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Register a new userMaking the best of mixed-field orientation of polar molecules: A recipe for achieving adiabatic dynamics in an electrostatic field combined with laser pulses
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Added by
Jens Hedegaard Nielsen
Publication date
2012
fields
Physics
and research's language is
English
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We have experimentally and theoretically investigated the mixed-field orientation of rotational-state-selected OCS molecules and we achieve strong degrees of alignment and orientation. The applied moderately intense nanosecond laser pulses are long enough to adiabatically align molecules. However, in combination with a weak dc electric field, the same laser pulses result in nonadiabatic dynamics in the mixed-field orientation. These observations are fully explained by calculations employing, both, adiabatic and non-adiabatic time-dependent models.
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The mixed-field orientation of an asymmetric-rotor molecule with its permanent dipole moment non-parallel to the principal axes of polarizability is investigated experimentally and theoretically. We find that for the typical case of a strong, nonresonant laser field and a weak static electric field complete 3D orientation is induced if the laser field is elliptically polarized and if its major and minor polarization axes are not parallel to the static field. For a linearly polarized laser field solely the dipole moment component along the most polarizable axis of the molecule is relevant resulting in 1D orientation even when the laser polarization and the static field are non parallel. Simulations show that the dipole moment component perpendicular to the most-polarizable axis becomes relevant in a strong dc electric field combined with the laser field. This offers an alternative approach to 3D orientation by combining a linearly-polarized laser field and a strong dc electric field arranged at an angle equal to the angle between the most polarizable axis of the molecule and its permanent dipole moment.
The induced polarization of a beam of polar clusters or molecules passing through an electric or magnetic field region differs from the textbook Langevin-Debye susceptibility. This distinction, which is important for the interpretation of deflection and focusing experiments, arises because instead of acquiring thermal equilibrium in the field region, the beam ensemble typically enters the field adiabatically, i.e., with a previously fixed distribution of rotational states. We discuss the orientation of rigid symmetric-top systems with a body-fixed electric or magnetic dipole moment. The analytical expression for their adiabatic-entry orientation is elucidated and compared with exact numerical results for a range of parameters. The differences between the polarization of thermodynamic and adiabatic-entry ensembles, of prolate and oblate tops, and of symmetric-top and linear rotators are illustrated and identified.
Molecular chirality is an omnipresent phenomenon of fundamental significance in physics, chemistry and biology. For this reason, search for novel techniques for enantioselective control, detection and separation of chiral molecules is of particular importance. It has been recently predicted that laser fields with twisted polarization may induce persistent enantioselective field-free orientation of chiral molecules. Here we report the first experimental observation of this phenomenon using propylene oxide molecules ($mathrm{CH_{3}CHCH_{2}O}$, or PPO) spun by an optical centrifuge - a laser pulse, whose linear polarization undergoes an accelerated rotation around its propagation direction. We show that PPO molecules remain oriented on a time scale exceeding the duration of the centrifuge pulse by several orders of magnitude. The demonstrated long-time field-free enantioselective orientation opens new avenues for optical manipulation, discrimination, and, potentially, separation of molecular enantiomers.
We report the first experimental observation of non-adiabatic field-free orientation of a heteronuclear diatomic molecule (CO) induced by an intense two-color (800 and 400 nm) femtosecond laser field. We monitor orientation by measuring fragment ion angular distributions after Coulomb explosion with an 800 nm pulse. The orientation of the molecules is controlled by the relative phase of the two-color field. The results are compared to quantum mechanical rigid rotor calculations. The demonstrated method can be applied to study molecular frame dynamics under field-free conditions in conjunction with a variety of spectroscopy methods, such as high-harmonic generation, electron diffraction and molecular frame photoemission.
The transition between two distinct mechanisms for the laser-induced field-free orientation of CO molecules is observed via measurements of orientation revival times and subsequent comparison to theoretical calculations. In the first mechanism, which we find responsible for the orientation of CO up to peak intensities of 8 x 10^13 W/cm^2, the molecules are impulsively oriented through the hyperpolarizability interaction. At higher intensities, asymmetric depletion through orientation-selective ionization is the dominant orienting mechanism. In addition to the clear identification of the two regimes of orientation, we propose that careful measurements of the onset of the orientation depletion mechanism as a function of the laser intensity will provide a relatively simple route to calibrate absolute rates of non-perturbative strong-field molecular ionization.
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