Calculated (top) and observed (bottom) spectra of the 1 _0,1–0 _0,0 transition of HOCN, showing the expected nitrogen hyperfine splitting. The spectrum is a synthesis of three settings of the Fabry–Perot cavity, which was tuned to be in resonance with each hyperfine transition. The double-peaked line profile is instrumental, caused by the Doppler shift of the supersonic molecular beam relative to the two traveling waves that compose the confocal mode of the Fabry–Perot cavity. The integration time was less than one minute for each spectrum.