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Resonant Coupling in the Heteronuclear Alkali Dimers for Direct Photoassociative Formation of X(0,0) Ultracold Molecules

William C. Stwalley, J. Raymajumder, Michael Bellos, Ryan Carollo, Matthew Recore, Michael Mastroianni

Published Nov 21, 2012• 38 citations
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Abstract

Promising pathways for photoassociative formation of ultracold heteronuclear alkali metal dimers in their lowest rovibronic levels (denoted X(0,0)) are examined using high quality ab initio calculations of potential energy curves currently available. A promising pathway for KRb, involving the resonant coupling of the 2 1 Π and 1 1 Π states just below the lowest excited asymptote (K(4s)+Rb(5p1/2)), is found to occur also for RbCs and less promisingly for KCs as well. The resonant coupling of the 3 1 Σ + and 1 1 Π states, also just below the lowest excited asymptote, is found to be promising for LiNa, LiK, LiRb, and less promising for LiCs and KCs. Direct photoassociation to the 11Π state near dissociation appears promising in the final dimers, NaK, NaRb, and NaCs, although detuning more than 100 cm−1 below the Research with ultracold atoms has been an amazingly fruitful area of frontier

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Promising pathways for photoassociative formation of ultracold heteronuclear alkali metal dimers in their lowest rovibronic levels (denoted X(0,0)) are examined using high-quality ab initio calculations of potential energy curves currently available. A promising pathway for KRb, involving the resonant coupling of the 2(1)Pi and 1(1)Pi states just below the lowest excited asymptote (K(4s) + Rb(5p(1/2))), is found to occur also for RbCs and less promisingly for KCs also. The resonant coupling of the 3(1)Sigma(+) and 1(1)Pi states, also just below the lowest excited asymptote, is found to be promising for LiNa, LiK, and LiRb and less promising for LiCs and KCs. Direct photoassociation to the 1(1)Pi state near dissociation appears promising in the final dimers NaK, NaRb, and NaCs, although detuning more than 100 cm(-1) below the lowest excited asymptote may be required.

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Experimental Studies of LiRb: Spectroscopy and Ultracold Molecule Formation by Photoassociation

Sourav Dutta

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.

•Jan 1, 2013•4 citations
Promising pathways for photoassociative formation of ultracold heteronuclear alkali metal dimers in their lowest rovibronic levels (denoted X(0,0)) are examined using high quality ab initio calculations of potential energy curves currently available. A promising pathway for KRb, involving the resonant coupling of the 2 1 Π and 1 1 Π states just below the lowest excited asymptote (K(4s)+Rb(5p1/2)), is found to occur also for RbCs and less promisingly for KCs as well. The resonant coupling of the 3 1 Σ + and 1 1 Π states, also just below the lowest excited asymptote, is found to be promising for LiNa, LiK, LiRb, and less promising for LiCs and KCs. Direct photoassociation to the 11Π state near dissociation appears promising in the final dimers, NaK, NaRb, and NaCs, although detuning more than 100 cm−1 below the Research with ultracold atoms has been an amazingly fruitful area of frontier
Promising pathways for photoassociative formation of ultracold heteronuclear alkali metal dimers in their lowest rovibronic levels (denoted X(0,0)) are examined using high-quality ab initio calculations of potential energy curves currently available. A promising pathway for KRb, involving the resonant coupling of the 2(1)Pi and 1(1)Pi states just below the lowest excited asymptote (K(4s) + Rb(5p(1/2))), is found to occur also for RbCs and less promisingly for KCs also. The resonant coupling of the 3(1)Sigma(+) and 1(1)Pi states, also just below the lowest excited asymptote, is found to be promising for LiNa, LiK, and LiRb and less promising for LiCs and KCs. Direct photoassociation to the 1(1)Pi state near dissociation appears promising in the final dimers NaK, NaRb, and NaCs, although detuning more than 100 cm(-1) below the lowest excited asymptote may be required.
Ultracold metastable RbCs molecules are observed in a double species magneto-optical trap through photoassociation near the Rb(5S(1/2)) + Cs(6P(3/2)) dissociation limit followed by radiative stabilization. The molecules are formed in their lowest triplet electronic state and are detected by resonance enhanced two-photon ionization through the previously unobserved (3)(3)Π ← a (3)Σ(+) band. The large rotational structure of the observed photoassociation lines is assigned to the lowest vibrational levels of the 0(+) or 0(-) excited states correlated to the Rb(5P(1/2)) + Cs(6S(1/2)) dissociation limit. This demonstrates the possibility of inducing direct photoassociation in heteronuclear alkali-metal molecules at a short internuclear distance, as pointed out earlier [J. Deiglmayr et al., Phys. Rev. Lett., 2008, 101, 13304].
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.
Obtaining ultracold samples of dipolar molecules is a current challenge which requires an accurate knowledge of their electronic properties to guide the ongoing experiments. In this paper, we calculate permanent dipole moments and transition dipole moments for excited states for alkali dimers NaK, NaRb and NaCs using a standard quantum chemistry approach based on pseudopotentials for atomic core representation, Gaussian basis sets, and effective terms for core polarization effects. We provide an extensive set of data concerning transitions among the first seven molecular states of each symmetry 1Σ+, 3Σ+, 1Π, 3Π. The accuracy of our results is generally improved compared to previous similar calculations on NaK, while they are found to be in good agreement with the few NaRb states calculated with other methods. Results for NaCs transition dipole moments are given here for the first time.
The photoassociation of alkali metal molecules (Rb2, Cs2, and RbCs) at short range is investigated in theory. Based on ab initio calculations to rationalize Franck–Condon filtering, the target states of photoassociation have been obtained. They correspond to vibrational transition levels from excited state (Rb2: v′{v}{\kern 1pt} 'v′ = 17–21, Cs2: v′{v}{\kern 1pt} 'v′ = 21–25, and RbCs: v′{v}{\kern 1pt} 'v′ = 18–22) to ground state. By using quantum wave-packet dynamic methods, the yields with time evaluation of the selected states are calculated interacting with a resonant laser pulse. Using gaussian pulse leads to the yield of Rb2 up to 82% at 860 fs. After a laser pulse, the positive chirp promotes the yield of vibrational states to increase, but the negative chirp inhibits it. That is to say, by changing the laser parameters and pulse shapes, it is much easier to control the photochemical process along desired direction. For photoassociation of alkali-metal dimers, homonuclear molecules have a better response to laser pulse than heteronuclear molecules. The efficiency of photochemical reaction of homonuclear molecules is much better and the photoassociative yield is much higher. Those conditions will provide an important reference and suggest a scheme for a feasible photoassociation for further experimental and theoretical research.
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.