Abstract
Impulsive orientation of symmetric-top molecules excited by two-color femtosecond pulses is considered. In addition to the well-known transient orientation appearing immediately after the pulse and then reemerging periodically due to quantum revivals, we report the phenomenon of field-free long-lasting orientation. Long-lasting means that the time averaged orientation remains non-zero until destroyed by other physical effects, e.g., intermolecular collisions. The effect is caused by the combined action of the field-polarizability and field-hyperpolarizability interactions. The dependence of degree of long-lasting orientation on temperature and pulse parameters is considered. The effect can be measured by means of second (or higher-order) harmonic generation, and may be used to control the deflection of molecules traveling through inhomogeneous electrostatic fields.
Highlights
Over the years, diverse optical methods have been developed to align and orient molecules of varying complexity and many applications related to studies of molecular and photon-induced processes are based on the ability to control the absolute orientation of the molecules
We have theoretically demonstrated a new phenomenon of longlasting orientation of symmetric-top molecules excited by a single two-color femtosecond pulse
The predictions of the formula are in full agreement with the results of numerical simulations in the limit of weak excitation
Summary
Diverse optical methods have been developed to align and orient molecules of varying complexity and many applications related to studies of molecular and photon-induced processes are based on the ability to control the absolute orientation of the molecules. Laser and THz pulses with twisted polarization were shown to be effective for inducing enantioselective orientation of chiral molecules [27,28,29,30,31,32,33]. The techniques listed above rely on the laser-dipole and/or laser-polarizability interactions Another route to molecular orientation stems from higher-order laser-molecule interactions, e.g., the laser field-hyperpolarizability interaction. The long-lasting orientation induced by a two-color pulse has an intricate dependence on both the molecular polarizability and hyperpolarizability. Related effects have been recently observed in chiral molecules excited by one-color laser pulses with twisted polarization [30, 32] and investigated in non-linear molecules excited by THz pulses [23, 33].
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