Controlling the mechanism of fulvene S1/S0 decay: switching off the stepwise population transferElectronic supplementary information (ESI) available: Cartesian coordinates and energies of all optimised geometries; frequency-mass-weighted components of the initial momentum vector giving the directions of the induced geometrical displacements; animations of the trajectory followed by the centre of the wavepacket for: (a) S1/S0 natural planar decay, (b) S1/S0 controlled decay preventing the initial

Direct quantum dynamics simulations were performed to model the radiationless decay of the first excited state S 1 of fulvene. The full space of thirty normal mode nuclear coordinates was explicitly considered. By default, ultrafast internal conversion takes place centred on the higher-energy planar region of the S 1 /S 0 conical intersection seam, giving the stepwise population transfer characteristic of a sloped surface crossing, and leading back to the ground state reactant. Two possible schemes for controlling whether stepwise population transfer occurs or not-either altering the initial geometry distribution or the initial momentum composition of the photo-excited wavepacket-were explored. In both cases, decay was successfully induced to occur in the lower-energy twisted/peaked region of the crossing seam, switching off the stepwise population transfer. This absence of re-crossing is a direct consequence of the change in the position on the intersection at which decay occurs (our target for control), and its consequences should provide an experimentally observable fingerprint of this system. No stepwise population transfer (re-crossing character) should be observed if controlled decay in the twisted region of the S 1 /S 0 conical intersection seam is successfully photo-induced as suggested here.