  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. The aim of this paper is to constrain the
physical and chemical conditions in the Orion Bar, a prototypical nearby
photon-dominated region. 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. 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 ~10^9 cm^-3, 10-100 times higher than previous
estimates, which is unlikely. However, electron impact excitation reproduces
our observations for T = 100 K and n(e) = 10 cm^-3, as expected for the
interclump gas. 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.
