Interstellar Nitrogen Monohydride - History

History

One of the earliest papers on the NH molecule was in 1976 by Richard M. Crutcher and William D. Watson. They were still trying to pinpoint the absorption line for NH. There was an already theoretical upper limit of 0.3mÅ, but they needed a more exact figure. They thought that NH should normally be produced on grains (directly or indirectly) at about the same rate per atom as OH (hydroxyl) and possibly CH (methine). However, its formation by gas phase reactions was extremely slow and could be neglected in comparison with the possible rate for surface reactions. The presence or absence of NH at a certain abundance level could then be interpreted as evidence concerning the importance of surface reactions. Their hope was to find the abundance ratio between NH and OH. They weren’t able to find the absorption line for NH. Therefore the observations were inconclusive.

Interstellar NH was discovered in outer space for the first time in 1991 by D. M. Meyer and K. C. Roth. NH is reported in the diffuse clouds toward Zeta Per and HD 27778 from high-resolution high S/N spectra of the NH Å 3Pi-X 3Sigma (0,0) absorption band near 3358 Å. These observations represent the first detection of this molecule anywhere in the interstellar medium.

Shortly after NH’s discovery, Wagenblast, R. Williams, D. A. Millar, T. J. Nejad, and L. A. M., did some work on NH in 1992. They noticed that the observation of NH in the interstellar clouds towards Per and HD 27778 could not be explained with conventional gas-phase chemistry models. They proposed a non-equilibrium model for the Per cloud, which incorporated grain-surface production of NH and OH would be able to reproduce the abundances of all the observed species (except CH+) accurately. A pure gas-phase model and cloud interface model, in which NH and CH+ were formed in a warm and tenuous environment, failed to explain the observed high abundance of CN. Hence, the observations of NH in Per and HD 27778 provided evidence for the presence of grain-surface reactions leading to molecules other than H2.

At the end of their research, they concluded that even though the gas-phase formation of NH is given a large rate coefficient, NH was calculated to be underabundant with respect to the observations by a factor of 30. From this they discovered that a cold environment with a temperature of about 30K favored an efficient production of CN from NH within the diffuse cloud.