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+u (6s+6p1/2) and (6s+6p3/2) states enables formation of ultracold molecules in vibrational levels of the ground state well below the 6s+6s dissociation limit. Such a scheme should be observable with many other electronic states and atomic species.

We present a strategy for post-pulse molecular orientation aiming both at efficiency and maximal duration within a rotational period. We first identify the optimally oriented states which fulfill both requirements. We show that a sequence of half-cycle pulses of moderate intensity can be devised for reaching these target states.PACS numbers: 32.80.Lg, 42.50.Hz Molecular orientation plays a crucial role in a wide variety of applications extending from chemical reaction dynamics, to surface processing, catalysis and nanoscale design [1,2,3,4]. Static electric field [5] and strong nonresonant long laser pulses [6,7] have been shown to yield adiabatic molecular orientation which disappears when the pulse is off. Noticeable orientation that persists after the end of the pulse (and even under thermal conditions) is of special importance for experiments requiring fieldfree transient orientation. It has recently been shown that very short pulses combining a frequency ω and its second harmonic 2ω excite a mixture of even and odd rotational levels and have the ability to produce such post-pulse orientation [8]. But even more decisive has been the suggestion to use half-cycle pulses (HCPs), that through their highly asymmetrical shape induce a very sudden momentum transfer to the molecule which orients under such a kick after the field is off [9,10]. Both the (ω + 2ω) and the kick mechanisms have received a confirmation from optimal control schemes [11]. The caveat is that the post-pulse orientation is maintained for only short times. Recently, the use of a train of kicks to increase the efficiency of the orientation has been suggested in optimal control strategies [11] and applied to molecular alignment [12] and orientation of a 2D rotor [13]. However, due to the strength of the kicks used, only the efficiency of the process has been optimized, its duration decreasing strongly. In the present letter, we propose a control strategy using specially designed series of kicks * Electronic address: dominique.sugny@u-bourgogne.fr † Electronic address: arne.keller@ppm.u-psud.fr ‡ Electronic address: ddaems@ulb.ac.be delivered by short HCPs, that allows to significantly enhance the duration of the orientation, maintaining a high efficiency. Our construction is first based on the identification of target states which fulfill the previous requirement. These states are characterized by the fact that they only involve a limited number of the lowest lying rotational levels and that they maximize the orientation efficiency within the corresponding restricted rotational spaces. At a second stage, we show that these selected states can be reached by a train of kicks, acting at appropriately chosen times. The choice of the strength of the pulses (taken equal for simplicity), together with the total number of kicks allow to approach these target states with good accuracy. The time evolution of the molecule (described in a 3D rigid rotor approximation) interacting with a linearly polarized field is governed by the time-dependent Schröding...

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We study the control by electromagnetic fields of molecular alignment and orientation, in a linear, rigid rotor model. With the help of a monotonically convergent algorithm, we find that the optimal field is in the microwave part of the spectrum and acts by resonantly exciting the rotation of the molecule progressively from the ground state, i.e., by rotational ladder climbing. This mechanism is present not only when maximizing orientation or alignment, but also when using prescribed target states that simultaneously optimize the efficiency of orientation/alignment and its duration. The extension of the optimization method to consider a finite rotational temperature is also presented.