  We study the evolution of chromospheric line and continuum emission during
the impulsive phase of the X-class SOL2014-09-10T17:45 solar flare. We extend
previous analyses of this flare to multiple chromospheric lines of Fe I, Fe II,
Mg II, C I, and Si II, observed with IRIS, combined with
radiative-hydrodynamical (RHD) modeling. For multiple flaring kernels, the
lines all show a rapidly evolving double-component structure: an enhanced,
emission component at rest, and a broad, highly red-shifted component of
comparable intensity. The red-shifted components migrate from 25-50 km s$^{-1}$
towards the rest wavelength within $\sim$30 seconds.
  Using Fermi hard X-ray observations, we derive the parameters of an
accelerated electron beam impacting the dense chromosphere, using them to drive
a RHD simulation with the RADYN code. As in Kowalski et al. 2017a, our
simulations show that the most energetic electrons penetrate into the deep
chromosphere, heating it to T$\sim$10,000 K, while the bulk of the electrons
dissipate their energy higher, driving an explosive evaporation, and its
counterpart condensation -- a very dense (n$_e \sim 2 \times 10^{14}$
cm$^{-3}$), thin layer (30--40 km thickness), heated to 8--12,000 K, moving
towards the stationary chromosphere at up to 50 km s$^{-1}$.
  The synthetic Fe II 2814.45A profiles closely resemble the observational
data, including a continuum enhancement, and both a stationary and a highly
red-shifted component, rapidly moving towards the rest wavelength. Importantly,
the absolute continuum intensity, ratio of component intensities, relative time
of appearance, and red-shift amplitude, are sensitive to the model input
parameters, showing great potential as diagnostics.
