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arXiv:0807.3272 · Physical Review Letters

Formation of Ultracold Polar Molecules in the Rovibrational Ground State

Johannes Deiglmayr, A. Grochola, M. Repp, Karin Moertlbauer, C. Glück, J. Lange, Olivier Dulieu, Roland Wester, Matthias Weidemüller

Published Sep 25, 2008• 588 citations
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Abstract

Ultracold LiCs molecules in the absolute ground state $X^{1}\ensuremath{\Sigma}^{+}$, ${v}^{\ensuremath{'}\ensuremath{'}}=0$, ${J}^{\ensuremath{'}\ensuremath{'}}=0$ are formed via a single photoassociation step starting from laser-cooled atoms. The selective production of ${v}^{\ensuremath{'}\ensuremath{'}}=0$, ${J}^{\ensuremath{'}\ensuremath{'}}=2$ molecules with a 50-fold higher rate is also demonstrated. The rotational and vibrational state of the ground state molecules is determined in a setup combining depletion spectroscopy with resonant-enhanced multiphoton ionization time-of-flight spectroscopy. Using the determined production rate of up to $5\ifmmode\times\else\texttimes\fi{}{10}^{3}\text{ }\text{ }\mathrm{\text{molecules}}/\mathrm{s}$, we describe a simple scheme which can provide large samples of externally and internally cold dipolar molecules.

DOI
10.1103/physrevlett.101.133004

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Structure, formation, and interactions of ultracold polar molecules

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Rotationally resolved depletion spectroscopy of ultracold KRb molecules

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We report on the use of depletion spectroscopy to detect ultracold ground-state KRb molecules with rotational resolution. The population of a specific ground-state vibrational level ${v}^{\ensuremath{''}}$, produced by photoassociation of ultracold atoms, is monitored by one-color two-photon pulsed-laser ionization. When a cw laser is resonant with a rovibrational transition to an excited state, the ground-state population, and hence the ion signal, is depleted. This narrow-band spectroscopic technique allows the individual rotational levels in both ground and excited states to be resolved, and thus the population of a single ground-state rovibrational level to be monitored. These resolved transitions are a necessary first step in transferring population from high vibrational levels of the ground state, such as produced by photoassociation, to the absolute molecular ground state $X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}^{+}$ (${v}^{\ensuremath{''}}=0$, ${J}^{\ensuremath{''}}=0$). This technique can also be used to determine binding energies of high-${v}^{\ensuremath{''}}$ molecules. Combining our binding energy measurement with the results of previous spectroscopy yields an improved dissociation energy of the $X$ state: ${D}_{e}=4217.822\ifmmode\pm\else\textpm\fi{}0.003\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. We also report on the two-color pulsed-laser ionization used to locate the depletion transitions.

