  We investigate experimentally the effect of quantum resonance in the
rotational excitation of the simplest quantum rotor - a diatomic molecule. By
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 its dependence on the train period. We show that the energy transfer is
significantly enhanced at quantum resonance, and use this effect for
demonstrating selective rotational excitation of two nitrogen isotopologues, $
^{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 spin-selective
rotational excitation of para- and ortho-isomers of $ ^{15}N_2$.
