Schematic energy-level diagram showing the fine- (J) and hyperfine-structure (F₁, F) splittings of the N = 1 ← 0 rotational transition of ¹⁵NH. Hyperfine level splittings shown are a hundred times the actual ones for clarity. The lower fine-structure component (J = 0 ← 1) occurring near 942.130 GHz recorded at PhLAM laboratory with partly resolved hyperfine-structure (see Fig. 4) is indicated by a solid line.