Reactive and inelastic collisions of H atoms with vibrationally excited water molecules

The dynamics of the collisions of H atoms with vibrationally excited H2O we re studied using classical mechanical reactive and quantum mechanical nonre active scattering calculations. The classical trajectory calculations were performed with the I5 potential surface of Isaacson. These results show the expected behavior for an endoergic reaction with a late barrier, with the cross section exhibiting a high threshold when the water is unexcited, and a much lower threshold if the asymmetric stretch of water is highly excited . Qualitatively this matches experimental results, although the threshold e nergy for reaction of water in the ground vibrational state is too low to r eproduce the measured rate coefficients. The rate coefficient is higher tha n for ground state water by six orders of magnitude when the asymmetric str etch mode is excited by four quanta. However the rate for reaction from thi s excited state is still two orders of magnitude smaller than the total rea ctive+inelastic rate coefficient obtained in recent measurements by Smith a nd co-workers. Quantum scattering calculations of the vibrational energy tr ansfer rate coefficients show that the pure stretch excited states can have very high deactivation rate coefficients, resulting from transitions to st ates that are separated by a small energy gap (<50 cm(-1)) from the initial state. The calculated rate coefficients for reactive+inelastic transitions are therefore dominated by inelastic scattering, and the results we obtain are in good agreement with the Smith data. (C) 1999 American Institute of Physics. [S0021-9606(99)00106-3].