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The Journal of Chemical Physics

Shaping molecular beams with intense light

Tamar Seideman

Published Dec 22, 1997• 61 citations
Publisher

Abstract

We discuss the possibility of manipulating the center-of-mass motion of molecules using the nonlinear interaction of a moderately intense, long-pulse laser field with the molecular polarizability tensor. Recent theoretical work demonstrating the possibility of focusing and trapping molecules is extended to consider the effects of circularly and elliptically polarized light and the effect of nonspherical laser optics. The aberrations affecting the quality of the “molecular lens” are analyzed and the means by which they can be minimized are discussed. Molecular focusing is extended to a general field of molecular optics; the possibilities of steering, reflecting, and collimating molecular beams are illustrated. Application of the mechanical force of light to disperse and separate species according to their mass, velocity, or quantum state is proposed.

DOI
10.1063/1.474206

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Physical Review Letters•Oct 5, 2009•48 citations
We discuss the possibility of manipulating the center-of-mass motion of molecules using the nonlinear interaction of a moderately intense, long-pulse laser field with the molecular polarizability tensor. Recent theoretical work demonstrating the possibility of focusing and trapping molecules is extended to consider the effects of circularly and elliptically polarized light and the effect of nonspherical laser optics. The aberrations affecting the quality of the “molecular lens” are analyzed and the means by which they can be minimized are discussed. Molecular focusing is extended to a general field of molecular optics; the possibilities of steering, reflecting, and collimating molecular beams are illustrated. Application of the mechanical force of light to disperse and separate species according to their mass, velocity, or quantum state is proposed.
The possibility of focusing and trapping molecules in the intensity gradient of a moderately intense laser field is shown. A fully quantum-mechanical and a hybrid quantum-classical model are developed and both the case of a near-resonance and that of a far-off-resonance field are studied numerically and analytically. Molecular focusing is obtained when the ratio between the kinetic energy and the field-matter interaction exceeds unity (but remains small). In this energy regime the laser beam serves as a lens for a beam of molecules. The properties of the “molecular lens” are analyzed and the experimental conditions required to focus molecules are examined. We propose the application of laser focusing to nanoscale deposition onto, and etching of a substrate and describe a possible setup to that end. Molecular trapping is obtained as the strong interaction limit of a general “molecular optics” scheme.
We develop a general framework for describing the interaction of molecules with a nonperturbative intensity gradient and note several limits, including the two-level Hamiltonian of atom trapping and the quasistatic Hamiltonian of molecular optics. The theory is used to propose a new molecular optics scheme which complements previous work in the area of spatial manipulation of molecules in intense fields by introducing the possibility of reflecting molecules with light. Repulsive interaction obtains in the limit of small level spacings as compared to the field frequency, attained by exciting the molecules to high Rydberg states. Some of the potential applications of the repulsive interaction are illustrated and others are noted. We address the problem of molecular alignment in intense laser fields and describe the possibility of simultaneously focussing the center-of-mass motion of molecules and aligning their figure axis after exit from the laser focus, under field-free conditions.
We measure the role of field-induced alignment on the dipole force for molecules using a single focused nonresonant laser beam. We show that through the alignment process we can modify the effective polarizability by field polarization and thus control the center-of-mass motion of the molecule. We observe a maximum change of 2020%20 in the dipole force on CS2{}_{2}2​ molecules when changing from linearly to circularly polarized light. Additionally, the effect of the dipole force on different vibrational states is also studied.
The optical dipole force from a singe focussed laser beam was used to study the role \nof laser-induced molecular alignment on the centre-of-mass motion of carbon disulphide molecules in a molecular beam. The translational, rotational and vibrational \ntemperatures of the carbon disulphide molecules were measured to be 3.4\\pm0.2 K, 35\\pm10 K and \n250\\pm14 K respectively. The velocity of the beam was measured to be 542\\pm22 m s ^{-1}. \nTime-of-flight mass spectroscopy was used to measure the acceleration and deceleration of the molecules. Maximum velocity changes of 7.5 m s ^{-1} and 10 m s ^{-1} were \nrecorded for linearly and circularly polarised light respectively. These results showed \nthat the dipole force, \\digamma \\alpha \\bigtriangledown [\\alpha_e_f_f(I)I(r)], where \\alpha_e_f_f is the effective polarisability and determined through laser-induced alignment, can be modified by changing the \nlaser polarisation. For linearly and circularly polarised light, a 12% difference in effective polarisability was measured to produce a 20% difference in dipole force. \nThe dipole force from a single focussed laser beam produces a molecular optical lens and the downstream density of the molecular focus was probed by measuring the \nion signal for both laser polarisations. The focal lengths for linearly and circularly polarised light were found to be separated by \\approx 100 \\mu m. By altering the laser polarisation from linearly through elliptically to circularly polarised light, the focal length \nof the molecular optical lens could be smoothly altered over the \\approx 100 \\mu m focal range. \nThe role of the effective polarisability of each rotational state was also studied \nnumerically. Separate rotational states were found to significantly alter the focal properties of a molecular optical lens. In carbon disulphide, higher rotational states (J > 10), exhibit less molecular alignment and when occupied, the focal length of the molecular \noptical lens for these states was increased by 60 % compared to the ground state.
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.
We report on the ability to tailor the optical dipole force for molecules by tuning their effective polarizability with strong field alignment using polarized fields. We have measured a difference of 20% in the dipole force on cold CS2 molecules when changing from linear to near-circular polarization using peak field intensities of 5.7x10(11) W cm(-2). A variation in the focal length with laser polarization of a molecular-optical lens formed by a single focused laser beam was also measured. This provides a new way of modifying this force for many molecules.