Energy level diagram (in K) for the first four rotational levels J of the ² Π_1∕2 spin sub-level of NS. The energy shift of the Λ-doubling states and the hyperfine levels with respect to the rotational level has been multiplied by 100 for the sake of visibility. The hyperfine quantum number F is indicated on the right of each hf level, where F = I + J with I the nuclear spin of the nitrogen atom (I = 1) and J the orbital angular momentum. Allowed transitions are restricted by the selection rules ΔJ = ±1, ΔF = 0, ±1, and + ↔−. The transitions shown in Figs. 3 and B.1–B.4, are outlined with arrows, where full lines indicate the 115 GHz lines, and the dashed lines those at 161 GHz.