Assembly of ultracold molecules from laser-cooled atoms

The tunable, anisotropic, long-range electric dipole-dipole interaction that exists between heteronuclear molecules is fundamentally different from the contact interaction that exists in atomic systems. In order to study this interaction, we are interested in creating and trapping a large sample of ultracold polar molecules. To achieve the ultracold temperatures necessary to observe novel behavior, we extend the mature techniques associated with atomic cooling to molecular systems.

We begin with cold (~100 μK), dense samples of atomic rubidium and cesium held in traditional magneto-optical traps. We illuminate these samples with a resonant laser beam that promotes colliding atomic pairs into an electronically-excited molecular state. By a judicious choice of photoassociation state, these molecules spontaneously decay directly to deeply-bound levels of the ground electronic state, including the vibrational ground state. Further, molecular selection rules constrain the available rotational levels such that we can continuously produce rovibronic (v=J=0) RbCs molecules.

As these molecules are created, we intend to accumulate them in an optical trap generated by an intense CO2 laser. The optical trap also contains unwanted vibrationally and rotationally excited RbCs molecules that can inelastically scatter with the absolute ground state molecules. This process liberates enough energy to eject the molecules and thereby deplete the sample. In order to achieve long trap lifetimes, we will intentionally load a high density sample of cesium atoms into the trap. The atoms will selectively remove the excited molecules via inelastic scattering while leaving the absolute ground state molecules, which only scatter elastically, unaffected. A resonant laser beam can then be used to remove the atoms, leaving a pure absolute ground state molecular sample.