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ChemPhysChem

Strong Optical Dipole Force Exerted on Molecules Having Low Rotational Temperature

Xing Nan Sun, Byung Gwun Jin, Lee Yeong Kim, Bong Jun Kim, Bum Suk Zhao

Published Sep 19, 2016• 2 citations
Publisher

Abstract

Abstract The optical dipole force acting on molecules is enhanced by decreasing the rotational temperature of the molecule and aligning the molecular axis with a linearly polarized nonresonant laser beam. The rotational temperature is decreased by increasing the source pressure from 2 to 81 bar. By using the effective polarizability directly pertaining to the optical dipole force, the force and the resulting change in the velocity of the molecules can be evaluated. Theoretical calculations are compared with measurements based on velocity map imaging techniques. If the rotational temperature is reduced from 295 to 1 K, the maximum alignment is increased from <cos2θ>=0.33 to 0.92, and the average optical force is enhanced by 74 %.

DOI
10.1002/cphc.201600838

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Physical Review Letters•Nov 24, 2015•6 citations
Abstract The optical dipole force acting on molecules is enhanced by decreasing the rotational temperature of the molecule and aligning the molecular axis with a linearly polarized nonresonant laser beam. The rotational temperature is decreased by increasing the source pressure from 2 to 81 bar. By using the effective polarizability directly pertaining to the optical dipole force, the force and the resulting change in the velocity of the molecules can be evaluated. Theoretical calculations are compared with measurements based on velocity map imaging techniques. If the rotational temperature is reduced from 295 to 1 K, the maximum alignment is increased from <cos2θ>=0.33 to 0.92, and the average optical force is enhanced by 74 %.
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.
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.
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.
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.
When a molecule is placed in a nonresonant laser field, the Stark interactions between the laser field and the induced molecular dipole result in a mechanical force on the molecule. This nonresonant dipole force is proportional to the intensity gradient of the laser, thus requiring a strong and focused pulsed laser for a sizable impact on the molecule. 36.4 mJ pulses of a 1064 nm Nd:YAG laser focused with a 17.5 cm focal length convex lens produced a 6.4 m/s change in the transverse velocity of a CS2 molecular beam. Using spherical mirrors with shorter focal lengths such as 10.0, 7.5, and 5.0 cm, dipole forces of similar magnitude were obtained with laser pulses of much lower energies. In particular the 5.0-cm focal length spherical mirror provided an 11.3 m/s change in the transverse velocity using 3.6 mJ laser pulses. This corresponds to 18-fold increase in the deflection efficiency, the ratio between the maximum velocity change and the pulse energy. From the improved efficiency, the nonresonant dipole force can be exerted with ease.
We report on the rotational-state-dependent, transverse acceleration of CS_{2} molecules affected by pulsed optical standing waves. The steep gradient of the standing wave potential imparts far stronger dipole forces on the molecules than propagating pulses do. Moreover, large changes in the transverse velocities (i.e., up to 80 m/s) obtained with the standing waves are well reproduced in numerical simulations using the effective polarizability that depends on the molecular rotational states. Our analysis based on the rotational-state-dependent effective polarizability can therefore serve as a basis for developing a new technique of state selection for both polar and nonpolar molecules.