Quantum interference between direct and indirect reaction paths in the photodissociation of HOD

The wave-like nature of quantum particles enables interference effects that can influence the outcomes of chemical reactions; however, most have been observed to occur from two spatially distinct reaction pathways, analogous to Young's double-slit experiment. Here we report detailed experiments on HOD photodissociation that reveal excitation-wavelength-dependent variations in the rotational state distribution of OD(X) product fragments. Full-dimensional quantum calculations reproduce the experimental observations semi-quantitatively, and our analysis attributes this behaviour to dynamical interferences between direct and indirect dissociation paths that both traverse the same conical intersection seam at collinear H-OD geometries, an interference analogous to 'single-slit diffraction' in optics. These observed dynamical signatures demonstrate that interference can manifest even within a single reaction pathway, suggesting a quantum mechanical means to control conical intersection-mediated nonadiabatic dynamics.