Heteronuclear Cold Atom Photoassociation Spectroscopy CAPAS

Clouds of neutral atoms, containing some 106 atoms, can be trapped in so-called magneto-optical traps by means of laser light. The interaction between light and atoms causes a transfer of photon momentum to the atoms and finally a transfer of kinetic energy to radiation energy. Finally the trapped atoms are cooled down to a temperature as low as 200 Mikrokelvin. That means the atoms have a very low relative velocity when they meet each other, and the long-distance part of the interaction potential becomes important for the collisional behaviour. The word heteronuclear in the title means that we are mainly interested in cold collisions between atoms of different species. First we had to build a trap which is able to trap simultaneously two different atomic species. We have chosen Sodium (Na) and Lithium (Li) since these are the two lightest alkali atoms, and since both necessary wavelengths (589 nm and 671 nm) can be generated by the dye laser systems available in our group. The atomic vapors are produced within the ultra-high-vacuum vessel by means of dispensers or are introduced from ovens. Cooling and trapping laser beams for both species are overlaid by dichroitic mirrors. The optical arrangement allows for easily and independent adjustment of the spatial place of both atomic clouds, so the can in a simple way be overlapped. With this experimental arrangement first cross-sections for optical ionization from the Na 3 2 P3/2 and from the Li 2 2P 3/2 have been measured, using as light source the UV lines of an Argon-ion laser or tunable radiation around 400 nm from a cw dye ring laser. The main difficulty here was a reliable measurement of the UV intensity at the place of the cold atoms, taking into account the beam profile, the overall beam power, absorption by the vacuum chamber windows etc. A publication is in press. In the last part of the project, the interaction between simultaneously captured clouds of Na and Li have been investigated. The difficulty here was the small interaction cross-section, so we had to improve significantly the trapping conditions, before we were able to see the Li loading curve influenced by the presence of a Na cloud. For the cross-section we derived a preliminary value of 10-11 cm3 /s, one order of magnitude as known for the influence Rb-Cs. Finally, all necessary steps were done concerning a change of the experimental arrangement for the following project, P 14645.