The λ 4502 Band of NH
From a consideration of the possible electron configurations in the NH molecule Mulliken predicted a stable 1 Ʃ + state lying about 1·9 volts above the ground state 3 Ʃ - . It is therefore to be anticipated that transitions to this level from the known 1 II level will be observed simultaneously with the known 1 II → 1 ∆ transition. Mulliken estimated the shape of the potential energy curve for the 1 Ʃ + state; this corresponds to the existence of a 1 II → 1 Ʃ + band degraded to the red and lying in the neighbourhood of λ 4100. The plates from which the 1 II → 1 ∆ band was analysed were therefore examined for evidence of the 1 II → 1 Ʃ + band. After eliminating the numerous lines of the intense secondary spectrum of hydrogen which occurs in this region there remained only five unidentified lines. These are now seen to be the most intense lines of the Q branch of the λ 4502 band. The plates record the spectrum of a heavy current discharge in hydrogen containing a trace of nitrogen.