Theory of Long-Range Ultracold Atom-Molecule Photoassociation

Abstract

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 light-assisted ultracold chemical reaction. The calculation of the photoassociation rate of ultracold Cs atoms with ultracold Cs2 molecules in their rovibrational ground state 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 Cs3 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.Ultracold dilute gases at temperatures much lower than 1 mK offer new opportunities for the study of elementary reactive processes between atoms and molecules. As emphasized in seminal review articles [1,2] this socalled ultracold chemistry is freed from averaging on large velocity distributions. Thus in this regime the quantum nature of the processes can be accessed, like the presence of resonances, or the sensitivity to long-range interactions which can induce anisotropic arrangements prior to the reaction. One of the most advanced experiments in this direction is undoubtedly under progress in JILA at Boulder, where quantum threshold collisions induced by tunneling through a centrifugal barrier between fermionic ultracold KRb molecules have been detected [3,4] [9] have been detected through atom trap losses. Three-body recombination in a Rb quantum gas has been characterized [10], and features associated to universal N -body (up to N = 5) resonances induced by collisions between atoms and weaklybound molecules have been observed in quantum degenerate gases [11][12][13][14][15][16].However the deep understanding of the collisional dynamics is still to come as none of these achievements were able to characterize the nature and the state distribution of the final products [17,18]. The heavy mass of alkalimetal species leads to a huge density of resonant states related to the numerous rovibrational levels accessible during an atom/molecule or a molecule/molecule collision, preventing them to be individually characterized. Models connecting the standard treatment of long-range interactions between particles to a statistical description of the resonances mostly resulting from short-range couplings have been developed [18][19][20][21]. The main quantitative uncertainty of such models arises from the estimation of the actual amount of resonant states which are indeed active during the collision.In this letter, we propose to use the well-known photoassociation (PA) process to get more insight into such collisions between ultracold atoms and molecules. PA of an ultracold atom pair is a laser-assisted collision creating an ...

Laser cooling of internal degrees of freedom of molecules

“…This method provides possible schemes for direct laser cooling of molecules [47,48]. Eventually, photo-association of ultracold cesium molecules in their ground ro-vibrational level with ultracold cesium atoms could lead to the creation of Cs 3 molecules [49] and the study of collisional processes so as to assess the efficiency of evaporative cooling or to investigate methods for achieving controlled chemistry.…”

Anomalous two-photon spectral features in warm rubidium vapor

“…Under the ambient conditions of the experiment, the lifetime of these states is predicted to be very short [31], and so the available density of molecules is likely to be insignificant. Furthermore, the binding energy of the state, typically tens of GHz [27], would be observed in the two-photon spectroscopic data as an offset in the crossing of the feature L −3 with respect to L ±1 . As the experimental data do not show any such offset, we rule out the triatomic collision hypothesis.…”

“…Figure 4 shows the steady-state excited-state probability P e versus detunings for this model (for a stationary velocity class). There are lines corresponding to the various processes corresponding to a frequency of ∼3 GHz [27], and so we would expect this mechanism to produce shifts for the L −3 line of this order in Fig. 2.…”

“…Interactions between atoms, mediated via long-range dipole-dipole coupling, have been observed in dense ultracold atomic clouds, with densities of ≈10 11 cm −3 , to alter the absorptive properties of atomic vapors [27][28][29].…”

“…Typically observed in ultracold atomic clouds [27][28][29], long-range dipole-dipole interactions have been recently observed in thermal rubidium and potassium vapors [24,26]. Such interactions have also been proposed for applications as broad as quantum information processing schemes [30] to atomic clocks [30] as well as for creation of novel quantum states of matter [31] and Rydberg-state quantum gates and molecules [28].…”