Herschel HIFI spectrum of o-NH₃(1₀ − 0₀)  toward the pre-stellar core L1544. The three groups of hyperfine components (with the 0−1 group having one single component) are detected for the first time in space. They are all highly self-absorbed (see Sect. 3). The hyperfine structure is shown by black vertical lines (with LTE relative intensities) and labelled as in Cazzoli et al. (2009). The vertical dotted line marks the core velocity. The root-mean-square noise level is 4.7 mK.