Production and loss of the water-related species H3O+, H2O and OH in dense interstellar clouds

Following the recent determinations of the fractional abundances of H3O+ and H2O in dense interstellar clouds, the production and loss processes for these species have been investigated in the laboratory. The major processes involved are dissociative electron recombination of H3O+ and proton transfer to and from H2O. The complete product distribution for the H3O+ recombination has now been determined and leads to the product channels OH + H-2(0.36), OH+2H(0.29) and O+H+H-2(0.30) with the channel H+H2O(0.05) being much smaller than previously assumed. It has been shown that loss of H3O+ by proton transfer to a series of sulphur-containing species with larger proton affinities is very efficient and also non-dissociative, further establishing the trend for proton transfer reactions involving this ion. These data have been used in a limited model to predict the fractional abundance of H2O and the relative abundances of H2O and H3O+ in Sgr B2. Predictions are also made for other dense interstellar clouds.