Ion-Molecule collisional Energy transfer I: Proton-Water interaction and Rotational excitations

The intermolecular potential between the ground electronic state of H2O (1A1) and the H+ projectile is obtained from SCF calculations of near-Hartree-Fock quality for the rigid rotor (RR) target molecule and within the Born-Oppenheimer (BO) approximation. The symmetry-adapted, multipolar expansion of the potential energy surface (PES) produces the leading coefficients which are further extrapolated via the perturbation expressions for the induction and polarization forces in the long-range region. The rotationally inelastic collision processes for this asymmetric top rotor are computed, at the energies examined in recent molecular beam experiments, in order to estimate the contribution of (T, R) energy transfer in the direct channels. The quantum dynamics was treated within the infinite-order-sudden (IOS) approximation and both rotationally summed and partial inelastic cross sections are analysed and discussed. The present computations suggest that the contribution of rotationally inelastic collisions under the experimental conditions of the target beam cannot be considered negligible and should significantly affect the shape of the time-of-flight, vibrationally inelastic peaks. Significant differences are, however, found with the behaviour of the same process in proton collision with HF molecules, in spite of the latter being polar diatomics with a very similar value for the dipole moment.