Vibrational excitation in He+(H₂O)₁₁ collisions: Quantum calculations and experiment

Helium atoms are scattered from a beam of water clusters with mean size n = 10 in an angular and velocity resolved collision experiment. The measured peaks are identified as elastic scattering, rotationally inelastic scattering of monomers, and vibrational excitation of the clusters. To interpret the latter processes quantum calculations are performed for He-(H₂O)₁₁ collisions using the TDSCF approximation which includes the anharmonic force field of the water clusters and energy transfer between the modes. By comparison of the calculated and experimental results, the most probable excitations correspond to energy transfer for around 7 meV and, with smaller intensities, up to 20 meV. The excitations correspond to shearing modes of the outer rings and the middle ring of the highly nonrigid cluster against each other.