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We report extensive ab-initio MRD-CI electronic structure calculations of the imidogen radical NH. Potential energy curves of all valence and many Rydberg states were calculated. Thirty nine states were computed including fifteen singlets, fifteen triplets and nine quintets. A basis set of DZ + POL augmented with $s-,p-$ and d-type Rydberg and bond functions together with an extensive treatment of electron correlation were employed. The present ab-initio study provides a considerable amount of new information on the excited states which is employed to discuss the spectroscopy and photochemistry of NH. Below 10.5 eV the electronic states are well separated so their relative positions have been well established; above this energy the high density of electronic states does not always permit to determine unambiguously the relative position of the states. However, overall, an excellent agreement is found with recent REMPI measurments by Ashfold and $collaborators1$. Vertical ionization potentials of NH are also reported.