  Trains of ultrashort laser pulses separated by the time of rotational revival
(typically, tens of picoseconds) have been exploited for creating ensembles of
aligned molecules. In this work we introduce a chiral pulse train - a sequence
of linearly polarized pulses with the polarization direction rotating from
pulse to pulse by a controllable angle. The chirality of such a train,
expressed through the period and direction of its polarization rotation, is
used as a new control parameter for achieving selectivity and directionality of
laser-induced rotational excitation. The method employs chiral trains with a
large number of pulses separated on the time scale much shorter than the
rotational revival (a few hundred femtosecond), enabling the use of
conventional pulse shapers.
