Imaging the inelastic scattering of water with helium. Comparison of experiment and theory.

State-to-state differential cross sections for rotationally inelastic He-H(2)O scattering have been measured at 53.3 meV (429 cm(-1)) collision energy, using the crossed molecular beam technique. The inelastic events are detected by velocity map imaging of nascent H(2)O(+) ions, which are formed by state-selective (2 + 1) resonance enhanced multiphoton ionization (REMPI) of the scattered H(2)O molecules. Raw density images are converted to flux images and the extracted differential cross sections are compared with full close-coupling calculations of state-to-state cross sections for rotational excitation based on a previously published ab initio potential. A hard-shell ellipsoid model is also employed to yield a more physical insight useful in interpreting the results. The excellent agreement of fully quantum theory and experiment found here for water collisions with helium at a collision energy relevant to that of the interstellar media place the theoretically determined potential energy surface and the collision cross sections extracted using this surface on a firmer basis.