  WASP-121b is a transiting gas giant exoplanet orbiting close to its Roche
limit, with an inflated radius nearly double that of Jupiter and a dayside
temperature comparable to a late M dwarf photosphere. Secondary eclipse
observations covering the 1.1-1.6 micron wavelength range have revealed an
atmospheric thermal inversion on the dayside hemisphere, likely caused by high
altitude absorption at optical wavelengths. Here we present secondary eclipse
observations made with the Hubble Space Telescope Wide Field Camera 3
spectrograph that extend the wavelength coverage from 1.1 micron down to 0.8
micron. To determine the atmospheric properties from the measured eclipse
spectrum, we performed a retrieval analysis assuming chemical equilibrium, with
the effects of thermal dissociation and ionization included. Our best-fit model
provides a good fit to the data with reduced chi^2=1.04. The data diverge from
a blackbody spectrum and instead exhibit emission due to H- shortward of 1.1
micron. The best-fit model does not reproduce a previously reported bump in the
spectrum at 1.25 micron, possibly indicating this feature is a statistical
fluctuation in the data rather than a VO emission band as had been tentatively
suggested. We estimate an atmospheric metallicity of [M/H]=1.09(-0.69,+0.57),
and fit for the carbon and oxygen abundances separately, obtaining
[C/H]=-0.29(-0.48,+0.61) and [O/H]=0.18(-0.60,+0.64). The corresponding
carbon-to-oxygen ratio is C/O=0.49(-0.37,+0.65), which encompasses the solar
value of 0.54, but has a large uncertainty.
