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arXiv:1409.2961 · The Journal of Chemical Physics

Formation of ultracold 7Li85Rb molecules in the lowest triplet electronic state by photoassociation and their detection by ionization spectroscopy

Adeel Altaf, Sourav Dutta, John Lorenz, Jesús Pérez‐Ríos, Yong P. Chen, Daniel S. Elliott

Published Mar 19, 2015• 20 citations
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Abstract

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.

DOI
10.1063/1.4914917

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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 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 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.
We present spectra of ultracold 7Li85Rb^{7}\mathrm{Li}^{85}\mathrm{Rb}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+\mathrm{LiRb}{}^{+}LiRb+ molecular ion.
We report the production of ultracold heteronuclear 7Li85Rb^{7}\mathrm{Li}^{85}\mathrm{Rb}7Li85Rb molecules in excited electronic states by photoassociation (PA) of ultracold 7Li^{7}\mathrm{Li}7Li and 85Rb^{85}\mathrm{Rb}85Rb atoms. PA is performed in a dual-species 7Li^{7}\mathrm{Li}7Li-85Rb^{85}\mathrm{Rb}85Rb magneto-optical trap (MOT) and the PA resonances are detected using trap loss spectroscopy. We identify several strong PA resonances below the Li (2sss 2{}^{2}2S1/2{S}_{1/2}S1/2​) + Rb (5ppp 2P3/2{}^{2}{P}_{3/2}2P3/2​) asymptote and experimentally determine the long range C6{C}_{6}C6​ dispersion coefficients. We find a molecule formation rate (PLiRb{P}_{\mathrm{LiRb}}PLiRb​) of 3.5 \ifmmode\times\else\texttimes\fi{} 107{}^{7}7 s${}^{\ensuremath{-}1}$ and a PA rate coefficient (KPA{K}_{\mathrm{PA}}KPA​) of 1.3 \ifmmode\times\else\texttimes\fi{} 10${}^{\ensuremath{-}10}$ cm3{}^{3}3/s, the highest among heteronuclear bi-alkali-metal molecules. At large PA laser intensity we observe the saturation of the PA rate coefficient (KPA{K}_{\mathrm{PA}}KPA​) close to the theoretical value at the unitarity limit.
We form ultracold ground-state Rb 2 molecules by photoassociating pairs of atoms in a magneto-optical trap into the 0 u + state, which decays radiatively into high vibrational levels of the X 1 Σ g + state. Sensitive and vibrationally state-selective detection is achieved by means of resonantly-enhanced two-photon ionization with a pulsed laser. Frequency scans of the detection laser reveal a long vibrational progression to a previously unobserved electronic excited state, which we identify as the 2 1 Σ u + state. Most of its vibrational spectrum is in excellent agreement with predictions based on ab initio potentials, although the lowest vibrational levels exhibit strong perturbative mixing with the triplet 2 3 Π u state. The detection method reported here, with minor variations, should be effective for the entire potential well of the X state. In this work no transitions are observed from vibrational levels above v = 118, but this turns out to be a limitation not of the detection method but rather of the photoassociative formation scheme, due to re-excitation of the highest- v levels by the same photoassociation laser that produces them.