Cross-Molecular Coupling in Combined Photoassociation and Feshbach Resonances (original) (raw)
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Physical Review A, 2006
The strong-coupling limit of molecule formation in an atomic Bose-Einstein condensate via two-mode one-color photoassociation or sweep across a Feshbach resonance is examined using a basic nonlinear time-dependent two-state model. For the general class of term-crossing models with constant coupling, a common strategy for attacking the problem is developed based on the reduction of the initial system of semiclassical equations for atommolecule amplitudes to a third order nonlinear differential equation for the molecular state probability. This equation provides deriving exact solution for a class of periodic levelcrossing models. These models reveal much in common with the Rabi problem. Discussing the strong-coupling limit for the general case of variable detuning, the equation is further truncated to a limit first-order nonlinear equation. Using this equation, the strong nonlinearity regime for the first Nikitin exponential-crossing model is analyzed and accurate asymptotic expressions for the nonlinear transition probability to the molecular state are derived. It is shown that, because of a finite final detuning involved, this model displays essential deviations from the Landau-Zener behavior. In particular, it is shown that in the limit of strong coupling the final conversion probability tends to 1/6. Thus, in this case the strong interaction limit is not optimal for molecule formation. We have found that if optimal field intensity is applied the molecular probability is increased up to 1/4 (i.e., the half of the initial atomic population).
Atom loss and the formation of a molecular Bose-Einstein condensate by Feshbach resonance
Physical Review A, 2000
, large loss rates were observed when a time-varying magnetic field was used to tune a molecular Feshbach resonance state near the state of a pair of atoms in the condensate. A collisional deactivation mechanism affecting a temporarily formed molecular condensate [see V. A. Yurovsky, A. Ben-Reuven, P. S. Julienne and C. J. Williams, Phys. Rev. A 60, R765 (1999)], studied here in more detail, accounts for the results of the slow-sweep experiments. A best fit to the MIT data yields a rate coefficient for deactivating atom-molecule collisions of 1.6 × 10 −10 cm 3 /s. In the case of the fast sweep experiment, a study is carried out of the combined effect of two competing mechanisms, the three-atom (atom-molecule) or fouratom (molecule-molecule) collisional deactivation vs. a process of two-atom trap-state excitation by curve crossing [F.
High-light-intensity photoassociation in a Bose-Einstein condensate
Physical Review A, 2004
We investigate theoretically the molecular yield in photoassociation of Bose-Einstein condensed sodium atoms for light intensities of the order of and above those applied in a recent experiment. Our results show that the rate at which ground state molecules may be formed saturates at high light intensities whereas the loss rate of condensate atoms does not. This is caused by the opposing roles of the short and long range pair correlations present near resonance under the influence of the laser and is crucial for the development of efficient photoassociation procedures in a condensate.
Photoassociation dynamics in a Bose-Einstein condensate
Physical Review A, 2004
A dynamical many body theory of single color photoassociation in a Bose-Einstein condensate is presented. The theory describes the time evolution of a condensed atomic ensemble under the influence of an arbitrarily varying near resonant laser pulse, which strongly modifies the binary scattering properties. In particular, when considering situations with rapid variations and high light intensities the approach described in this article leads, in a consistent way, beyond standard mean field techniques. This allows to address the question of limits to the photoassociation rate due to many body effects which has caused extensive discussions in the recent past. Both, the possible loss rate of condensate atoms and the amount of stable ground state molecules achievable within a certain time are found to be stronger limited than according to mean field theory. By systematically treating the dynamics of the connected Green's function for pair correlations the resonantly driven population of the excited molecular state as well as scattering into the continuum of non-condensed atomic states are taken into account. A detailed analysis of the low energy stationary scattering properties of two atoms modified by the near resonant photoassociation laser, in particular of the dressed state spectrum of the relative motion prepares for the analysis of the many body dynamics. The consequences of the finite lifetime of the resonantly coupled bound state are discussed in the two body as well as in the many body context. Extending the two body description to scattering in a tight trap reveals the modifications to the near resonant adiabatic dressed levels caused by the decay of the excited molecular state.
Resonant Coupling in the Formation of Ultracold Ground State Molecules via Photoassociation
Physical Review Letters, 2001
We demonstrate the existence of a new mechanism for the formation of ultracold molecules via photoassociation of cold cesium atoms. The experimental results, interpreted with numerical calculations, suggest that a resonant coupling between vibrational levels of the 0 1 u ͑6s 1 6p 1͞2 ͒ and ͑6s 1 6p 3͞2 ͒ states enables formation of ultracold molecules in vibrational levels of the ground state well below the 6s 1 6s dissociation limit. Such a scheme should be observable with many other electronic states and atomic species.
