  We report measurements of the isotope shifts of the 3d6 4s2 a 5D3 - 3d6 4s4p
z 5F4o FeI resonance line at 373.7 nm between all four stable isotopes 54Fe,
56Fe, 57Fe and 58Fe, as well as the complete hyperfine structure of that line
for 57Fe, the only stable isotope having a non-zero nuclear spin. The field and
specific mass shift coefficients of the transition have been derived from the
data, as well as the experimental value for the hyperfine structure magnetic
dipole coupling constant A of the excited state of the transition in 57Fe :
A(3d6 4s4p z 5F4o) = 68.21(69) MHz. The measurements were carried out by means
of high-resolution Doppler-free laser saturated absorption spectroscopy in a
Fe-Ar hollow cathode discharge cell using both natural and enriched iron
samples. The measured isotope shifts and hyperfine constants are reported with
uncertainties at the percent level.
