Ultraviolet Absorption Spectrum of Ammonia in Solid Argon at 4.2°K

A series of absorption bands in the region 1600 to 1900 A has been observed in thin films of solid argon containing between 0.3% and 3% of ammonia. The bands are attributed to isolated NH3, and the absence of alternating bands in the observed vibrational progression shows that in the ground state only the J = 0 level is appreciably populated. Thermal equilibration of NH3 at 4.2°K involves the interconversion of nuclear spin species. It is suggested that the observed rapid equilibration is due to coupling between the proton spins and the spin of the nitrogen nucleus.