Herschel/HIFI observations of the Horsehead and Orion Bar PDRs. The green horizontal lines display the ±3σ rms levels. Red curves are single-slab models for N(NH) = 1.3 × 10¹³ cm⁻³. These data show a 3σ detection (at the line peak) at 974.479 GHz. The pink spectrum shows the relative strengths of the individual HFS components for optically thin line emission. The model of the Bar is also consistent with the nondetection of the fainter NH lines at ~946 and ~1000 GHz.