Physical Review A•Mar 20, 2007•53 citations
Ultracold LiCs molecules in the absolute ground state X1\ensuremathΣ+X^{1}\ensuremath{\Sigma}^{+}X1\ensuremathΣ+, v\ensuremath′\ensuremath′=0{v}^{\ensuremath{'}\ensuremath{'}}=0v\ensuremath′\ensuremath′=0, J\ensuremath′\ensuremath′=0{J}^{\ensuremath{'}\ensuremath{'}}=0J\ensuremath′\ensuremath′=0 are formed via a single photoassociation step starting from laser-cooled atoms. The selective production of v\ensuremath′\ensuremath′=0{v}^{\ensuremath{'}\ensuremath{'}}=0v\ensuremath′\ensuremath′=0, J\ensuremath′\ensuremath′=2{J}^{\ensuremath{'}\ensuremath{'}}=2J\ensuremath′\ensuremath′=2 molecules with a 50-fold higher rate is also demonstrated. The rotational and vibrational state of the ground state molecules is determined in a setup combining depletion spectroscopy with resonant-enhanced multiphoton ionization time-of-flight spectroscopy. Using the determined production rate of up to 5\ifmmode×\else\texttimes\fi103  molecules/s5\ifmmode\times\else\texttimes\fi{}{10}^{3}\text{ }\text{ }\mathrm{\text{molecules}}/\mathrm{s}5\ifmmode×\else\texttimes\fi103  molecules/s, we describe a simple scheme which can provide large samples of externally and internally cold dipolar molecules.
In this thesis properties of ultracold alkali dimers are studied both theoretically and experimentally. In the first, theoretical, part, the prospects for the formation of ultracold LiCs dimers in deeply bound vibrational levels using photoassociation (PA) are investigated. Furthermore a previous systematic investigation of the structure of all alkali dimers is extended by the ab initio calculation of static dipole polarizabilities. An extensive comparison with other published results supports the reliability of the chosen method. Based on these results an experimental scheme for the creation of an sample of oriented ultracold dipolar molecules is developed. In the second, experimental part the formation of LiCs ground state molecules by PA is investigated. PA spectroscopy is used to improve the accuracy of the potential energy curve for the B1P and the value of the dissociation energy of the X1S+ state significantly. The rovibrational distribution of ground state molecules is probed in a combination of resonance-enhanced multi-photon ionization and depletion spectroscopy. A simple scheme is found to form ultracold polar LiCs molecules in the absolute ground state X1S+,v''=0,J''=0. In a quasi-electrostatic trap created by a CO2 laser, LiCs molecules are formed and stored. A lifetime of 24(3)s, limited by collisions with background gas, is observed and rate coefficients for inelastic collisions between LiCs in high vibrational states and Cs atoms are determined. The long storage time facilitates the observation of radiative decay of polar molecules towards lower vibrational levels.
We recently reported the formation of ultracold LiCs molecules in the rovibrational ground state X1sigma+, v" = 0,J" = 0 (J. Deiglmayr et al., Phys. Rev. Lett., 2008, 101, 133004). Here we discuss details of the experimental setup and present a thorough analysis of the photoassociation step including the photoassociation line shape. We predict the distribution of produced ground state molecules using accurate potential energy curves combined with an ab initio dipole transition moment and compare this prediction with experimental ionization spectra. Additionally we improve the value of the dissociation energy for the X1sigma+ state by high resolution spectroscopy of the vibrational ground state.
We have produced ultracold, polar RbCs∗{\mathrm{R}\mathrm{b}\mathrm{C}\mathrm{s}}^{*}RbCs∗ molecules via photoassociation in a laser-cooled mixture of Rb and Cs atoms. Using a model of the RbCs∗{\mathrm{R}\mathrm{b}\mathrm{C}\mathrm{s}}^{*}RbCs∗ molecular interaction which reproduces the observed rovibrational structure, we infer decay rates in our experiments into deeply bound $X\text{ }^{1}\ensuremath{\Sigma}^{+}$ ground-state RbCs vibrational levels as high as $5\ifmmode\times\else\texttimes\fi{}{10}^{5}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ per level. Population in such deeply bound levels could be efficiently transferred to the vibrational ground state using a single stimulated Raman transition, opening the possibility to create large samples of stable, ultracold polar molecules.
Ultracold polar RbCs molecules are produced via photoassociation in a dual dark-SPOT magneto-optical trap, in which ultracold rubidium and cesium atoms with high densities are formed by laser cooling and trapping. The a3∑+ triplet metastable state ultracold RbCs molecules with 104/s production rate are detected using resonance-enhanced two-photon ionization. The ultra-high resolution rovibrational spectra of (2)0+, (3)0- and (2)0- rovibrational states are observed by scanning the photoassociation laser frequency. The rotational constants of (2)0+, (3)0- and (2)0- vibrational states are 0.00349 cm-1, 0.00649 cm-1 and 0.00372 cm-1 by fitting respectively.
Using resonance-enhanced two-photon ionization, we detect ultracold, metastable RbCs molecules formed in their lowest triplet state $a\text{ }^{3}\ensuremath{\Sigma}^{+}$ via photoassociation in a laser-cooled mixture of 85Rb^{85}\mathrm{Rb}85Rb and 133Cs^{133}\mathrm{Cs}133Cs atoms. We obtain extensive bound-bound excitation spectra of these molecules, which provide detailed information about their vibrational distribution, as well as spectroscopic data on several RbCs molecular states including $a\text{ }^{3}\ensuremath{\Sigma}^{+}$, $(2)\text{ }^{3}\ensuremath{\Sigma}^{+}$, and $(1)\text{ }^{1}\ensuremath{\Pi}$. Analysis of this data allows us to predict strong transitions from observed levels to the absolute vibronic ground state of RbCs, potentially allowing the production of stable, ultracold polar molecules at rates in excess of ${10}^{6}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$.
We report on the use of depletion spectroscopy to detect ultracold ground-state KRb molecules with rotational resolution. The population of a specific ground-state vibrational level ${v}^{\ensuremath{''}}$, produced by photoassociation of ultracold atoms, is monitored by one-color two-photon pulsed-laser ionization. When a cw laser is resonant with a rovibrational transition to an excited state, the ground-state population, and hence the ion signal, is depleted. This narrow-band spectroscopic technique allows the individual rotational levels in both ground and excited states to be resolved, and thus the population of a single ground-state rovibrational level to be monitored. These resolved transitions are a necessary first step in transferring population from high vibrational levels of the ground state, such as produced by photoassociation, to the absolute molecular ground state $X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}^{+}$ (${v}^{\ensuremath{''}}=0$, ${J}^{\ensuremath{''}}=0$). This technique can also be used to determine binding energies of high-${v}^{\ensuremath{''}}$ molecules. Combining our binding energy measurement with the results of previous spectroscopy yields an improved dissociation energy of the XXX state: ${D}_{e}=4217.822\ifmmode\pm\else\textpm\fi{}0.003\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. We also report on the two-color pulsed-laser ionization used to locate the depletion transitions.