Reaching optimally oriented molecular states by laser kicks

Abstract

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...

Bell-Type Inequalities for Cold Heteronuclear Molecules

“…(3). This result can be very useful in the realization of an experiment, and it is a consequence of two facts: high orientation (high values of λ max ) can be obtained in reduced angular momentum subspaces [22] and entanglement enhances two particle orientation correlations [13].…”

Control of mixed-state quantum systems by a train of short pulses

“…From the physical point of view, this reduction can be qualitatively justified by the fact that moderate perturbations, i.e. a finite number of applied pulses with moderate amplitude, can only transfer finite amounts of energy to the system, which stays thus essentially confined in a finite-dimensional subspace [11]. This reduction drastically simplifies the study of both kinematical constraints and dynamical realizability as we will show below.…”

“…This has roughly an overall similitude with the scheme followed for a pure quantum state [11,12], but the two approaches have to be contrasted at the level of the target state. In the pure-state case, it is a specific linear combination of rotational states, constituting a wavepacket evolving through the Schrödinger equation.…”

“…The specific properties of the target density matrix cannot be inferred from a Boltzmann superposition of wavefunctions, separately time-propagated, i.e. it cannot be obtained from the analysis of Refs [11,12].…”

“…We have recently proposed a laser control strategy [11,12], for a molecular system initially in a pure quantum state, aiming at the maximalization (or minimalization) of the expectation value of a given observable, which does not commute with the field-free Hamiltonian. This control scheme consists in applying sudden pulses each time a given quantity (such as the expectation value of the observable under consideration) reaches its maximum.…”