Low-Energy Water–Hydrogen Inelastic Collisions

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New molecular beam scattering experiments are reported for the water–hydrogen system. Integral cross sections of the first rotational excitations of para - and ortho -H 2 O by inelastic collisions with normal -H 2 were determined by crossing a beam of H 2 O seeded in He with a beam of H 2 . H 2 O and H 2 were cooled in the supersonic expansion down to their lowest rotational levels. Crossed-beam scattering experiments were performed at collision energies from 15 cm –1 (below the threshold for the excitation to the lowest excited rotational state of H 2 O: 18.6 cm –1 ) up to 105 cm –1 by varying the beam crossing angle. The measured state-to-state cross-sections were compared to the theoretical cross-sections (close-coupling quantum scattering calculations): the good agreement found further validates both the employed potential energy surface describing the H 2 O–H 2 van der Waals interaction and the state-to-state rate coefficients calculated with this potential in the very low temperature range needed for the modeling of interstellar media.