In this thesis, we present the creation, via stimulated Raman adiabatic passage, and pre- liminary lifetime measurements of deeply bound triplet molecules of ⁶Li₂ in several rovi- brabrational levels of the a(³Σ⁺u ) molecular potential, including the lowest lying state in this potential. In addition to being the first experimental demonstration of the formation of these dimers, these results will serve as the basis for our ongoing efforts to reliably determine the rate constants responsible for the finite lifetimes of these ultracold molecules and may shed light on some of the mysteries surrounding the quantum state dependence of chemi- cal reactions in the ultracold regime. Moreover, all of the tools - including a robust laser cooling system for ⁶Li, an ultra-low phase noise Raman laser system and various software recipes for the automation of data collection and analysis - and experimental techniques developed in this study can also be used for the creation of heteronuclear LiRb molecules. Unlike our ⁶Li₂ homonuclear molecules which only have a magnetic dipole moment, the LiRb polar molecules are predicted to also have a large electric dipole in the lowest lying triplet state. These characteristics combined with a three dimensional optical lattice would give us precise control over several degrees of freedom and enable us to perform quantum simulations of exotic condensed matter systems. We also present the design and performance of a coherent source of Lyman-α radiation that was used by the ALPHA collaboration at CERN for laser cooling anti-hydrogen for the purpose of experimentally verifying the predictions of the standard model.
In this thesis, we present the creation, via stimulated Raman adiabatic passage, and pre- liminary lifetime measurements of deeply bound triplet molecules of ⁶Li₂ in several rovi- brabrational levels of the a(³Σ⁺u ) molecular potential, including the lowest lying state in this potential. In addition to being the first experimental demonstration of the formation of these dimers, these results will serve as the basis for our ongoing efforts to reliably determine the rate constants responsible for the finite lifetimes of these ultracold molecules and may shed light on some of the mysteries surrounding the quantum state dependence of chemi- cal reactions in the ultracold regime. Moreover, all of the tools - including a robust laser cooling system for ⁶Li, an ultra-low phase noise Raman laser system and various software recipes for the automation of data collection and analysis - and experimental techniques developed in this study can also be used for the creation of heteronuclear LiRb molecules. Unlike our ⁶Li₂ homonuclear molecules which only have a magnetic dipole moment, the LiRb polar molecules are predicted to also have a large electric dipole in the lowest lying triplet state. These characteristics combined with a three dimensional optical lattice would give us precise control over several degrees of freedom and enable us to perform quantum simulations of exotic condensed matter systems. We also present the design and performance of a coherent source of Lyman-α radiation that was used by the ALPHA collaboration at CERN for laser cooling anti-hydrogen for the purpose of experimentally verifying the predictions of the standard model.
Alkali hydride molecules are polar, exhibiting large ground-state dipole moments. As ultracold sources of alkali atoms, as well as hydrogen, have been created in the laboratory, we explore theoretically the feasibility of forming such molecules from a mixture of the ultracold atomic gases, employing various photoassociation schemes. In this work we use lithium and sodium hydride as benchmark systems to calculate molecule formation rates through stimulated one-photon radiative association directly from the continuum as well as two-photon stimulated radiative association (Raman transfer) and excitation to bound levels of an excited state followed by spontaneous emission to the ground state. Using accurate molecular potential energy curves and dipole transition moments and with laser intensities and MOT densities that are easily attainable experimentally, we have found that substantial molecule formation rates can be realized even after the effect of back-stimulation has been accounted for. We examine the spontaneous emission cascade which takes place from the upper vibrational levels of the singlet ground state on a time scale of milliseconds. Because photon emission in the cascade process does not contribute to trap loss, a sizable population of molecules in the lowest vibrational level can be achieved. The triplet ground electronic state is of particular interest for experimental efforts since, although it has never been observed experimentally, molecular structure calculations of the a3Σ+ state for LiH and NaH predict a small van der Waals attraction, with a potential energy well so shallow that it can support only one bound rotational-vibrational level. Any molecule formed in the triplet ground state would then be immediately in the lowest and most stable level of that state and would be quite long-lived, unlike molecules in high vibrational levels which have significantly shorter lifetimes. As an analysis of our method of calculating molecule formation rates, we investigate more thoroughly the two-photon stimulated photoassociation process by employing the exact treatment of Dalgarno & Lewis to solve the Raman scattering problem. ^
Heteronuclear polar molecules have recently attracted enormous attention owing to their ground state having a large electric dipole moment. The long range anisotropic dipole-dipole interaction in such systems is the basis for a variety of applications including quantum computing, precision measurements, ultracold chemistry and quantum simulations. Heteronuclear bi-alkali molecules, only a small subset of polar molecules, have received special attention mainly because the constituent alkali atoms are easy to laser cool and can be relatively easily associated to form molecules at ultracold temperatures. Our choice, the LiRb molecule, is motivated by the relatively high dipole moment (4.1 Debye) of the LiRb molecule in its rovibronic ground state. In this thesis, we study the LiRb molecule using laser spectroscopy and report, for the first time, the production of ultracold LiRb molecules by photoassociation (PA). The LiRb molecule is the least studied among all bi-alkali molecules and the first spectroscopic measurements on hot vapor phase LiRb molecules were reported only in 2011. We describe these measurements and their significance in chapter 2, after a brief introduction to the field of ultracold polar molecules in chapter 1. In chapter 3, we discuss our apparatus for simultaneous cooling and trapping of Li and Rb atoms in a dual-species magneto-optical trap (MOT) and report the measurement of interspecies collision-induced losses. In chapter 4, we describe the production of ultracold LiRb molecules in excited electronic states by photoassociation (PA). We report the measurements of the C6 coefficients for the Li (2s 2S1/2) + Rb (5p 2P1/2) and the Li (2s 2S1/2) + Rb (5p 2P3/2) asymptotes. We find a molecule formation rate (PLiRb) as high as 3.5x107 s-1 and a PA rate coefficient (KPA) as high as 1.3x10-10 cm3/s, the highest among heteronuclear bi-alkali molecules. In chapter 5, we discuss results on two-photon PA and we conclude, in chapter 6, with a road roadmap of future experiments for the production and detection of ultracold LiRb molecules in their rovibronic ground state.
