1722 ISSN 1054-660X, Laser Physics, 2006, Vol. 16, No. 12, pp. 1722–1725. © MAIK “Nauka / Interperiodica” (Russia), 2006. Original Text © Astro, Ltd., 2006. INTRODUCTION Laser cooling is a technique routinely used to cool many different atomic species down to temperatures below mK. The extension of this technique to mole- cules is not straightforward due to the lack of closed optical transitions. An alternative may well be the pro- duction of cold molecules from cold trapped atoms. The possibility to produce and to store samples com- posed of cold molecules, at temperatures in the μK–mK range, is opening up new perspectives in chemistry, metrology, and quantum physics [1]. In addition, their production is still in the early stages. Up to now, mole- cule formation in the μK range and below could only be performed either by Fesbach resonance in condensates [2] or by photoassociation of cold atoms in optical and magneto-optical traps [3–5]. Photoassociation (PA) was the first technique to be proposed [6] and demon- strated [3], where cold Cs 2 molecules in the ground triplet state were obtained by photoassociation at long range of a colliding pair of cold atoms with a laser red- detuned from the principal atomic transition. The ground state molecules produced were translationally and rotationally cold but were distributed among many vibrationally excited levels. In a slightly different scheme, PA of cold potassium atoms has produced cold K 2 dimers in deeply bound levels of the ground singlet state [4]. Finally, ground triplet state rubidium mole- cules have also been observed as the result of PA of cold atoms in a MOT [5, 7]. In order to use cold trapped atoms as an efficient source of cold molecules, it is necessary to know the rate of molecular formation and to find possible ways of enhancing the molecule production. One possibility is to enhance the atomic flux in a specific channel which is part of the photoassociation process. The flux enhancement of the atomic pairs that reach short inter- nuclear separation induced by a long-range excitation was first observed by Sanchez-Villicana et al. [8], where the authors determined a significant enhance- ment in the collisional flux (up to a factor of ~3) at short range when observing trap loss measurement. More recently [9], control of ultracold collisions, which results in a enhancement-type effect on ultracold colli- sions, has been demonstrated using frequency-chirped light, allowing excitation of atomic pairs over a wide range of internuclear separation. A flux-enhancement model has also been used to explain observations of fine-structure changing collisions in a magneto-optical trap of Cs atoms [10]. The flux enhancement can be explained in the following way: the atomic pair is excited at large internuclear separation by the trapping laser to an attractive excited potential. In this potential, the atoms pair are accelerated against each other, but after some time the pair decays to the ground state. They continue the inward motion but with a larger velocity, resulting in a large number of pairs available for the photoassociation laser resonant at short internu- clear separation. In this work, we apply the flux enhancement con- cept to demonstrate an increase in cold ground state molecule production due to a photoassociation laser. We measured the molecular formation rate constant induced by the photoassociation laser as a function of the trapping laser intensity. The obtained results are compared with a semi-classical model [11]. We first present our experimental setup, followed by the exper- imental results and discussions, and finally the conclud- ing remarks. Enhancing Molecular Formation in a Rb-MOT L. G. Marcassa a , V. A. Nascimento a , L. L. Caliri a , A. Caires a , D. V. Magalhães a , and V. S. Bagnato a, b a Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, São Carlos-SP, 13560-970 Brazil b Brazilian Academy of Science, Rua Anfilyfio de Carvalho, 29, 3o Andar, Rio de Janeiro, RJ, 20030-060, Brazil e-mail: lgmarcassa@uol.com.br Received May 15, 2006 Abstract—In this paper, we report observation of an enhancement of the ground state molecule Rb 2 formation in a Rb magneto-optical trap due to a photoassociation laser. Such an enhancement effect is due to flux enhance- ment of atomic pairs at short internuclear distances. Our experimental observations consist in the measurement of the molecular formation rate constant due to a probe laser as a function of the trap laser intensity. The results are compared with a simple semi-classical model, showing good agreement. We conclude that the production of cold Rb 2 may be enhanced using appropriate laser parameters; this may be useful for future experiments involving production and trapping of ultracold ground state molecules. PACS numbers: 34.50.Rk, 34.10.+x, 32.80.Pj DOI: 10.1134/S1054660X06120206 PHYSICS OF COLD TRAPPED ATOMS