Quantum resonances in selective rotational excitation of molecules with a sequence of ultrashort laser pulses

We experimentally investigate the effect of quantum resonance in the rotational excitation of the simplest quantum rotor--a diatomic molecule. Using the techniques of high-resolution femtosecond pulse shaping and rotational state-resolved detection, we measure directly the amount of energy absorbed by molecules interacting with a periodic train of laser pulses, and study their dependence on the train period. We show that the energy transfer is significantly enhanced at quantum resonance, and use this effect to demonstrate selective rotational excitation of two nitrogen isotopologs, (14)N(2) and (15)N(2). Moreover, by tuning the period of the pulse train in the vicinity of a fractional quantum resonance, we achieve selective rotational excitation of para- and ortho-isomers of (15)N(2).