  Nonradiative energy dissipation in electronically excited polyatomic
molecules proceeds through conical intersections, loci of degeneracy between
electronic states. We observe a marked enhancement of laser-induced double
ionization in the vicinity of a conical intersection during a non-radiative
transition. We measured double ionization by detecting the kinetic energy of
ions released by laser-induced strong-field fragmentation during the
ring-opening transition between 1,3-cyclohexadiene and 1,3,5-hexatriene. The
enhancement of the double ionization correlates with the conical intersection
between the HOMO and LUMO orbitals.
