  Control over various fragmentation reactions of a series of polyatomic
molecules (acetylene, ethylene, 1,3-butadiene) by the optical waveform of
intense few-cycle laser pulses is demonstrated experimentally. We show both
experimentally and theoretically that the responsible mechanism is inelastic
ionization from inner-valence molecular orbitals by recolliding electron
wavepackets, whose recollision energy in few-cycle ionizing laser pulses
strongly depends on the optical waveform. Our work demonstrates an efficient
and selective way of pre-determining fragmentation and isomerization reactions
in polyatomic molecules on sub-femtosecond time-scales.
