measurements of the interspecies scattering length for 133Cs and Yb isotopes. We present two approaches that are being developed in tandem: rethermalisation in a conservative trap, and two-photon photoassociation. Progress towards rethermalisation measurements has focussed on developing systems for the efficient transfer of Cs to an optical trap. For photoassociative measurements, a laser system has been developed and tested by producing one-photon photoassociation spectra of Cs2.
We investigate quantum-mechanical interactions between ultracold rubidium and cesium in an optical trap at temperatures of a few microkelvin. Our results provide, on the one hand, an experimental key to understand the collisional properties and, on the other hand, a tool to control the interspecies interactions. By performing loss measurements we locate several Feshbach resonances, which provide insight into the energy structure of weakly bound RbCs molecules near the dissociation threshold and allow for the production of such heteronuclear Feshbach molecules. In the future we will transfer these loosely-bound molecules into the absolute internal ground state. The availability of ultracold heteronuclear ground state molecules will open the door to investigate phenomena associated with ultracold polar quantum gases. In our new experimental set-up we are able to trap and cool rubidium and cesium atoms in their lowest internal states. First we load both species into a two-color magneto-optical trap, having full control over the single-species atom number. We extend the technique of degenerate Raman-sideband cooling to a two-color version, which is able to simultaneously cool and polarize both rubidium and cesium. Thereafter we load the atoms into a levitated crossed optical dipole trap. Because of the presence of the gradient magnetic field the trap is highly state selective and consequently provides perfect spin-polarization of the sample. Furthermore, a coincidence of the magnetic-moment-to-mass ratios of the two species allows for simultaneous levitation of both, which assures an almost perfect spatial overlap between the species. We perform Feshbach spectroscopy in two different spin channels of the mixture within a magnetic field ranging from 20 to 300G. In the lowest spin combination of the species we locate 23 interspecies Feshbach resonances, while in a higher spin mixture we find 2 resonances. The high number of resonances found within this range of magnetic field is unusual for alkali mixtures. The presence of many resonances points to scattering properties of the mixture, which include higher-order coupling mechanisms. The obtained data on the Feshbach spectroscopy provide, on one hand, fundamental experimental input to characterize the Rb-Cs scattering properties and, on the other hand, identification of possible starting points for the association of ultracold heteronuclear RbCs molecules. In addition we show preliminary results on spectroscopy of the binding energy of RbCs dimers, based on a modulation of the magnetic field. The recently obtained Bose-Einstein condensate of Cs atoms, which represents a benchmark for the performance of the present apparatus, will be discussed as well as the potential pathways towards a double-degenerate mixture. We follow two main goals, first the production of a double Bose-Einstein condensate and, second, the transfer of shallow bound Feshbach molecules to the absolute internal ground state. The production of heteronuclear ground state molecules allows to enter the world of ultracold polar quantum systems.