Photoassociation Rates Research Articles

Modeling Photoassociative Spectra of Ultracold NaK + K.

A model for photoassociation of ultracold atoms and molecules is presented and applied to the case of 39K and 23Na39K bosonic particles. The model relies on the assumption that photoassociation is dominated by long-range atom-molecule interactions well outside the chemical bond region. The frequency of the photoassociation laser is chosen close to a bound-bound rovibronic transition from the X1Σ+ ground state toward the metastable b3Π lowest excited state of 23Na39K, allowing us to neglect any other excitation, which could hinder the photoassociation detection. The energy level structure of the long-range 39K···23Na39K excited super-dimer is computed in the space-fixed frame by solving coupled-channel equations, involving the coupling between the 23Na39K internal rotation and the mechanical rotation of the super-dimer complex. A quite rich structure is obtained, and the corresponding photoassociation rates are presented. Other possible photoassociation transitions are discussed in the context of the proposed model.

Heteronuclear Long-Range Rydberg Molecules.

We present the formation of homonuclear Cs_{2}, K_{2}, and heteronuclear CsK long-range Rydberg molecules in a dual-species magneto-optical trap for ^{39}K and ^{133}Cs by one-photon UV photoassociation. The different ground-state-density dependence of homo- and heteronuclear photoassociation rates and the detection of stable molecular ions resulting from autoionization provide an unambiguous assignment. We perform bound-bound millimeter-wave spectroscopy of long-range Rydberg molecules to access molecular states not accessible by one-photon photoassociation. Calculations based on the most recent theoretical model and atomic parameters do not reproduce the full set of data from homo- and heteronuclear long-range Rydberg molecules consistently. This shows that photoassociation and millimeter-wave spectroscopy of heteronuclear long-range Rydberg molecules provide a benchmark for the development of theoretical models.

Proposal for the formation of ultracold deeply bound RbSr dipolar molecules by all-optical methods

Ultracold paramagnetic and polar diatomic molecules are among the promising systems for quantum simulation of lattice-spin models. Unfortunately, their experimental observation is still challenging. Based on our recent \textit{ab-initio} calculations, we analyze the feasibility of all-optical schemes for the formation of ultracold $^{87}$Rb$^{84}$Sr bosonic molecules. First, we have studied the formation by photoassociation followed by spontaneous emission. The photoassociation rates to levels belonging to electronic states converging to the $^{87}$Rb$(5s\,^2S)$+$^{84}$Sr($5s5p\,^3P_{0,1,2}$) asymptotes are particularly small close to the asymptote. The creation of molecules would be more interesting by using deeply levels that preferentially relaxes to the $v''=0$ level of the ground state. On the other hands, the photoassociation rates to levels belonging to electronic states converging to the Rb$(5p\,^2P_{1/2,3/2})$+Sr($5s^2\,^1S$) asymptotes have high value close to the asymptote. The relaxation from the levels close to the asymptotes creates weakly-bound molecules in mosty only one vibrational level. Second, stimulated Raman adiabatic passage (STIRAP) achieved in a tight optical trap efficiently creates weakly-bound ground-state molecules in a well-defined level, thus providing an alternative to magnetic Feshbach resonances to implement several schemes for an adiabatic population transfer toward the lowest ground-state level of RbSr. Finally, we have studied STIRAP process for transferring the weakly-bound molecules into the $v''=0$ level of the RbSr ground state.

Theory of Long-Range Ultracold Atom-Molecule Photoassociation.

The creation of ultracold molecules is currently limited to diatomic species. In this Letter, we present a theoretical description of the photoassociation of ultracold atoms and molecules to create ultracold excited triatomic molecules, thus being a novel example of a light-assisted ultracold chemical reaction. The calculation of the photoassociation rate of an ultracold Cs_{2} molecule in its rovibrational ground state with an ultracold Cs atom at frequencies close to its resonant excitation is reported, based on the solution of the quantum dynamics involving the atom-molecule long-range interactions and assuming a model potential for the short-range physics. The rate for the formation of excited Cs_{3} molecules is predicted to be comparable with currently observed atom-atom photoassociation rates. We formulate an experimental proposal to observe this process relying on the available techniques of optical lattices and standard photoassociation spectroscopy.

Enhancing photoassociation rates by nonresonant-light control of shape resonances

Photoassociation, assembling molecules from atoms using laser light, is limited by the low density of atom pairs at sufficiently short interatomic separations. Here, we show that non-resonant light with intensities of the order of 10^10 W/cm^2 modifies the thermal cloud of atoms, enhancing the Boltzmann weight of shape resonances and pushing scattering states below the dissociation limit. This leads to an enhancement of photoassociation rates by several orders of magnitude and opens the way to significantly larger numbers of ground state molecules in a thermal ensemble than achieved so far.

Dynamical hole in ultrafast photoassociation: Analysis of the compression effect

Photoassociation of a pair of cold atoms by excitation with a short chirped laser pulse creates a dynamical hole in the initial continuum wave function. This hole is manifested by a void in the pair wave function and a momentum kick. Photoassociation into loosely bound levels of the external well in ${\mathrm{Cs}}_{2}\phantom{\rule{0.3em}{0ex}}{0}_{g}^{\ensuremath{-}}(6S+6{P}_{3∕2})$ is considered as a case study. After the pulse, the free evolution of the ground triplet state $a^{3}\ensuremath{\Sigma}_{u}^{+}$ wave packet is analyzed. Due to a negative momentum kick, motion to small distances is manifested and a compression effect is pointed out, markedly increasing the density of atom pairs at short distance. A consequence of the hole is the redistribution of the vibrational population in the $a^{3}\ensuremath{\Sigma}_{u}^{+}$ state, with population of the last bound level and creation of pairs of hot atoms. The physical interpretation makes use of the time dependence of the mass current and population on each channel to understand the role of the parameters of the photoassociation pulse. By varying such parameters, optimization of the compression effect in the ground-state wave packet is demonstrated. Due to an increase of the short-range probability density by more than two orders of magnitude, we predict significant gain in the photoassociation rates into deeply bound levels of the excited state by a second pulse, red-detuned relative to the first one, and conveniently delayed.

