  The Migdal effect in a dark-matter-nucleus scattering extends the direct
search experiments to the sub-GeV mass region through electron ionization with
sub-keV detection thresholds. In this paper, we derive a rigorous and
model-independent "Migdal-photoabsorption" relation that links the sub-keV
Migdal process to photoabsorption. This relation is free of theoretical
uncertainties as it only requires the photoabsorption cross section as the
experimental input. Validity of this relation is explicitly checked in the case
of xenon with an state-of-the-arts atomic calculation that is well-benchmarked
by experiments. The predictions based on this relation for xenon, argon,
semiconductor silicon and germanium detectors are presented and discussed.
