Session U3: Cold Molecules and Photoassociative Spectroscopy

Photoassociation spectroscopy enhanced by optical pumping

We present our results from photoassociation (PA) spectroscopy of ultracold polar NaCs molecules enhanced through the use of our optical pumping (OP) technique. PA produces molecules in a range of vibrational levels in both the singlet and triplet electronic states, which are then detected through resonant multi-photon ionization (REMPI) of a chosen vibrational level. OP has been shown to transfer population from both the singlet and triplet states very efficiently to the singlet rovibrational ground state, thus making potentially all the molecules accessible with a single REMPI detection line. We use this technique to search for missing PA lines and enhance existing ones.   

Accurate, predictive model of ultracold three-body recombination of Cs atoms

We describe an accurate numerical model to calculate the three-body recombination and atom-dimer scattering rates of three ultracold atoms. The model uses scaled length and energy units based on the long-range van der Waals potential and accurately represents the multichannel two-body physics when a magnetic field is tuned due to a Feshbach resonance. Only pairwise atom-atom interactions are assumed; these are characterized by a van der Waals long range potential with an inner wall chosen to give the known background scattering length of the resonance. The model sets up the multichannel interactions to correspond to the known dimensionless pole strength $s_{res}$ of the resonance [1] and has no adjustable parameters except the number of bound states in the two-body potentials. The calculated three body recombination rate coefficient for three Cs atoms agrees very well with the measured one [2] as the scattering length is tuned across its full range of variation, including the range where the scattering length is small and two minima occur in the recombination coefficient. The model additionally predicts the observed position of the atom-dimer resonance.\\[4pt] [1] C. Chin, et al., Rev. Mod. Phys. 82, 1225 (2010).\\[0pt] [2] Kraemer, et al., Nature 440, 315 (2006).   

Long Range Interactions of Ytterbium in Mixed Quantum Gases

A first-principles relativistic method is developed for an accurate calculation of the van der Waals coefficients of dimers involving excited state atoms with strong decay channel to the ground state. Accurate values of long-range interaction parameters are needed for efficient production, cooling, and control of molecules. We used the developed methodology to calculate a number of $C_6$ and $C_8$ coefficients for Yb-Yb, Yb-Rb, and Yb-Li dimers which are of particular interest for development of optical lattice clocks, studies of fundamental symmetries and quantum gas mixtures, and practical realization of quantum simulation proposals. Our calculations include $C_6$ coefficients for the Yb--Yb $^1\!S_0+\,^3\!P^o_{0,1}$ and $^3\!P^o_0+\,^3\!P^o_{0}$ dimers, $C_8$ coefficients for the $^1\!S_0+\,^1\!S_0$ and $^1\!S_0+\,^3\!P^o_1$ dimers, $C_6$ coefficients for the Yb--Rb $^3\!P_1^o+5s\, ^2\!S_{1/2}$ and $^1\!S_0+5p\, ^2\!P^o_{1/2}$ dimers, and the $C_8$ coefficients for the Yb--Li $^1\!S_0+2s\, ^2\!S_{1/2}$ and Yb--Rb $^1\!S_0+5s\, ^2\!S_{1/2}$ dimers. We performed detailed uncertainty analysis and provided stringent bounds on all quantities obtained in this work to allow future benchmark tests of experimental methodologies and theoretical molecular models.   

Continuous Vibrational Cooling of Ground State Rb$_2$

The process of photoassociation generally results in a distribution of vibrational levels in the electronic ground state that is energetically close to the dissociation limit. Several schemes have appeared that aim to transfer the population from the higher vibrational levels to lower ones, especially the ground vibrational state. We demonstrate continuous production of vibrationally cooled Rb$_2$ using optical pumping. The vibrationally cooled molecules are produced in three steps. First, we use a dedicated photoassociation laser to produce molecules in high vibrational levels of the $X^1\Sigma_g^+$ state. Second, a broadband fiber laser at 1071 nm is used to transfer the molecules to lower vibrational levels via optical pumping through the $A^1\Sigma_u^+$ state. This process transfers the molecules from vibrational levels around $\nu\simeq$ 113 to a distribution of levels where $\nu<$35. The molecules may then be further cooled using a broadband superluminescent diode near 685 nm that has its frequency spectrum shaped. The resulting vibrational distributions are probed using resonance-enhanced multiphoton ionization with a pulsed dye laser near 670 nm. The results are presented and compared with theoretical simulations. This work was supported by Fapesp and INCT-IQ.   

