Upper limit and significance of the NH molecule in diffuse interstellar clouds

An upper limit of 0.3 mA in the direction of Omicron Persei has been obtained for the strength of the possible absorption line at 3358 A due to interstellar NH. A column density of less than 700 billion per sq cm is derived for NH - a factor of 10 below the previous upper limit for diffuse clouds. This is also less than 1% that of OH in the same direction. Interstellar NH is expected to be produced at about the same rate per atom as OH if their formation occurs on grain surfaces. The absence of NH at the observed sensitivity is shown to be strong evidence in favor of pure gas-phase formation for molecules other than H2 in diffuse interstellar clouds.