We investigate experimentally the effect of quantum resonance in therotational excitation of the simplest quantum rotor - a diatomic molecule. Byusing the techniques of high-resolution femtosecond pulse shaping androtational state-resolved detection, we measure directly the amount of energyabsorbed by molecules interacting with a periodic train of laser pulses, andstudy its dependence on the train period. We show that the energy transfer issignificantly enhanced at quantum resonance, and use this effect fordemonstrating selective rotational excitation of two nitrogen isotopologues, $^{14}N_2$ and $ ^{15}N_2$. Moreover, by tuning the period of the pulse train inthe vicinity of a fractional quantum resonance, we achieve spin-selectiverotational excitation of para- and ortho-isomers of $ ^{15}N_2$.
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