Near-Threshold and Resonance Effects in Rotationally Inelastic Scattering of D 2 O with Normal -H 2 .

We present a combined experimental and theoretical study on the rotationally inelastic scattering of heavy water, D 2 O, with normal -H 2 . Crossed-molecular beam measurements are performed in the collision energy range between 10 and 100 cm -1 , corresponding to the near-threshold regime in which scattering resonances are most pronounced. State-to-state excitation cross-sections are obtained by probing three low-lying rotational levels of D 2 O using the REMPI technique. These measurements are complemented by quantum close-coupling scattering calculations based on a high-accuracy D 2 O-H 2 interaction potential. The agreement between experiment and theory is within the experimental error bars at 95% confidence intervals, leading to a relative difference of less than 7%: the near-threshold rise and the overall shape of the cross-sections, including small undulations due to resonances, are nicely reproduced by the calculations. Isotopic effects (D 2 O versus H 2 O) are also discussed by comparing the shape and magnitude of the respective cross-sections.