A state-selected study of the ion–molecule reactions O+(2D,2P)+H2O

Abstract

State-selected absolute cross sections for H2O+ and OH+ formed by the O+(2D,2P)+H2O reactions have been measured in the center-of-mass collision energy (Ec.m.) range of ≈0.10–30 eV. The charge transfer cross sections for O+(2D)+H2O are significantly higher than those for O+(4S)+H2O. This observation is attributed to the increased number of accessible exothermic product channels for O+(2D)+H2O. While the H2O+ cross sections for O+(2P)+H2O are comparable to those from O+(4S)+H2O at Ec.m.⩾1 eV, the H2O+ cross sections for O+(2P)+H2O at Ec.m.<1 eV are substantially lower than those for O+(4S)+H2O. The lower H2O+ cross sections observed for O+(2P)+H2O are rationalized as due to further dissociation of excited charge transfer H2O+ ions and/or the efficient competition of the OH++OH product channel. The cross sections for OH+ from O+(2D,2P)+H2O are significantly greater than those from O+(4S)+H2O. The majority of OH+ ions from O+(2D,2P)+H2O are associated with exothermic channels corresponding to the formation OH+(X 3Σ−,1Δ,A 3Π)+OH. The comparison of the sum (σT) of the cross sections for H2O+ and OH+ from O+(4S)+H2O to those from O+(2D)+H2O and O+(2P)+H2O shows that σT’s for O+(4S)+H2O and O+(2P)+H2O are comparable, whereas the σT values for O+(2D)+H2O are greater than those for O+(4S)+H2O and O+(2P)+H2O. The σT values are found to conform with the 1/Ec.m. dependence at low Ec.m.’s, indicating that the ion–dipole interaction plays an important role in the formation of the long-lived collision complexes. The high cross sections for H2O+ and OH+ from O+(2D,2P)+H2O observed here suggest that these reactions should be included in the simulation of the H2O+ and H3O+ ion density data obtained in space-borne mass spectrometric experiments.