Theory has established the importance of geometric phase (GP) effects in the adiabatic dynamics of molecular systems with a conical intersection connecting the ground- and excited-state potential energy surfaces, but direct observation of their manifestation in chemical reactions remains a major challenge. Here, we report a high-resolution crossed molecular beams study of the H + HD → H2+ D reaction at a collision energy slightly above the conical intersection. Velocity map ion imaging revealed fast angular oscillations in product quantum state–resolved differential cross sections in the forward scattering direction for H2products at specific rovibrational levels. The experimental results agree with adiabatic quantum dynamical calculations only when the GP effect is included.

Accurate measurements of product state-resolved angular distributions are central to fundamental studies of chemical reaction dynamics. Yet, fine quantum-mechanical structures in product angular distributions of a reactive scattering process, such as the fast oscillations in the forward-scattering direction, have never been observed experimentally and the nature of these oscillations has not been fully explored. Here we report the crossed-molecular-beam experimental observation of these fast forward-scattering oscillations in the product angular distribution of the benchmark chemical reaction, H + HD → H + D. Clear oscillatory structures are observed for the H(v' = 0, j' = 1, 3) product states at a collision energy of 1.35 eV, in excellent agreement with the quantum-mechanical dynamics calculations. Our analysis reveals that the oscillatory forward-scattering components are mainly contributed by the total angular momentum J around 28. The partial waves and impact parameters responsible for the forward scatterings are also determined from these observed oscillations, providing crucial dynamics information on the transient reaction process.

We have calculated the rovibronic structures which arise from the simultaneous radial and angular couplings among the lowest nine excited 1Σ+g, 1Πg, and 1Δg states in H2, D2, and T2. Using ab initio potential curves, adiabatic corrections, and nonadiabatic coupling functions wherever possible, we have solved the coupled vibronic equations for all bound states below the 1s+2l dissociation limit up to J=5 in H2 and D2, and up to J=7 in T2. The good agreement of the bulk of the data with spectroscopic results enables us to eliminate spurious assignments in old spectroscopic work (e.g., all old assignments involving the 4s O 1Σ+g state are spurious), and to make numerous new assignments. We present updated tables of spectroscopic term values for H2 and D2. The absolute energy errors of the ab initio Born–Oppenheimer energies around the minima of the potential curves appear to be at most of the order of the radiative energy corrections (≂0.1 cm−1) in the states 1σg2sσg E and (1σu)2 F, less than 1 cm−1 in the states 3dσ G, 3s H, and 3dπ I, less than 2 cm−1 in the 4s O state, and approximately 4 cm−1 in the 4dσ P state.