  Mid-infrared emission features probe the properties of ionized gas, and hot
or warm molecular gas. The Orion Bar is a frequently studied photodissociation
region (PDR) containing large amounts of gas under these conditions, and was
observed with the MIRI IFU aboard JWST as part of the "PDRs4All" program. The
resulting IR spectroscopic images of high angular resolution (0.2") reveal a
rich observational inventory of mid-IR emission lines, and spatially resolve
the substructure of the PDR, with a mosaic cutting perpendicularly across the
ionization front and three dissociation fronts. We extracted five spectra that
represent the ionized, atomic, and molecular gas layers, and measured the most
prominent gas emission lines. An initial analysis summarizes the physical
conditions of the gas and the potential of these data. We identified around 100
lines, report an additional 18 lines that remain unidentified, and measured the
line intensities and central wavelengths. The H I recombination lines
originating from the ionized gas layer bordering the PDR, have intensity ratios
that are well matched by emissivity coefficients from H recombination theory,
but deviate up to 10% due contamination by He I lines. We report the observed
emission lines of various ionization stages of Ne, P, S, Cl, Ar, Fe, and Ni,
and show how certain line ratios vary between the five regions. We observe the
pure-rotational H$_2$ lines in the vibrational ground state from 0-0 S(1) to
0-0 S(8), and in the first vibrationally excited state from 1-1 S(5) to 1-1
S(9). We derive H$_2$ excitation diagrams, and approximate the excitation with
one thermal (~700 K) component representative of an average gas temperature,
and one non-thermal component (~2700 K) probing the effect of UV pumping. We
compare these results to an existing model for the Orion Bar PDR and highlight
the differences with the observations.
