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) over bar = 10 in an angular and velocity resolved collision experiment. Th e measured peaks are identified as elastic scattering, rotationally inelast ic scattering of monomers, and vibrational excitation of the clusters. To i nterpret the latter processes quantum calculations are performed for He+(H2 O)(11) collisions using the TDSCF approximation which includes the anharmon ic force field of the water clusters and energy transfer between the modes. By comparison of the calculated and experimental results, the most probabl e excitations correspond to energy transfer for around 7 meV and, with smal ler intensities, up to 20 meV. The excitations correspond to shearing modes of the outer rings and the middle ring of the highly nonrigid cluster agai nst each other. (C) 1999 American Institute of Physics. [S0021-9606(99)0154 6-9].