Grid methods for cold molecules: Determination of photoassociation line shapes and rate constants

A description of photoassociation by cw laser is formulated in the framework of grid methods. The Hamiltonian describing one or several bound states coupled to a multiple of continuum manifolds via a radiative field is written in the energy representation and diagonalized. The generality of the treatment allows us to compute accurately and efficiently physical properties such as nonlinear high-intensity energy shifts, line shapes, and photoassociation rates both for isolated and nonisolated resonances. Application is given to sodium photoassociation in the experimental conditions of McKenzie et al. [Phys. Rev. Lett. 88, 090403 (2002)]. Inverted regions for the dependency of the rate vs the intensity and nonsymmetric line shapes are predicted to occur above the saturation limit. Comparison with the model of Bohn and Julienne [Phys. Rev. A 60, 414 (1999)] is discussed.

Ultracold photoassociation spectroscopy: Long-range molecules and atomic scattering

Photoassociation is the process in which two colliding atoms absorb a photon to form an excited molecule. The development of laser-cooling techniques for producing gases at ultracold !!1 mK temperatures allows photoassociation spectroscopy to be performed with very high spectral resolution. Of particular interest is the investigation of molecular states whose properties can be related, with high precision, to the properties of their constituent atoms with the “complications” of chemical binding accounted for by a few parameters. These include bound long-range or purely long-range vibrational states in which two atoms spend most or all of their time at large internuclear separations. Low-energy atomic scattering states also share this characteristic. Photoassociation techniques have made important contributions to the study of all of these. This review describes what is special about photoassociation spectroscopy at ultracold temperatures, how it is performed, and a sampling of results including the determination of scattering lengths, their control via optical Feshbach resonances, precision determinations of atomic lifetimes from molecular spectra, limits on photoassociation rates 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. Progress in the field is already past the point where this review can be exhaustive, but an introduction is provided on the capabilities of photoassociation spectroscopy and the techniques presently in use.

Local Pair Correlations in One-Dimensional Bose Gases

We measure photoassociation rates in one-dimensional Bose gases, and so determine the local pair correlation function over a wide range of coupling strengths. As bosons become more strongly coupled, we observe a tenfold decrease in their wave function overlap, thus directly observing the fermionization of bosons.

Prospects for the formation of ultracold ground state polar molecules from mixed alkali atom pairs

Photoassociation rates of mixed cold alkali atom pairs and formation rates of cold heteronuclear alkali dimers in their ground state are computed within an approach where the wave functions are calculated exactly, and the laser field is treated as a perturbation. These rates are predicted to have the same magnitude for all heteronuclear species involving either Rb or Cs atoms. Moreover, these rates are found slightly smaller than, or similar to, the same rates for Cs2 molecules, which is encouraging for future experiments. We studied the specific case of the photoassociation into excited molecular states coupled with spin-orbit interaction, emphasizing the role of the so-called resonant coupling in the formation of stable ultracold molecules with low vibrational levels. The comparison with recent experimental results on RbCs photoassociation and cold molecule formation show their reasonable agreement with our calculations.

Formation of ultracold molecules via photoassociation with blue detuned laser light

We propose a new possibility to form ultracold molecules, via photoassociation of a pair of cold atoms into vibrational levels of the external well of an excited electronic state located at intermediate interatomic distance ( ≈ 20 Bohr radii), and embedded in the dissociation continuum above its dissociation limit. The existence of such a well is demonstrated by conventional free-free absorption spectroscopy at thermal energies. Estimation for cold atom photoassociation and cold molecule formation rates are obtained within a perturbative approach [Drag et al., IEEE J. Quant. Electr. 36, 1378 (2000)], and are found observable for usual conditions of photoassociation experiments.

Theory of coherent photoassociation of a Bose-Einstein condensate

We study coherent photoassociation, phenomena analogous to coherent optical transients in few-level systems, which may take place in photoassociation of an atomic Bose-Einstein condensate but not in a nondegenerate gas. We develop a second-quantized Hamiltonian to describe photoassociation, and apply the Hamiltonian both in the momentum representation and in the position representation (field theory). Solution of the two-mode problem including only one mode each for the atomic and molecular condensates displays analogs of Rabi oscillations and rapid adiabatic passage. A classical version of the field theory for atoms and molecules is used to demonstrate that, in the presence of photoassociating light, a joint-atom molecule is unstable against growth of density fluctuations. Experimental complications, including spontaneous emission and unwanted "rogue" photodissociation from a photoassociated molecule are analyzed. A two-color Raman scheme is studied as a method to set up an effective two-mode scheme with reduced spontaneous emission losses. We discuss photoassociation rates and photoassociation Rabi frequencies for high-lying vibrational states in alkalis both on the basis of molecular-structure calculations, and by comparing with an experiment [Wynar et al., Science 287, 1016 (2000)].