Reports: G6

47586-G6 Energy Transfer in a Trapped Gas of NH Molecules

Heather Lewandowski, University of Colorado (Boulder)

The past year has seen our experiments move from the design and development stage to producing initial results. Below is a brief description of our experiments that were supported by the Petroleum Research Fund.

Controlled molecular beams and trapping

First, we have developed a new slowing protocol that greatly decreases the energy spread of a molecular packet produced from a Stark decelerator. The decreased energy spread is critical for studying collisions with narrow resonances and thresholds. We published an article on these results in a special issue of the New Journal of Physics, which was dedicated to cold and ultracold molecules.

Second, we implemented an electrostatic trap for slowed ammonia molecules. The trap design allows for efficient trapping and ion detection of ND3 as well as access for the rubidium atoms and associated absorption imaging. Using this new design, we demonstrated trapping of ND3 for times on the order of several seconds. We believe our lifetime is limited by non-adiabatic transitions to untrapped states near the center of the trap where the electric field is zero.

Atom-molecule collisions

One of the main goals of our Stark deceleration experiments is to study interactions between ultracold atoms and cold molecules in a co-trapped environment. Individually trapping the atoms with magnetic fields and the molecules with electric fields allows us excellent control over the reactants. In addition, the co-trapped configuration allows us to reach unprecedentedly low collision energies for ammonia molecule interactions. The collision energies we are currently exploring are around 0.1 cm-1 (3 GHz). In this regime, the rubidium hyperfine energy (6.8 GHz) can contribute significantly to the interaction.

We first saw evidence of interactions between ultracold rubidium atoms and cold ammonia molecules this past summer. By measuring the trap loss rate and temperature evolution of the ammonia sample, we are able to extract both elastic and inelastic collision rates. Example data are shown in Figs 1 and 2. There is still considerable work to be done to accurately calibrate our measurements. We also hope to examine the effects of different internal states and isotopes of the reactants on the interactions.  These measurements will not only elucidate the primary interactions between cold atoms and molecules but also help to constrain the theory that predicts interactions at lower collision energies. We are planning on submitting our initial collision results for publication early in 2010.

Fig. 1 Peak density of the trapped ammonia cloud as a function of time. The presence of the ultracold rubidium atoms increases the loss rate of the molecules via inelastic collisions.

Fig. 2 Density of ammonia molecules as a function of position in the trap. The size of the cloud corresponds to a temperature of around 60 mK. However, the sample is not in thermodynamic equilibrium.