Low-Energy Water-Hydrogen Inelastic Collisions

New molecular beam scattering experiments are reported for the water-hydrogen system. Integral cross sections of the first rotational excitations of para- and ortho-H₂O by inelastic collisions with normal-H₂ were determined by crossing a beam of H₂O seeded in He with a beam of H₂. H₂O and H₂ 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 H₂O: 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 H₂O-H₂ 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.