Selective Rotational Excitation of Molecular Isotopes and Nuclear Spin Isomers

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.