Quantum Interference Between H(1s)+H(2s) and H(1s)+H(2p) Channels in Photodissociation of H₂(J″=0)

The photodissociation of H₂ near its second dissociation threshold yields two competing channels: H(1s) + H(2s) and H(1s) + H(2p). While cosine oscillations in the branching ratio, signatures of quantum interference between the two pathways, were previously observed in HD and D₂, they remained undetected in H₂ due to the rotational state distribution of conventional molecular beams. Here, we isolate the ground rotational state (J″ = 0) of para-H₂ and combine delay-time-curve measurements with velocity map imaging to directly resolve the branching ratios, confirming universal quantum interference across all isotopologues. The oscillations are quantitatively described by a p-wave scattering model with effective spherical potentials, where fitted parameters for H₂ agree with those derived from HD, D₂, and ab initio calculations. The interference arises from a phase difference between two dissociation channels, which is determined by the product of the fragment wavevector and the width difference between the two effective potentials. These results establish quantum interference as a fundamental driver of molecular dissociation and provide benchmark data for refining theoretical models of multichannel dynamics.