Session P4: Cold Collisions of Atoms and Heteronuclear Molecules

Collisional cooling and trapping of molecules

Using a crossed molecular beam apparatus, we have shown that under the correct criteria, collisions between supersonic atoms and molecules can yield post-collision molecules at rest in the laboratory frame. This technique was first demonstrated in nitric oxide (NO), and has since been extended to other non- radical diatomics, strong polar molecules, and weak polar molecules. The cooling technique only relies on the momentum of the molecule or atom to be cooled and mass of the collision partner. However, it suffers from the fact that the cold molecules are produced at the crossing of two intense supersonic beams, which leads to glancing and secondary collisions which in turn heat the cold molecules. We have modified the apparatus so that the secondary collisions and heating of the cold molecules is minimized. Our observation time of the cold molecules is limited by the latent velocity distribution of the molecules, which is approximately 50mK. Under these conditions, we are currently attempting to implement a trap in the collision region to confine the cold molecules.   

Population transfer in heteronuclear molecules

Heteronuclear molecules are an interesting field of research due to their large electric dipole moment, which can be used in a variety of experiments (e.g. test of fundamental symmetries, dipolar BECs, quantum information processing,..). All of these experiments require control over the molecular states. In this talk we will discuss ways of transferring population between various molecular levels. By solving the time-dependent Schroedinger equation using a split-operator approach, we calculate transfer efficiencies for a STIRAP type process. We will consider the examples of KRb (transfer between triplet and singlet ground state) and NaCs (transfer from higher excited vibrational levels of the singlet ground state to lower vibrational levels).   

Electrostatic trapping of ultracold NaCs molecules

We present an electrostatic trap for ultracold polar molecules. As the molecules are created inside the trap via photoassociation, a continous accumulation of polar molecules is straightforward. Careful choice of experimental parameters allows detection of specific rovibrational states.   

Feshbach resonances and photoassociation in heteronuclear systems

Feshbach resonnaces have been observed in recent experiments [1] with two different atomic species involving Bose-Fermi mixtures. Such experiments allow the study of new phenomena such as boson-mediated Cooper pairing, the formation of heteronuclear molecules, or degenerate gases with long range dipole-dipole interactions. We studied the scattering properties of mixed alkali gases in the presence of a magnetic field, focusing our attention on systems that include Li, such as Li-Na, Li-Cs or Li-Rb. Several Feshbach resonances were found for experimentally accessible magnetic fields. We also analyzed the production of ultracold ground state molecules via photoassociation in the vicinity of those Feshbach resonances, and found a substantial enhancement of molecule formation in deeply bound levels. For this work, new accurate \emph{ab initio} potentials have been used. [1] S. Inouye et al.Phys. Rev. Lett. \textbf{93}, 183201 (2004). C.A. Stan et al. Phys. Rev. Lett. \textbf{93}, 143001 (2004)   

Production and Trapping of Ultra-cold RbCs Molecules

Our lab has recently demonstrated the production of ultracold polar RbCs molecules in their vibronic ground state, via photoassociation of laser-cooled atoms followed by a laser-stimulated state transfer process. The resulting sample of X$^1\Sigma^+$(v = 0) molecules has a translational temperature of $\sim$100 $\mu$K and a narrow distribution of rotational states. With this method it should be possible to produce samples even colder in all degrees of freedom, as well as other bialkali species. Currently, we are implementing a quasi-electrostatic trap (QUEST) to collect the photoassociated molecules for further observation. Specifically, we wish to observe the strong, anisotropic collisions of these polar molecules as a function of an additional static, external electric field. We will report on our progress towards observing these collisions as well as our efforts to implement stimulated Raman adiabatic passage to improve the transfer efficiency of the molecules to the absolute ground state.   

Determination of the scattering length of the a$^3\Sigma^+$ potential of $^{87}$RbCs

We have determined the scattering length of the a$^3\Sigma^+$ potential of $^{87}$RbCs based on experimental observations from the literature and the known value for the long-range dispersion coefficient. Our analysis uses quantum defect theory and analytical solutions of the Schr\"odinger equation for a van der Waals potential. We find that the scattering length is either 700$^{+700}_{-300}$ $a_0$ or 176$\pm$2 $a_0$ with more confidence associated to the first value, where $a_0$=0.05292 nm is a Bohr radius. An independent value of the van der Waals coefficient could not be determined and the best theoretically determined $C_6$ value was used.   

Magneto-electrostatic trapping of Stark decelerated OH

Cold molecules promise to impact research on precision measurement, quantum physics, and controlled chemistry. To accomplish this goal, our research employs a Stark decelerator to slow a supersonic expansion of OH in its rovibronic ground state. At the decelerator's terminus, a $<$50 mK OH packet of density 10$^{4}$ cm$^{-3}$ is caught and confined in a magnetic quadrupole trap. An adjustable electric field of sufficient magnitude to completely polarize the OH is superimposed on the trap in either a quadrupole or homogenous field geometry. The trap dynamics deviate from that governed by simple addition of the fields' forces on OH's magnetic and electric dipoles. Rather, the OH is confined by potentials modified by molecular state mixing induced by the crossed electric and magnetic fields, which we model via an effective molecular Hamiltonian that includes Stark and Zeeman terms. Confinement of cold polar molecules in a magnetic trap, leaving large, adjustable electric fields for control, is an important step towards the study of low energy dipole-dipole collisions.   

