Scattering resonances in the rotational excitation of HDO by Ne and normal-H2: theory and experiment†

The rotational excitation of a singly deuterated water molecule (HDO) by a heavy atom (Ne) and a light diatomic molecule (H₂) is investigated theoretically and experimentally in the near-threshold regime. Crossed-molecular-beam measurements with a variable crossing angle are compared to close-coupling calculations based on high-accuracy potential energy surfaces. The two lowest rotational transitions, 0₀₀ → 1₀₁ and 0₀₀ → 1₁₁, are probed in detail and a good agreement between theory and experiment is observed for both transitions in the case of HDO + Ne, where scattering resonances are however blurred out experimentally. In the case of HDO + H₂, the predicted theoretical overlapping resonances are faithfully reproduced by experiment for the 0₀₀ → 1₁₁ transition, while the calculated strong signal for the 0₀₀ → 1₀₁ transition is not detected. Future work is needed to reconcile this discrepancy.