We present the results of a spectroscopic study LiRb molecules created via photoassociation (PA) to vibration levels of the 4(1) potential. Atoms are first cooled and trapped in a dual species 7Li/ 85Rb magneto-optical trap (MOT) at temperatures of ≈ 1 mK or less. In a cold sample of sufficient density, PA occurs when a resonant laser field induces a free-bound transition to create molecules in an excited electronic potential. After spontaneous emission the molecules decay to a mixture of free atoms and bound molecules depending on the Franck-Condon factors (FCFs) of the excited and ground state vibrational wavefunctions. While excited LiRb * has been previously detected by trap loss fluorescence detection, ionization spectroscopy is required to determine the population of ground state levels after decay. We detect ground state molecules via resonantly enhanced multiphoton ionization (REMPI) where a two photon ionization transition is enhanced by an intermediate resonance. The intermediate resonances match progressions from the D1Π and possibly B1&Pgr; potentials, many of which have known energies obtained from previous heat pipe experiments. This is contrary to the expectation that weakly bound LiRb * should primarily decay to weakly bound levels of the ground state and could point to a possible path for creating molecules in the rovibronic ground state.
We present the results of a spectroscopic study LiRb molecules created via photoassociation (PA) to vibration levels of the 4(1) potential. Atoms are first cooled and trapped in a dual species 7Li/ 85Rb magneto-optical trap (MOT) at temperatures of ≈ 1 mK or less. In a cold sample of sufficient density, PA occurs when a resonant laser field induces a free-bound transition to create molecules in an excited electronic potential. After spontaneous emission the molecules decay to a mixture of free atoms and bound molecules depending on the Franck-Condon factors (FCFs) of the excited and ground state vibrational wavefunctions. While excited LiRb * has been previously detected by trap loss fluorescence detection, ionization spectroscopy is required to determine the population of ground state levels after decay. We detect ground state molecules via resonantly enhanced multiphoton ionization (REMPI) where a two photon ionization transition is enhanced by an intermediate resonance. The intermediate resonances match progressions from the D1Π and possibly B1&Pgr; potentials, many of which have known energies obtained from previous heat pipe experiments. This is contrary to the expectation that weakly bound LiRb * should primarily decay to weakly bound levels of the ground state and could point to a possible path for creating molecules in the rovibronic ground state.
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.
We present spectra of ultracold 7Li85Rb molecules in their electronic ground state formed by spontaneous decay of weakly bound photoassociated molecules. Beginning with atoms in a dual-species magneto-optical trap, weakly bound molecules are formed in the 4(1) electronic state, which corresponds to the ${B}^{1}\ensuremath{\Pi}$ state at short range. These molecules spontaneously decay to the electronic ground state and we use resonantly enhanced multiphoton ionization to determine the vibrational population distribution in the electronic ground states after spontaneous emission. Many of the observed lines from the spectra are consistent with transitions from the ${X}^{1}{\ensuremath{\Sigma}}^{+}$ ground electronic state to either the ${B}^{1}\ensuremath{\Pi}$ or the ${D}^{1}\ensuremath{\Pi}$ electronic state that has been previously observed, with levels possibly as low as ${X}^{1}{\ensuremath{\Sigma}}^{+}({v}^{\ensuremath{'}\ensuremath{'}}=2)$ being populated. We do not observe decay to weakly bound vibrational levels of the ${X}^{1}{\ensuremath{\Sigma}}^{+}$ or ${a}^{3}{\ensuremath{\Sigma}}^{+}$ electronic state in the spectra. We also deduce a lower bound of $3900 \mathrm{cm}{}^{\ensuremath{-}1}$ for the dissociation energy of the LiRb+ molecular ion.
