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arXiv:1410.4862 · Physical Review A

Formation of deeply bound ultracold LiRb molecules via photoassociation near the Li 2 S 1 / 2 + Rb 5 P 3 / 2 asymptote

John Lorenz, Adeel Altaf, Sourav Dutta, Yong P. Chen, Daniel S. Elliott

Published Dec 19, 2014• 12 citations
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Abstract

We present spectra of ultracold $^{7}\mathrm{Li}^{85}\mathrm{Rb}$ 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 $\mathrm{LiRb}{}^{+}$ molecular ion.

DOI
10.1103/physreva.90.062513

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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 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 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.
The formation of ultracold metastable RbCs molecules is observed in a double species magneto-optical trap through photoassociation below the 85 Rb(5S 1/2 ) + 133 Cs(6P 3/2 ) dissociation limit followed by spontaneous emission. The molecules are detected by resonance enhanced two-photon ionization. Using accurate quantum chemistry calculations of the potential energy curves and transition dipole moment, we interpret the observed photoassociation process as occurring at short internuclear distance, in contrast with most previous cold atom photoassociation studies. The vibrational levels excited by photoassociation belong to the 5th 0 + or the 4th 0 − electronic states correlated to the Rb(5P 1/2, 3/2 ) + Cs(6S 1/2 ) dissociation limit. The computed vibrational distribution of the produced molecules shows that they are stabilized in deeply bound vibrational states of the lowest triplet state. We also predict that a noticeable fraction of molecules is produced in the lowest level of the electronic 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 investigate the formation of weakly bound, electronically excited, heteronuclear 6 Li 40 K * molecules by single-photon photoassociation in a magneto-optical trap. We performed trap loss spectroscopy within a range of 325 GHz below the Li(2 S 1/2 )+K(4 P 3/2 ) and Li(2 S 1/2 )+K(4 P 1/2 ) asymptotic states and observed more than 60 resonances, which we identify as rovibrational levels of 7 of 8 attractive long-range molecular potentials. The long-range dispersion coefficients and rotational constants are derived. We find large molecule formation rates of up to ∼3.5 × 10 7 s − 1 , which are shown to be comparable to those for homonuclear 40 K 2 * . Using a theoretical model we infer decay rates to the deeply bound electronic ground-state vibrational level X 1 Σ + ( v '=3) of ∼5 × 10 4 s −1 . Our results pave the way for the production of ultracold bosonic ground-state 6 Li 40 K molecules which exhibit a large intrinsic permanent electric dipole moment.