  The enrichment of Fe, relative to alpha-elements such as O and Mg, represents
a potential means to determine the age of quasars and probe the galaxy
formation epoch.
  To explore how \ion{Fe}{2} emission in quasars is linked to physical
conditions and abundance, we have constructed a 830-level \ion{Fe}{2} model
atom and investigated through photoionization calculations how \ion{Fe}{2}
emission strengths depend on non-abundance factors. We have split \ion{Fe}{2}
emission into three major wavelength bands, \ion{Fe}{2} (UV),
\ion{Fe}{2}(Opt1), and \ion{Fe}{2}(Opt2), and explore how the
\ion{Fe}{2}(UV)/\ion{Mg}{2}, \ion{Fe}{2}(UV)/\ion{Fe}{2}(Opt1) and
\ion{Fe}{2}(UV)/\ion{Fe}{2}(Opt2) emission ratios depend upon hydrogen density
and ionizing flux in broad-line regions (BLR's) of quasars. Our calculations
show that: 1) similar \ion{Fe}{2}(UV)/\ion{Mg}{2} ratios can exist over a wide
range of physical conditions; 2) the \ion{Fe}{2}(UV)/\ion{Fe}{2}(Opt1) and
\ion{Fe}{2}(UV)/\ion{Fe}{2}(Opt2) ratios serve to constrain ionizing luminosity
and hydrogen density; and 3) flux measurements of \ion{Fe}{2} bands and
knowledge of ionizing flux provide tools to derive distances to BLR's in
quasars. To derive all BLR physical parameters with uncertainties, comparisons
of our model with observations of a large quasar sample at low redshift ($z<1$)
is desirable.
  The STIS and NICMOS spectrographs aboard the Hubble Space Telescope (HST)
offer the best means to provide such observations.
