  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's 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.
