  We present model calculations of ionization fractions for elements in the
warm (log T = 4), low-density photoionized interstellar medium (WIM) of the
Milky Way. We model the WIM as a combination of overlapping low-excitation H II
regions having n(HII)/n(H) > 0.8. Our standard model incorporates an intrinsic
elemental abundance pattern similar to that found for warm neutral clouds in
the Galaxy and includes the effects of interstellar dust grains. The radiation
field is characterized by an ionizing spectrum of a star with T ~ 35,000 K and
an ionization parameter log(q) ~ -4.0. The emergent emission line strengths are
in agreement with the observed ratios of [SII]/Halpha, [NII]/Halpha,
[SII]/[NII], [OI]/Halpha, [OIII]/Halpha, and He I/Halpha in the Galactic WIM.
Although the forbidden emission-line intensities depend strongly upon the input
model parameters, the ionization fractions of the 20 elements studied in this
work are robust over a wide range of physical conditions considered in the
models. These ionization fractions have direct relevance to absorption-line
determinations of the elemental abundances in the warm neutral and ionized
gases in the Milky Way and other late-type galaxies. We demonstrate a method
for estimating the WIM contributions to the observed column densities of singly
and doubly ionized atoms used to derive abundances in the warm neutral gas. We
apply this approach to study the gas-phase abundances of the warm interstellar
clouds toward the halo star HD 93521.
