This dissertation describes two-photon photoassociation to the least-bound vibrational level of the X
1Σg+ electronic ground state of the
86Sr
2 dimer, which represents the first observation of this naturally occurring halo molecule. We measure the binding energy of this state to be
Eb=83.00(7)(20)\,kHz. Using the precise determination of the halo state binding energy and the universal theory for a very weakly-bound state on a potential that asymptotes to a van der Waals form, we determine
s-wave scattering lengths for all strontium isotopes. Our results are consistent with, but substantially more accurate than the previously determined values found from spectroscopic determination of long-range coefficients using other strontium isotopes. With a radial expectation value of
⟨r⟩≈21\,nm the halo state extends well into the classically forbidden region, which results in a large sensitivity of the dimer binding energy to light near-resonant with a bound-bound transition on the metastable
1S0−3P1 potential. This suggests that
86Sr may be a promising candidate for manipulating atomic interactions via optical coupling of the molecular bound states and for probing naturally occurring Efimov states. Furthermore, we observe novel multi-photon spectral loss features due to strong coupling and small detuning of the photoassociation lasers. Numerical simulations of a simple three-level model undergoing a simultaneous two-frequency drive, shows that solutions of the time-dependent Schr\"{o}dinger equation accurately reproduce these additional loss features. A Floquet analysis of this model, yields analytic formulas for predicting the AC Stark shift of the halo molecule. Additionally, we report on the current state and recent modifications of our ultracold neutral strontium apparatus with a particular emphasis on the installation of a
532\,nm optical lattice. Characterization of this laser system and applications are discussed for further investigation of the strontium-
86 halo molecule and additional photoassociation experiments.
We report on the creation of
86Sr
2 molecules in the halo state and neighboring weakly bound states. Efficient molecule production via one-photon photoassociation relies on sufficient wavefunction overlap between the target vibrational states in the electronic excited- and ground-state potentials. Using Autler-Townes spectroscopy, transition strengths are measured to identify optimal pathways for production of weakly bound molecules. Vibrational splittings for the three least-bound vibrational states are measured, and dominant systematic uncertainties are evaluated with uncertainties below 100 Hz. From these splittings, absolute binding energies for these weakly bound vibrational states are determined. The results pave the way to a molecular isotope shift measurement with Sr
2.
We investigate two-frequency photoassociation of a weakly bound molecular state, focusing on a regime where the AC Stark shift is comparable to the halo-state energy. In this ``high-intensity'' regime, we observe features absent in low-intensity two-frequency photoassociation. We experimentally measure the spectra of
86Sr atoms coupled to the least bound state of the
86Sr2 ground electronic channel through an intermediate electronically excited molecular state. We compare the spectra to a simple three-level model that includes a two-frequency drive on each leg of the transition. With numerical solution of the time-dependent Schr\"odinger equation, we show that this model accurately captures (1) the existence of experimentally observed satellite peaks that arise from nonlinear processes, (2) the locations of the two-photon peak in the spectrum, including AC Stark shifts, and (3) in some cases, spectral line shapes. To better understand these numerical results, we develop an approximate treatment of this model, based on Floquet and perturbation theory, that gives simple formulas that accurately capture the halo-state energies. We expect these expressions to be valuable tools to analyze and guide future two-frequency photoassociation experiments.
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.