Search for Interstellar Water in the Translucent Molecular Cloud toward HD 154368

We report an upper limit of 9 x 10^{12} cm-2 on the column density of waterin the translucent cloud along the line of sight toward HD 154368. This resultis based upon a search for the C-X band of water near 1240 \AA carried outusing the Goddard High Resolution Spectrograph of the Hubble Space Telescope.Our observational limit on the water abundance together with detailed chemicalmodels of translucent clouds and previous measurements of OH along the line ofsight constrain the branching ratio in the dissociative recombination of H_3O+to form water. We find at the $3\sigma$ level that no more than 30% ofdissociative recombinations of H_3O+ can lead to H_2O. The observed spectrumalso yielded high-resolution observations of the Mg II doublet at 1239.9 \AAand 1240.4 \AA, allowing the velocity structure of the dominant ionizationstate of magnesium to be studied along the line of sight. The Mg II spectrum isconsistent with GHRS observations at lower spectral resolution that wereobtained previously but allow an additional velocity component to beidentified.Comment: Accepted by ApJ, uses aasp