We demonstrate optical tuning of the scattering length in a Bose-Einstein condensate as predicted by Fedichev et al. [Phys. Rev. Lett. 77, 2913 (1996)]. In our experiment, atoms in a 87Rb condensate are exposed to laser light which is tuned close to the transition frequency to an excited molecular state. By controlling the power and detuning of the laser beam we can change the atomic scattering length over a wide range. In view of laser-driven atomic losses, we use Bragg spectroscopy as a fast method to measure the scattering length of the atoms.

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Momentum transfer to atoms by a standing light wave is measured in the presence of spontaneous decay. As the number of spontaneous decays is increased, the well-known Kapitza-Dirac diffractive structure observed in the absence of spontaneous decay evolves into a smooth diffusive pattern.Theoretical treatments based on a diffusive model adequately describe the rms momentum transfers and the envelopes of the momentum distributions. However, we observe significant persistence of diffractive structure at our long interaction times and find that details of the deflection profiles are accurately predicted only when several spontaneous decays occur during the interaction.The forces on atoms by light have recently received much theoretical and experimental attention, ' not only because of interest in the basic atom-light interaction, but also because these forces offer ways to slow, cool, and trap neutral atoms. %'e present here high-resolution measurements of the deflection of an atomic beam by a plane standing light wave that show how the momentum transfer to the atom changes from diffractive to diffusive as the number of spontaneous decays during the interaction is increased. These measurements represent a quantitative test of diffusion-based light force theories that must be applied when spontaneous emission plays an important role. Such diffusive theories do not predict the considerable persistence of diffractive structure that we observe at our long interaction times. On the other hand, these theories predict the rrns momentum transfer within the 15% experimental uncertainties and agree qualitatively with diffractive-averaged envelopes of the observed defiection profiles. Quantitative agreement between measured and predicted momentum distributions is obtained under conditions where many spontaneous decays occur.Forces on atoms due to light are generally separated into two types, the spontaneous force (radiation pressure) and the dipole force. The dipole force is due to the interaction of the atom's induced dipole moment with (the gradient of) the light amplitude and is by far the more significant force in a standing light wave, the configuration used in our experiment. (In fact, the average spontaneous force vanishes everywhere in a standing wave -only its fiuctuations remain. ) Recent experiments involving the dipole force in a standing wave have focused on the velocity dependence of the force and the ability of the standing wave to redistribute and channel atoms. In the present work, we probe the effects of spontaneous decay on the spatially dependent dipole force, thereby extending our previous measurements of diffraction ' into the diffusive regime. The dressed-atom approach provides the most physical description of dipole light forces. The two "dressed" states have an induced dipole moment aligned parallel and antiparallel to the oscillating light field, resulting in an attraction towards higher and lower light intensities, respectively. In the absence of spontaneous emission the atoms remain in one dressed s...