We report the formation of ultracold (7)Li(85)Rb molecules in the a(3)Σ(+) electronic state by photoassociation (PA) and their detection via resonantly enhanced multiphoton ionization (REMPI). With our dual-species Li and Rb magneto-optical trap apparatus, we detect PA resonances with binding energies up to ∼62 cm(-1) below the (7)Li 2s (2)S1/2 + (85)Rb 5p (2)P1/2 asymptote. In addition, we use REMPI spectroscopy to probe the a(3)Σ(+) state and excited electronic 3(3)Π and 4(3)Σ(+) states and identify a(3)Σ(+) (v″ = 7-13), 3(3)Π (vΠ' = 0-10), and 4(3)Σ(+) (vΣ' = 0-5) vibrational levels. Our line assignments agree well with ab initio calculations. These preliminary spectroscopic studies on previously unobserved electronic states are crucial to discovering transition pathways for transferring ultracold LiRb molecules created via PA to deeply bound rovibrational levels of the electronic ground state.
We report the production of ultracold 7 Li 85 Rb molecules by photoassociation (PA) below the Li (2 s 2 S 1/2 ) + Rb(5 p 2 P 1/2 ) asymptote. We perform PA spectroscopy in a dual-species 7 Li- 85 Rb magneto-optical trap (MOT) and detect the PA resonances using trap loss spectroscopy. We observe several strong PA resonances corresponding to the last few bound states, assign the lines and derive the long-range C 6 dispersion coefficients for the Li (2 s 2 S 1/2 ) + Rb(5 p 2 P 1/2 ) asymptote. We also report an excited-state molecule formation rate of and a PA rate coefficient of , which are both among the highest observed for heteronuclear bi-alkali molecules. These suggest that PA is a promising route for the creation of ultracold ground-state LiRb molecules.
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.
In this work, a reaction microscope with a magneto-optical trap for 6Li atoms, was extended by an optical dipole trap in order to be able to investigate in detail laser-induced atomic and molecular ionization dynamics in a cold quantum gas. \nThe optical dipole trap was operated at a full trap depth of 2.3 mK for 6Li atoms and about \n1% of the atoms in the MOT could be transferred into the dipole trap, with 1/e storage times \nexceeding 5 s. The optically trapped ensemble of 6Li atoms was used as a target for ionisation with intensive and broadband femtosecond laser pulses (λ = 750−820 nm, P = 10^11−10^14 W/cm2, Δt = 30 fs) and allowed to perform kinematically complete experiments, in which 6Li+ ions \nas well as photoelectrons were measured coincidentically. \nAs a first application, in this work, a series of association- and ionization mechanisms, which \nled to production of molecular 6Li2+ ions, were investigated with trapped lithium atoms. \nIn photoassociative ionization, two atoms collide, which were previously lifted into the asymptotic 2p − 3s potential energy curve by ladder excitation. During the collision the atoms autoionize into the 12Σg+(6Li2+) groundstate of the molecular ion, since these two potential energy curves exhibit an avoided crossing. This process was observed when magneto-optically trapped atoms were illuminated with the femtosecond laser. \nIn the dipole trap, using single-color photoassociation, excited state molecules were produced \nin high-lying vibrational states 11Σg+(ν = 65) and 13Σg+(v = 57) and spectroscopically investigated. A fraction of the excited state molecules decay via fluorescence into the molecular ground state. The 11Σg+(ν = 38)(6Li2) ground state molecules created via the singlet res- \nonance 11Σu+(ν = 65)(Li∗2) were detected via direct 3 photon ionisation. The momentum spectra show very low kinetic energies for the photo electrons of below 100 meV. Therefore \nin the molecular ion only vibrational states of Li2+ are getting populated, which are directly below the 3-photon transition energy. \nFinally, a stepwise ionization mechanism was identified, which leads into the continuum via \nan intermediate molecular state of 6Li2∗ after photoassociation. The starting point is a pho- \ntoassociated excited state molecule 11Σu+(ν = 65)(6Li∗2), which absorbs two photons of the dipole trap laser (λ = 1070 nm ± 2 nm). This happens via an intermediate molecular state \n31Σg+(2s + 3s), after which it leads into the 12Σg+(6Li2+) potential.