Fig. 7. The potential energy surfaces of CHD allow for reversion to the ground state of the reactant molecule, favored when the wavepacket is close to the plane of symmetry, or ring opening to the HT product molecule, favored when the wavepacket is off symmetry.

The path through the 2A/1B CIs can either deflect the wavepacket away from the symmetry plane (1B excitation, leading preferentially to the HT product) or focus it onto the symmetry plane (3p_x,y excitation, leading preferentially to the hot CHD).