Influence of external fields in cold collisions of OH with Rb

OH molecules in their ground electronic state have been successfully slowed to temperatures of the order of 10 mK by Stark deceleration in at least two laboratories. Cooling the molecules further using ultracold Rb (``sympathetic cooling'') seems an attractive possibility, since Rb is easily cooled and trapped in copious quantities. In previous work, we studied Rb + OH collision processes in the absence of external fields and showed that the cross sections are likely to unfavorable for sympathetic cooling. Nevertheless, the effects of external magnetic and electric fields are of considerable interest. Here we discuss the results of quantum collision calculations on Rb + OH, accounting for the hyperfine structure of both partners. We use a system of coupled diabatic potential energy surfaces, built from accurate {\em ab initio} electronic structure calculations, and expand the scattering wave function in a set of channels suitable for representing the OH levels in the presence of electric and/or magnetic fields. The large number of scattering channels involved is managed through the use of a frame-transformation procedure.   

Cold collisions of magnetically oriented YbF molecules in an electric field

The sensitivity of spectroscopic experiments to measure the electric dipole moment of the electron can be greatly enhanced by employing dense cold ensembles of heavy polar molecules such as YbF [1]. In order to elucidate the collisional stability of Zeeman states of heavy polar molecules, we have performed a rigourous quantum study of YbF--He collisions in the presence of superimposed electric and magnetic fields. It is shown that the interaction between the ground $N=0$ and the second excited $N=2$ rotational levels is responsible for simultaneous collisional depolarization of electronic and nuclear spins. The nuclear spin-conserving electronic spin relaxation occurs by a two-step mechanism, via the coupling with the $N=1$ rotationally excited state. Both processes are influenced by Feshbach resonances whose positions and lifetimes can be manipulated by varying external electric and magnetic fields. Our results suggest that buffer gas cooling of heavy polar molecules in a magnetic trap may be easier than was previously expected. J.J. Hudson {\it et al.}, Phys. Rev. Lett. 89, 023003 (2002).   

Ultracold vibrational relaxation of H$_2$ molecules

The success in creating Bose-Einstein condensates of molecules has spurred much interest in atom-molecule and molecule-molecule collisions at cold and ultracold temperatures. To understand the effect of rotational and vibrational relaxation in molecular collisions at ultracold temperatures we have performed quantum scattering calculations taking the H$_2$-H$_2$ system as an illustrative example. We have used a time-independent quantum formalism based on Jacobi coordinates in space fixed frame implemented in a new quantum scattering code [1] that includes all six internal degrees of freedom. Elastic and inelastic cross sections including state-to-state cross sections in cold and ultracold H$_2$(v=1,j=0) + H$_2$(v=0,j=0) and H$_2$(v=1,j=0) + H$_2$(v=1,j=0) collisions will be presented. \footnotesize{ [1] R. V. Krems, TwoBC - quantum scattering program, University of British Columbia, Vancouver, Canada, (2006) }   

Pseudo-potential treatment of two aligned dipoles under external harmonic confinement

Dipolar Bose and Fermi gases, which are currently being studied extensively experimentally and theoretically, interact through anisotropic, long-range potentials. Here, we replace the long-range potential by a zero-range pseudo-potential that simplifies the theoretical treatment of two dipolar particles in a harmonic trap. Our zero-range pseudo-potential description reproduces the energy spectrum of two dipoles interacting through a shape-dependent potential under external confinement very well, provided that sufficiently many partial waves are included, and readily leads to a classification scheme of the energy spectrum in terms of approximate angular momentum quantum numbers. The results may be directly relevant to the physics of dipolar gases loaded into optical lattices.   

Ultracold collisional properties of weakly-bound $p$-wave molecules in a gas of spin-polarized fermions

Abstract: We study three-atom collisional physics relevant to spin-polarized fermionic molecules, under conditions where the interatomic interactions are strongly modified by the presence of a Feshbach resonance [1]. We have explored how both the size and binding energy of $p$-wave molecules modify their collisional properties. We also have studied the effects of a non-negligible energy dependence, and of the finite atom-atom $p$-wave scattering length, on the $s$-wave elastic atom-dimer scattering length. We then speculate about the relevance of these results to ultracold spin-polarized fermi gas experiments. [1] H. Suno, B. D. Esry, and C. H. Greene, Phys. Rev. Lett. 90, 053202 (2003). This work was supported in part by the National Science Foundation.   

Cold collisions between NH molecules and Rubidium atoms

In the past decade, cooling and trapping of atoms has allowed physicists to probe the nature of quantum mechanics on a macroscopic scale. Recently, ground state molecules have been cooled into the milli-Kelvin regime using a variety of techniques. However, these methods do not produce molecular samples with the required densities and temperatures to see quantum statistical effects. One method that may make this possible is sympathetically cooling the molecules through collisions with laser-cooled atoms. To this end, we are investigating the interactions of cold NH ($^{1}\Delta$) radicals with laser-cooled rubidium. A beam of cold NH radicals is created by supersonic expansion and decelerated using time varying inhomogeneous electric fields. The cold NH is then loaded into an electrostatic trap, which is overlaid spatially with a magnetic trap containing cold rubidium atoms for subsequent collision studies.