Photoassociation Spectroscopy of Ultracold Atoms and the Study of Physicists' Molecules
Reviews of Modern Physics, 2006
is the process where two colliding atoms absorb a photon to form an excited molecule. The development of laser cooling techniques for producing gasses at ultracold (< 1 mK) temperatures has allowed photoassociation spectroscopy to be performed with very high spectral resolution. In particular, it has allowed the probing of "purely long range" molecular states and the investigation of such "physicist's molecules,"-molecules whose properties can be derived with high precision from the properties of their constituent atoms. This presentation describes what is special about photoassociation spectroscopy at ultracold temperatures, how it is performed, and how it is used to investigate cold atomic collisions and extract atomic and molecular properties. We discuss the extraction of scattering lengths, their control via optical Feshbach resonances, precision determinations of atomic lifetimes, rate limits in a Bose-Einstein condensate, and briefly, production of cold molecules. Discussions are illustrated with examples on alkali-metal atoms as well as other species. This work has recently been accepted by the Review of Modern Physics.
Dynamics of a molecular Bose-Einstein condensate near a Feshbach resonance
2005
We consider the dissociation of a molecular Bose-Einstein condensate during a magnetic-field sweep through a Feshbach resonance that starts on the molecular side of the resonance and ends on the atomic side. In particular, we determine the energy distribution of the atoms produced after the sweep. We find that the shape of the energy distribution strongly depends on the rate of the magnetic-field sweep, in a manner that is in good agreement with recent experiments.
Photoassociative frequency shift in a quantum degenerate gas
Physical Review A, 2001
A light-induced frequency shift is observed in single-photon photoassociative spectra of magnetically trapped, quantum degenerate 7 Li. The shift is a manifestation of the coupling between the threshold continuum scattering states and discrete bound levels in the excited-state molecular potential induced by the photoassociation laser. The frequency shift is observed to be linear in the laser intensity with a measured proportionality constant that is in good agreement with theoretical predictions. This phenomenon has important implications for a scheme to alter the interactions between atoms in a Bose-Einstein condensate using photoassociation resonances.
Atom loss from Bose-Einstein condensates due to Feshbach resonance
Physical Review A, 1999
In recent experiments on Na Bose-Einstein condensates [S. Inouye et al, Nature 392, 151 (1998); J. Stenger et al, Phys. Rev. Lett. 82, 2422], large loss rates were observed when a time-varying magnetic field was used to tune a molecular Feshbach resonance state near the state of pairs of atoms belonging to the condensate many-body wavefunction. A mechanism is offered here to account for the observed losses, based on the deactivation of the resonant molecular state by interaction with a third condensate atom, with a deactivation rate coefficient of magnitude ∼ 10 −10 cm 3 /s. 03.75.Fi, 32.80.Pj, 32.60.+i, 34.50.Ez, 34.90.+q Experiments have been carried out recently [1,2] (see also the review [3]) in order to control the interatomic interaction underlying the properties of a Bose-Einstein condensate (BEC). One way to achieve this [1,2] is by tuning a magnetic field B to modify the two-atom scattering length as predicted for a B-dependent Feshbach resonance . The experiments carried out on a Na optical trap measured two distinct features: (a) the change in scattering length and large collisional atom losses with a slow sweep of B that stopped short of the resonant field B 0 , and (b) a near-catastrophic loss of atom density with a fast sweep of B through the B 0 region. Two groups of investigators have recently proposed a unimolecular mechanism to explain the latter feature as due to a fast sweep-induced transfer of population from the ground trap state of an atom pair to excited states.
Observation of Feshbach-Like Resonances in Collisions between Ultracold Molecules
Physical Review Letters, 2005
We observe magnetically tuned collision resonances for ultracold Cs2 molecules stored in a CO2laser trap. By magnetically levitating the molecules against gravity, we precisely measure their magnetic moment. We find an avoided level crossing which allows us to transfer the molecules into another state. In the new state, two Feshbach-like collision resonances show up as strong inelastic loss features. We interpret these resonances as being induced by Cs4 bound states near the molecular scattering continuum. The tunability of the interactions between molecules opens up novel applications such as controlled chemical reactions and synthesis of ultracold complex molecules.