Photoassociation Rate Research Articles

Photoassociation and optical Feshbach resonances in an atomic Bose-Einstein condensate: Treatment of correlation effects

In this paper we formulate the time-dependent many-body theory of photoassociation in an atomic Bose-Einstein condensate with realistic interatomic interactions, using and comparing two approximations: the first-order cumulant approximation, originally developed by Kohler and Burnett [Phys. Rev. A 65, 033601 (2002)], and the reduced pair wave approximation, based on a previous paper [Phys. Rev. A 68 033612 (2003)] generalizing to two channels the Cherny-Shanenko approach [Phys. Rev. E 62, 1046 (2000)]. The two approximations differ only by the way a pair of condensate atoms is influenced by the mean field at short interatomic separations. For these approximations we identify two different regimes of photoassociation: the adiabatic regime and the coherent regime. The threshold for the so-called "rogue dissociation" [Phys. Rev. Lett. 88, 090403 (2002)] (where mean-field theory breaks down) is found to be different in each regime, which sheds new light on the experiment of McKenzie et al [Phys. Rev. Lett. 88, 120403 (2002)] and previous theoretical calculations. We then use the two approximations to investigate numerically the effects of rogue dissociation in a sodium condensate under conditions similar to the McKenzie et al experiment. We find two different effects: reduction of the photoassociation rate at short times, and creation of correlated pairs of atoms, confirming previous works. We also observe that the photoassociation line shapes become asymmetric in the first-order cumulant approximation, while they remain symmetric in the reduced pair wave approximation, giving the possibility to experimentally distinguish between the two approximations.

Photoassociation inside an optical dipole trap: absolute rate coefficients and Franck–Condon factors

We present quantitative measurements of the photoassociation of cesium molecules inside a far-detuned optical dipole trap. A model of the trap depletion dynamics is derived which allows to extract absolute photoassociation rate coefficients for the initial single-photon photoassociation step from measured trap-loss spectra. The sensitivity of this approach is demonstrated by measuring the Franck-Condon modulation of the weak photoassociation transitions into the low vibrational levels of the outer well of the 0g- state that correlates to the 6s+6p3/2 asymptote. The measurements are compared to theoretical predictions. In a magneto-optical trap these transitions have previously only been observed indirectly through ionization of ground state molecules.

Photoassociation dynamics in a Bose-Einstein condensate

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 one 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 noncondensed 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.

Photoassociation of cold atoms with chirped laser pulses: Time-dependent calculations and analysis of the adiabatic transfer within a two-state model

This theoretical paper presents numerical calculations for photoassociation of ultracold cesium atoms with a chirped laser pulse and detailed analysis of the results. In contrast with earlier work, the initial state is represented by a stationary continuum wavefunction. In the chosen example, it is shown that an important population transfer is achieved to $\approx 15$ vibrational levels in the vicinity of the v=98 bound level in the external well of the $0_g^-(6s+6p_{3/2})$ potential. Such levels lie in the energy range swept by the instantaneous frequency of the pulse, thus defining a ``photoassociation window''. Levels outside this window may be significantly excited during the pulse, but no population remains there after the pulse. Finally, the population transfer to the last vibrational levels of the ground $a^3\Sigma_u^+$(6s + 6s) is significant, making stable molecules. The results are interpreted in the framework of a two state model as an adiabatic inversion mechanism, efficient only within the photoassociation window. The large value found for the photoassociation rate suggests promising applications. The present chirp has been designed in view of creating a vibrational wavepacket in the excited state which is focussing at the barrier of the double well potential.

Saturation in heteronuclear photoassociation of6Li7Li

We report heteronuclear photoassociation spectroscopy in a mixture of magneto-optically trapped ${}^{6}\mathrm{Li}$ and ${}^{7}\mathrm{Li}.$ The laser-induced decrease in the ${}^{7}\mathrm{Li}$ steady-state particle number, only appearing in the presence of ${}^{6}\mathrm{Li},$ gives clear evidence of photoassociation to form ${}^{6}{\mathrm{Li}}^{7}\mathrm{Li}.$ Hyperfine resolved spectra of the vibrational level $v=83$ of the singlet state ${\mathrm{A}}^{1}{\ensuremath{\Sigma}}_{u}^{+}$ have been taken up to intensities of $1000{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}.$ The absolute resonance frequencies and the rotational constant have been measured. Saturation of the photoassociation rate has been observed for two hyperfine transitions, which can be shown to be due to saturation of the rate coefficient near the unitarity limit. Saturation intensities on the order of $40{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ can be determined.

Photoassociation of sodium in a Bose-Einstein condensate.

We form ultracold Na2 molecules by single-photon photoassociation of a Bose-Einstein condensate, measuring the photoassociation rate, linewidth, and light shift of the J = 1, v = 135 vibrational level of the A1 Sigma (+)(u) molecular state. The photoassociation rate constant increases linearly with intensity, even where it is predicted that many-body effects might limit the rate. Our observations are in good agreement with a two-body theory having no free parameters.

Photoassociation as a probe of Feshbach resonances in cold-atom scattering

We investigate theoretically the influence of magnetically tunable Feshbach collision resonances on the photoassociation spectra of ultracold atoms. As an example we consider recently predicted resonances for ${}^{85}\mathrm{Rb}$ atoms. For excitation to the ${}^{85}{\mathrm{Rb}}_{2}{\mathrm{ }0}_{g}^{\ensuremath{-}}{(S}_{1/2}{+P}_{1/2})$ electronic state, we predict that the photoassociation rate is resonantly enhanced by at least two orders of magnitude. We find that photoassociation could serve as a useful probe with which to study Feshbach resonances in ultracold collisions. In turn, these resonances should be very important to Bose-condensed gases, and more generally, to coherent atom optics.

Photoassociation in a gas of cold alkali atoms: I. Perturbative quantum approach

We develop a quantum-mechanical approach to the photoassociation phenomenon in a sample of cold atoms initially at thermal equilibrium with temperature T. We calculate the rate of formation of molecules in the weak field regime using a perturbative approach. Analytical expressions are obtained for the overlap between the initial and final molecular wavefunctions, and are shown to agree with numerical calculations. The photoassociation rate is found to decrease with increasing atomic mass m, from Li to Cs. Also considered is the heating of the atomic sample through spontaneous photodissociation.