Controlling Product Selection in the Photodissociation of Formaldehyde: Direct Quantum Dynamics from the S₁ Barrier

Controlling the selectivity between H₂+CO and H+HCO in the S₁/S₀ nonadiabatic photodissociation of formaldehyde has been investigated using direct quantum dynamics. Simulations started from the S₁ transition state have suggested that a key feature for controlling the branching ratio of ground-state products is the size of the momentum given to the wavepacket along the transition vector. Our results show that letting the wavepacket fall down from the barrier to the conical intersection with no initial momentum leads to H₂+CO, while extra momentum toward products favors the formation of H+HCO through the same conical intersection. Quantum dynamics results are interpreted in semiclassical terms with the aid of a Mulliken-like analysis of the final population distribution among both products and the reactant on each electronic state.