The wave nature of quantum particles underpins interference phenomena, which can influence chemical reactions by producing oscillatory patterns in product states or angular distributions. While such effects have been observed in several molecular systems, they are typically attributed to interference between spatially distinct paths. Whether interference can occur within a single reaction path—analogous to optical single-slit diffraction—has remained an open question.

To address this knowledge gap, a research team led by Prof. YUAN Kaijun from the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences, in collaboration with researchers from Nanjing University, has revealed quantum interference between direct and indirect reaction pathways in the photodissociation of partially deuterated water (HOD). 

Their findings were recently published in Nature Chemistry.

Experimental results revealed wavelength-dependent rotational state population distributions of the OD(X) product from the photodissociation of HOD molecules at excitation wavelengths around 121 nm.

Full-dimensional quantum calculations semi-quantitatively reproduced the experimental observations and uncovered the underlying mechanism. The results showed that the experimentally observed phenomenon arises from dynamical interferences between direct and indirect dissociation pathways—both of which traverse the same conical intersection (CI) seam at collinear H–OD geometries, analogous to "single-slit diffraction" in optics.

These observed dynamical signatures further demonstrate that interference can occur even within a single reaction path, suggesting a quantum mechanical route to control CI-mediated nonadiabatic dynamics.