Starting from an ultracold ensemble of 6Li2 Feshbach molecules, we produce deeply bound triplet $a(1{\phantom{\rule{0.16em}{0ex}}}^{3}{\mathrm{\ensuremath{\Sigma}}}_{u}{}^{+})$ molecules in a single quantum state by stimulated Raman adiabatic passage. The ensemble lifetime is found to be limited by two-body molecule-molecule collisions that depend on both the vibrational and rotational states. The loss rate observed is near the universal rate for the $|v=0,5,8;N=0,2\ensuremath{\rangle}$ states and, remarkably, below universality for the $|v=9,N=0\ensuremath{\rangle}$ state. We also observe a strong rotational state dependence of the v=9 reaction rate. Consistent with theoretical predictions, we observe that molecules in the absolute lowest triplet level are unstable and likely decay due to trimer formation.
This thesis presents an experimental apparatus capable of producing and studying ultra-cold mixtures of ⁶Li and Rb, and progress towards the creation of ultra-cold ground state ⁶Li₂ and LiRb molecules. Ultra-cold experiments with molecules have applications in many quantum computation and simulation experiments. We discuss elements of the apparatus which are important to these experiments, including electric field plates in air capable of producing fields up to 18 kV/cm, and a laser system for photoassociation spectroscopy based on two Ti:Sapphire lasers phase locked to the same optical frequency comb. With respect to ⁶Li and ⁸⁵Rb mixtures, we report on the observation of six Feshbach resonances, which represent an important step towards molecule production and future experiments in this heteronuclear mixture. In addition, we demonstrate the production of a BEC of ⁶Li₂ Feshbach molecules, a degenerate Fermi gas of ⁶Li and the formation of BCS pairs. In the ⁶Li system, we report on the high resolution spectroscopy of the v'=20-26 vibrational levels of the c(1³∑⁺g) potential and the v'=29-35 vibrational levels of the A(1¹∑⁺u) potential. In the A(1¹∑⁺u) potential, we find that the v'=31 and v'=35 levels have the largest transition strength and are therefore good candidates to use as intermediate states in molecule formation. We demonstrate atom-molecule dark states in the BEC-BCS crossover regime and additionally use dark-state spectroscopy to make extremely high resolution measurements of the least bound N''=0 ro-vibrational levels in the X(1¹∑⁺g) and a(1³∑⁺u) potential. In addition, we present spectroscopy of all ten N''=0 and N''=2 ro-vibrational levels in the a(1³∑⁺u) potential and furnish a preliminary interpretation of the observed energy structure. Finally, we report on the observation of anomalous Autler-Townes and dark-state spectrum. Using an extension to the standard three level model, we show that these anomalous profiles are due to degeneracies that exist in the bound molecular states, and to the choice of polarization of the coupling fields. These results have a direct impact on molecule formation, and provide a clear guide to future experiments.
We report the creation of ultracold ground state ^{6}Li^{40}K polar molecules with high efficiency. Starting from weakly bound molecules, stimulated Raman adiabatic passage is adopted to coherently transfer the molecules to their singlet rovibrational ground state |X^{1}Σ^{+},v=0,J=0⟩. By employing a singlet stimulated Raman adiabatic passage pathway and low-phase-noise narrow-linewidth lasers, we observed a one-way transfer efficiency of 96(4)%. Held in an optical dipole trap, the lifetime of the ground state molecules is measured to be 5.0(3) ms. The large permanent dipole moment of LiK is confirmed by applying a dc electric field on the molecules and performing Stark shift spectroscopy of the ground state. With recent advances in the quantum control of collisions, our work paves the way for exploring quantum many-body physics with strongly interacting ^{6}Li^{40}K molecules.
We present spectroscopic studies of ultracold LiRb molecules using ionization detection. The molecules are created by cooling and trapping Li and Rb atoms in overlapping magneto-optical traps (MOTs) and using light resonant with a free-bound transition to create weakly bound excited state molecules in a process known as photoassociation (PA). We explore weakly bound vibrational levels of LiRb with PA spectroscopy using ionization detection and, where possible, compare our results with earlier measurements performed in our lab using trap-loss spectroscopy. In addition, we also probe vibrational levels of the ground triplet electronic state and excited electronic states using resonantly enhanced multiphoton ionization (REMPI) spectroscopy. We identify several vibrational levels of the α3Σ+, (3)3ΠΩ and (4)3Σ + states and compare our observations with theoretical calculations. As LiRb is one of the least studied heteronuclear diatomic molecules, these studies are aimed towards exploring the molecular structure. The spectroscopic work is also in line with the long-term project goals of transferring ultracold LiRb molecules into the lowest rotational and vibrational levels of the ground singlet electronic state. Molecules in this rovibronic ground state possess a large electric dipole moment, which is essential for application of ultracold molecules in various quantum computation schemes. The rovibronic ground state will also be an ideal starting point for investigating molecular entangled states.