Long-Range Forces between Cold Atoms

Abstract

Long-range interactions can substantially modify collision or interaction processes among cold atoms. We are able to monitor their effects on two different systems: Cs atoms which undergo photoassociation and highly excited Cs Rydberg atoms. Although the length and the time scales of the interactions are quite different, their effects can be described with the same resonant dipole picture.[S0031-9007(99)08564-6] PACS numbers: 34.50. Rk, 32.80.Pj, 34.60. + z Collisions between two cold atoms are extremely sensitive to the long-range part of interatomic potentials. For example, recent experiments in cold optical collisions observed in real time [1,2] have shown the effects of the resonant dipole-dipole interaction. In these experiments, respectively, with rubidium and xenon trapped atoms, photoexcitation of pairs of atoms at relatively short internuclear distance [R ϳ ͑10 400͒a 0 ] gives rise, respectively, to trap loss [1] and associative ionization [2]. As such processes depend on the flux of colliding particles arriving at short distance, they can be enhanced (shielded) by exciting at large distance [R exc ϳ ͑1000 3000͒a 0 ] the atomic pairs to an attractive (repulsive) branch of the excited molecular potential curve.The dynamical effects of long-range forces should be ubiquitous in experiments with cold atoms. Here we report two very different manifestations of essentially similar resonant dipole-dipole forces, in which the modified atomic dynamics at long range clearly affects the outcome of the reactions.The first effect is the photoassociation (PA) of cold Cs atoms [3] into relatively high rotational states of the Cs 2 excited molecule. In alkali PA spectroscopy [4], two free colliding atoms resonantly absorb one photon to form an excited dimer in a rovibrational state. The rotational number J is a direct signature of the partial waves involved in the collisional process. In our experiment, by exciting at long range atom pairs to an attractive excited state, we can increase the number of partial waves participating in the collision and observe PA into higher rotational levels.The second effect is the distortion in the resonances of dipole-dipole energy transfer between cold Cs Rydberg atoms [5,6]. In this case pairs of atoms, separated by roughly 2 3 10 4 a 0 , are moved slightly either closer together or farther apart by the dipole-dipole force over several microseconds. The resulting change in the dipoledipole interaction strength distorts the resonances.Although the systems and magnitudes of the involved quantities are very different, the dynamical effects are comparable and are due to the same resonant dipole-dipole interaction with the R 23 dependence.The paper is organized as follows. The essential idea of the long-range forces is developed for the PA experiment, followed by a description of the experiment. Then we describe the cold Rydberg atom system and the effects of the resonant dipole force in this case.Long-range forces in the PA of Cs can be understood with the help of the level diagram in F...

Ionization due to the interaction between two Rydberg atoms

“…This effect was observed in cold Rydberg collisions [1][2][3][4]. The cross section for ionization at somewhat higher energy was investigated in [5].…”

Zeros of Rydberg–Rydberg Föster interactions

“…Dramatic enhancements of collisional interactions due to such resonant couplings have been demonstrated for Rydberg excitation in a magnetooptical trap [3]. The quantum nature of these types of interactions was recently used to resolve molecules 12 nm apart in an organic solid [4].…”

Electric-Field Induced Dipole Blockade with Rydberg Atoms

“…One example is the efficiency of photoassociation of cold atoms and the formation of cold molecules [1]. In the case of a Rydberg atomic ensemble, the range of the dipole-dipole interactions can exceed several micrometers, leading to many-body effects [2,3,4]. An interesting application of the dipole-dipole interaction is the dipole blockade (DB) in Rydberg excitation.…”