Collisional studies of an ultracold mixture of lithium and excited state ytterbium atoms

Ultracold mixtures of alkali and alkaline-earth systems are one promising avenue to achieve ultracold paramagnetic ground state molecules. However, alkaline-earth atoms are insensitive to magnetic fields in their ground state, rendering Feshbach molecule creation techniques extraordinarily difficult. By using a long-lived and magnetic excited state of the alkaline-earth, this deficiency may be overcome. We perform collisional studies of alkali lithium and the metastable $^3$P$_2$ excited state of alkaline-earth-like ytterbium in the presence of a controllable background magnetic field. We present measurements of the field dependent inter and intra-species inelastic interactions. We assess the potential for a magnetic Feshbach resonance to be used in LiYb dimer creation.   

Cooling of Electronically-Excited Diatomic Molecules in a Supersonic Microplasma Jet

Electronically-excited diatomic molecules having natural lifetimes as short as 16 ns have been cooled in a microplasma jet. By integrating a cylindrical microcavity plasma device with a nozzle, rotational cooling of He$_2$ molecules in the $d^3\Sigma_u^+$, $e^3\Pi_g$ and $f^3\Sigma_u^+$ states (radiative lifetimes of 25 ns, 67 ns, and 16 ns, respectively) have been observed. Cooling of He dimers in the $d^3\Sigma_u^+$ state from $\sim$1200 K to temperatures below 250 K has been realized and, as a result of the sub-10 $\mu$m spatial resolution of our present optical imaging capability, the temporal evolution of the non-equilibrium rotational distribution can be monitored with $<$10 ns resolution. Furthermore, evidence of excitation transfer between rotational levels of the $f^3\Sigma_u^+$ state and the lower lying $d^3\Sigma_u^+$ and $e^3\Pi_g$ states has been observed. Dynamics of the non-equilibrium cooling process, and the extension of the method to other electronically-excited diatomics with internal energies of $>$5 eV, will be discussed.   

Born-Oppenheimer study of two-component few-particle systems under one-dimensional confinement

The energy spectrum, atom-dimer scattering length, and atom-trimer scattering length for systems of three and four ultracold atoms with $\delta$-function interactions in one dimension are presented as a function of the relative mass ratio of the interacting atoms. The Born-Oppenheimer approach is used to treat three-body (``HHL'') systems of one light and two heavy atoms, as well as four-body (``HHHL'') systems of one light and three heavy atoms. Zero-range interactions of arbitrary strength are assumed between different atoms, but the heavy atoms are assumed to be noninteracting among themselves. Both fermionic and bosonic heavy atoms are considered.   

Quantum defect theory with full close-coupling method at short-range: Applications to ro-vibrational transitions in ultracold H$_2$-H$_2$ collisions and beyond

An accurate characterization of both short-range dynamics and long-range physics continues to be a major challenge in describing ultracold collisions. While quantum close-coupling (CC) is the preferred approach, it is computationally intractable for many systems of current experimental interest. A well-tested and computationally tractable approach for ultracold collisions is multi-quantum defect theory (MQDT). It is adept in handling both long-range forces and external field dependencies. Here we describe a hybrid approach that uses explicit CC method at short-range and the MQDT formalism at long range. The CC calculations needs to be carried out only at one energy, essentially at zero collision energy, to yield a short-range K-matrix from which accurate cross sections can be evaluated for energies up to a kelvin using the MQDT formalism. We illustrate the approach for the benchmark case of rovibrational transitions in H$_2$-H$_2$ collisions. Potential applicability of the approach to reactive collisions is discussed.   

Nonadiabatic effects in ultracold strontium ``physicist's molecules'' via anomalous linear, quadratic, and higher order Zeeman shifts

Weakly bound ultracold Sr$_2$ molecules provide a rich testing ground for gaining insight into the nature of molecular bonding. We present measurements of quadratic Zeeman shifts for weakly bound molecules which scale roughly cubically with molecule size and are over a millionfold enhanced compared to free strontium atoms. From the anomalous linear shifts we obtain mixing angles for the molecular states. The nature of long-range molecular potentials and nonadiabatic Coriolis coupling explain how these non-intuitive effects arise. At large magnetic fields, cubic and quartic Zeeman shifts are clearly present, revealing higher-order mixing with nearby states. Methods of precision spectroscopy and imaging of the molecules in an optical lattice will be discussed.   

Forbidden transitions of ultracold Sr$_2$ molecules

We present precision studies of forbidden molecular transitions with ultracold Sr$_2$ in an optical lattice. The molecules are photoassociated from $^{88}$Sr atoms near the $^1S_0$-$^3P_1$ intercombination line. We strongly enable forbidden $\Delta J > 1$ transitions using small magnetic fields, and report doubly forbidden optical magnetic-dipole and electric-quadrupole transitions to subradiant excited states. In magic-wavelength optical lattices, these subradiant states provide very high-Q molecular spectra.