Polarization Dependence of Optical Suppression in Photoassociative Ionization Collisions in a Sodium Magneto-optic Trap

Abstract

Three-dimensional quantum scattering calculations predict that the degree of optical shielding of cold collisions saturates with increasing intensity very differently from two-state models and is sensitive to the polarization of the shielding light. We report measurements showing how suppression of Na photoassociative ionization (PAI) rates is sensitive to the intensity and polarization of the shielding laser. At higher intensities, circular polarization suppresses PAI rates an order of magnitude more effectively than linear polarization, in qualitative agreement with our model calculations. PACS numbers: 32.80.Pj, 33.80.Ps, 34.50.Rk, 34.80.Qb The recently developed ability to optically cool and confine atoms [1] has opened the door to a new regime of ultracold collision processes [2][3][4] in which the frequency and intensity of externally applied fields can be used to switch on or turn off inelastic channels. The suppression of inelastic collision rates by optical shielding of ultracold, trapped atoms has been demonstrated experimentally [5][6][7][8][9] and calculated theoretically with two-state models [5,10]. In the shielding process, a blue detuned laser excites a repulsive state of the colliding atoms and causes the approaching atoms to turn around and separate, never getting closer together than the Condon point R C , where the quasimolecular system is in resonance with the light. In this way, the rate of an inelastic collision process due to interactions at much shorter range than R C can be strongly diminished. Shielding is important to investigate, since it can turn off "bad" collisions which lead to the loss of trapped atoms, while, in principle, minimizing heating due to excited state production [10,11]. Although the shielding phenomenon has been semiquantitatively explained by two-state Landau-Zener (LZ) models [5,10,11], these models are greatly oversimplified and can be misleading.In order to obtain a more realistic understanding of the optical shielding, we have carried out an exact threedimensional quantum scattering calculation for a simplified model problem of the collision in a strong radiation field [12]. Our calculations predict very different qualitative variation with increasing light intensity from existing two-state models, due to the role of multiphoton processes which couple different relative angular momentum states of the colliding ground state atoms via their mutual coupling through the excited state. As intensity increases, these differences are as follows: (1) greatly reduced shielding and greatly increased excited state production compared to LZ models, and (2) a strong polariza-tion dependence in these effects. The reduced shielding is consistent with observations on metastable Xe [8,13].Here we describe new observations on trapped Na which confirm our prediction that the optical shielding mechanism itself is sensitive to the polarization of the light field. At low intensities, below saturation, shielding can be interpreted in terms of the LZ avoided-crossing model that is in...

Cold polar molecules in two-dimensional traps: Tailoring interactions with external fields for novel quantum phases

“…In particular, we show below that the shielding efficiency depends strongly on the chosen polarization q of the AC field (see Fig. 11 and text), a characteristic which was found both in theory and in experiments with cold atoms [39,41].…”

Optical trapping and manipulation of neutral particles using lasers

“…Ultra-cold collisions play a role in optical traps where they are one of the principal loss mechanisms (59,60). Of interest, it was shown that one can suppress photoassociative collisions in traps (64).…”

Photo induced collisions with laser cooled He* atoms

“…If the molecular complex is excited to a repulsive energy state the atoms will be pushed apart thus preventing them from reaching small internuclear distances where reactions may take place. This effect is called optical shielding of collisions and has been observed in several systems [6,10]. In the case of a light field detuned below the atomic resonance, excitation to an attractive potential curve will take place.…”