Sideways scattering oscillations reveal geometric phase effect and isotope effect in the H + D2 → HD + D reaction.

Quantum interference between reaction pathways plays a crucial role in understanding the microscopic mechanisms of chemical reactions. The hydrogen exchange reaction, involving a well-characterized conical intersection, exhibits rich dynamics arising from interference between multiple reactive pathways. Here, we report high-resolution scattering measurements at a collision energy of 2.38 eV, which reveal pronounced sideways angular oscillations in the HD product distribution. Together with exact quantum theoretical calculations, these results provide unambiguous evidence of geometric phase-induced quantum interference in the H + D2 reaction. Detailed analysis reveals that the observed sideways scattering oscillations, hallmarks of quantum interference, are strongly influenced by transient D-D bond elongation in D2 reactants. This behavior, in contrast to the reaction with HD, highlights a pronounced isotopic effect and reflects the unique dynamic competition along the roaming insertion pathway. Collectively, these findings underscore the critical role of pathway interference and provide new mechanistic insights into how the isotopic effect modulated the geometric phase in an elementary chemical reaction.