Experimental and modeling study of the ion-molecule association reaction H3O++H2O(+M)-> H5O2+(+M)

Abstract

Experimental results for the rate of the association reaction H3O++H2O(+M)-> H5O2+(+M) obtained with the Cinetique de Reactions en Ecoulement Supersonique Uniforme flow technique are reported. The reaction was studied in the bath gases M=He and N-2, over the temperature range of 23-170 K, and at pressures between 0.16 and 3.1 mbar. At the highest temperatures, the reaction was found to be close to the limiting low-pressure termolecular range, whereas the limiting high-pressure bimolecular range was approached at the lowest temperatures. Whereas the low-pressure rate coefficients can satisfactorily be reproduced by standard unimolecular rate theory, the derived high-pressure rate coefficients in the bath gas He at the lowest temperatures are found to be markedly smaller than given by simple ion-dipole capture theory. This result differs from previous observations on the related reaction NH4++NH3(+M)-> N2H7+(+M). This observation is tentatively attributed to more pronounced contributions of the valence part of the potential-energy surface to the reaction in H5O2+ than in N2H7+. Falloff curves of the reaction H3O++H2O(+M)-> H5O2+(+M) are constructed over wide ranges of conditions and represented in compact analytical form. (C) 2005 American Institute of Physics.