Context. The clumpy density structure of photon-dominated regions is well established, but the physical properties of the clumps and of the surrounding interclump medium are only approximately known.
Aims: The aim of this paper is to constrain the physical and chemical conditions in the Orion Bar, a prototypical nearby photon-dominated region.
Methods: We present observations of the HF J = 1-0 line, which appears in emission toward the Orion Bar, and compare the brightness of the line to non-LTE radiative transfer calculations.
Results: The large width of the HF line suggests an origin of the emission in the interclump gas, but collisional excitation by H2 in the interclump gas underpredicts the observed line intensity by factors of 3-5. In contrast, an origin of the line in the dense clumps requires a density of ~109 cm-3, 10-100 times higher than previous estimates, which is unlikely. However, electron impact excitation reproduces our observations for T = 100 K and ne = 10 cm-3, as expected for the interclump gas.
Conclusions: We conclude that HF emission is a signpost of molecular gas with a high electron density. Similar conditions may apply to active galactic nuclei, where HF also appears in emission.
