  We have studied the photo-induced isomerization from 1,3-cyclohexadiene to
1,3,5-hexatriene in the presence of an intense ultrafast laser pulse. We find
that the laser field maximally suppresses isomerization if it is both polarized
parallel to the excitation dipole and present 50 fs after the initial
photoabsorption, at the time when the system is expected to be in the vicinity
of a conical intersection that mediates this structural transition. A modified
ab initio multiple spawning (AIMS) method shows that the laser induces a
resonant coupling between the excited state and the ground state, i.e., a
light-induced conical intersection. The theory accounts for the timing and
direction of the effect.
