  Following excitation by a strong ultra-short laser pulse, molecules develop
coordinated rotational motion, exhibiting transient alignment along the
direction of the laser electric field, followed by periodic full and fractional
revivals that depend on the molecular rotational constants. In mixtures, the
different species undergo similar rotational dynamics, all starting together
but evolving differently with each demonstrating its own periodic revival
cycles. For a bimolecular mixture of linear molecules, at predetermined times,
one species may attain a maximally aligned state while the other is
anti-aligned (i.e. molecular axes are confined in a plane perpendicular to the
laser electric field direction). By a properly timed second laser pulse, the
rotational excitation of the undesired species may be almost completely removed
leaving only the desired species to rotate and periodically realign, thus
facilitating further selective manipulations by polarized light. In this paper,
such double excitation schemes are demonstrated for mixtures of molecular
isotopes (isotopologues) and for nuclear spin isomers.
