Low Saturation Intensities in Two-Photon Ultracold Collisions

Interactions between ultracold atoms are of consider- able interest because of the appearance of novel light- induced dynamics at low temperature and the onset of quantum behavior (1). Ultracold collisions proved to be crucial in attaining Bose-Einstein condensation, and op- tical control of ultracold collisions appears to provide a promising technique for producing large ensembles of ul- tracold molecules (2). In this Letter, we report on a study of light-induced ultracold atomic collisions which, among other striking observations, display saturation intensities for two-photon processes that are much lower than ob- served for single-photon processes. Any realistic theoretical treatment of excited state col- lisions must include spontaneous emission as collision times are comparable to excited state lifetimes (3). Fur- thermore, at ultracold temperatures— unlike thermal col- lisions —the hyperfine interaction plays an important role in the collision dynamics (4), though the analysis of virtu- ally all experiments to date has ignored hyperfine interac- tions. For the two-photon experiment reported here, we analyzed the measurements using a semiclassical model incorporating all hyperfine energy levels. This allows us to elucidate the interplay between the various processes of flux depletion, spontaneous emission, optical shielding, and Franck-Condon overlap. Furthermore, we can quan- titatively explain the striking observation of very low sat- uration intensities in the two-photon collision process. Ultracold collisions begin when two atoms approach each other in the ground state. The ensuing dynamics de- pend on the number of photons absorbed and emitted by the colliding atom pair. Absorption of one photon creates a singly excited state (SES). Acceleration on the SES pro- duces trap loss (1). Collisions involving the absorption of two photons, producing doubly excited states (DES), allow greater control over the collision dynamics because the relative energy of the colliding atoms is set by the difference of the photon frequencies. Here we show that DES collisions allow insight into some processes which are masked in SES trap loss measurements. Analysis of studies of some of the important dynamical processes of DES collisions in Na (5) were hampered by the complex- ity of the hyperfine structure in the P3y2 fine-structure level, thus emphasizing the importance of exploring a simpler, more tractable system such as the P1y2 mani- fold in Rb (6). Further simplification results when the ini- tial excitation is to the lowest energy molecular hyperfine manifold, i.e., from the lowest ground state hyperfine level to the lowest hyperfine levels of the 5S1y2 1 5P1y2 SES. This pathway avoids the complex behavior which results from entangled attractive and repulsive potential curves. In Rb, the collisional photoassociative ionization chan- nel is closed (7), so we employ a fluorescence detection method which takes advantage of an energy pooling re- action. Energy pooling occurs when the atom pair in the DES reaches small separation and yields a violet photon with l › 421 nm. We have probed the dynamics of the collision by measuring the relative violet photon produc- tion rate R as a function of laser detuning and intensity when two ir laser fields (both with l , 795 nm) are ap- plied. Referring to Fig. 1, one field, of frequency v1 (in- tensity I1), is tuned below resonance while the other, v2 sI2d, is tuned above resonance. We find a strong depen- dence of R on v2, including the surprising appearance of deep modulations for 87Rb, but not for 85Rb. Even more striking, we observe saturation at laser intensities much lower than in any previous ultracold collisions ex- periment. We identify saturation associated with v1 as resulting from a depletion of incoming flux (8), the first such observation of this effect. Depletion results when excitation at larger interatomic separation removes ground state flux available for excitation at shorter distances. Sat- uration of v2 is also observed at small detuning, arising from an optical shielding process (9). Our model indi- cates that the modulations in the v2 dependence origi- nate from two sources: the onset of new collisional channels due to hyperfine structure in the DES, and Franck-Condon (FC) overlap factors. Our data also demonstrate the strong influence of spontaneous emission and optical shielding when the detuning is near the onset of the two-photon collision. The spectrum of the spontaneously emitted photons gives important information about the collision process.