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arXiv:1006.1734 · Physical Review A

Controlling molecular scattering by laser-induced field-free alignment

Erez Gershnabel, Ilya Sh. Averbukh

Published Sep 2, 2010• 31 citations
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Abstract

We consider deflection of polarizable molecules by inhomogeneous optical fields and analyze the role of molecular orientation and rotation in the scattering process. It is shown that molecular rotation induces spectacular rainbowlike features in the distribution of the scattering angle. Moreover, by preshaping molecular angular distribution with the help of short and strong femtosecond laser pulses, one may efficiently control the scattering process, manipulate the average deflection angle and its distribution, and reduce substantially the angular dispersion of the deflected molecules. We study the problem both classically and quantum mechanically and arrive at the same conclusions in both treatments. The effects of strong deflecting field on the scattering of rotating molecules are considered by the means of the adiabatic invariants formalism. The suggested control scheme opens new ways for many applications involving molecular focusing, guiding, and trapping by optical and static fields.

DOI
10.1103/physreva.82.033401

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We consider deflection of polarizable molecules by inhomogeneous optical fields and analyze the role of molecular orientation and rotation in the scattering process. It is shown that molecular rotation induces spectacular rainbowlike features in the distribution of the scattering angle. Moreover, by preshaping molecular angular distribution with the help of short and strong femtosecond laser pulses, one may efficiently control the scattering process, manipulate the average deflection angle and its distribution, and reduce substantially the angular dispersion of the deflected molecules. We study the problem both classically and quantum mechanically and arrive at the same conclusions in both treatments. The effects of strong deflecting field on the scattering of rotating molecules are considered by the means of the adiabatic invariants formalism. The suggested control scheme opens new ways for many applications involving molecular focusing, guiding, and trapping by optical and static fields.
We consider deflection of polarizable molecules by inhomogeneous optical fields, and analyze the role of molecular orientation and rotation in the scattering process. We show that by preshaping molecular angular distribution with the help of short and strong femtosecond laser pulses, one may efficiently control the scattering process, manipulate the average deflection angle and its distribution, and reduce substantially the angular dispersion of the deflected molecules. This opens new ways for many applications involving molecular focusing, guiding, and trapping by optical and static fields.
We consider deflection of rotating symmetric top molecules by inhomogeneous optical and static electric fields, compare results with the case of linear molecules, and find new singularities in the distribution of the scattering angle. Scattering of the prolate/oblate molecules is analyzed in detail, and it is shown that the process can be efficiently controlled by means of short and strong femtosecond laser pulses. In particular, the angular dispersion of the deflected molecules may be dramatically reduced by laser-induced molecular prealignment. We first study the problem by using a simple classical model, and then find similar results by means of more sophisticated methods, including the formalism of adiabatic invariants and direct numerical simulation of the Euler-Lagrange equations of motion. The suggested control scheme opens new ways for many applications involving molecular focusing, guiding, and trapping by optical and static fields.
Several laser techniques have been suggested and demonstrated recently for preparing polarizable molecules in rapidly spinning states with a disk-like angular distribution. We consider motion of these spinning disks in inhomogeneous fields and show that the molecular trajectories may be precisely controlled by the tilt of the plane of the laser-induced rotation. The feasibility of the scheme is illustrated by optical deflection of linear molecules twirled by two delayed cross-polarized laser pulses. These results open new ways for many applications involving molecular focusing, guiding, and trapping and may be suitable for separating molecular mixtures by optical and static fields.
Summary form only given. Optical manipulation of atoms is a well-developed and established field. Its successful use of resonant processes is difficult to mimic for molecules due to the complexity of the molecular energy spectra. However, intense non-resonant fields can provide forces similar to or even stronger than the resonant weak fields used in atom optics. These forces, long utilized in the manipulation of microscopic particles, were recently demonstrated for molecules in optical deflection and trapping experiments. We propose to use strong infrared laser field to create an "optical twister" a field that would exert very large optical torques on anisotropic molecules, leading to a controlled molecular rotation induced with a simple pulse. Molecular dissociation via rotations is used to demonstrate our method. The scheme distinguishes molecules with different moments of inertia, and thus acts as an optical centrifuge.
We introduce a new scheme for controlling the sense of molecular rotation. By varying the polarization and the delay between two ultrashort laser pulses, we induce unidirectional molecular rotation, thereby forcing the molecules to rotate clockwise/counterclockwise under field-free conditions. We show that unidirectionally rotating molecules are confined to the plane defined by the two polarization vectors of the pulses, which leads to a permanent anisotropy in the molecular angular distribution. The latter may be useful for controlling collisional cross-sections and optical and kinetic processes in molecular gases. We discuss the application of this control scheme to individual components within a molecular mixture in a selective manner.
The properties of molecule-optical elements such as lenses or prisms based on the interaction of molecules with optical fields depend in a crucial way on the molecular quantum state and its alignment created by the optical field. Herein, we consider the effects of state-dependent alignment in estimating the optical dipole force acting on the molecules and, to this end, introduce an effective polarizability which takes proper account of molecular alignment and is directly related to the alignment-dependent optical dipole force. We illustrate the significance of including molecular alignment in the optical dipole force by a trajectory study that compares previously used approximations with the present approach. The trajectory simulations were carried out for an ensemble of linear molecules subject to either propagating or standing-wave optical fields for a range of temperatures and laser intensities. The results demonstrate that the alignment-dependent effective polarizability can serve to provide correct estimates of the optical dipole force, on which a state-selection method applicable to nonpolar molecules could be based. We note that an analogous analysis of the forces acting on polar molecules subject to an inhomogeneous static electric field reveals a similarly strong dependence on molecular orientation.