  The ultraviolet-visible wavelength range holds critical spectral diagnostics
for the chemistry and physics at work in planetary atmospheres. To date,
exoplanet time-series atmospheric characterization studies have relied on
several combinations of modes on Hubble's STIS/COS instruments to access this
wavelength regime. Here for the first time, we apply the Hubble WFC3/UVIS G280
grism mode to obtain exoplanet spectroscopy from 200-800 nm in a single
observation. We test the G280 grism mode on the hot Jupiter HAT-P-41b over two
consecutive transits to determine its viability for exoplanet atmospheric
characterization. We obtain a broadband transit depth precision of 29-33ppm and
a precision of on average 200ppm in 10nm spectroscopic bins. Spectral
information from the G280 grism can be extracted from both the positive and
negative first order spectra, resulting in a 60% increase in the measurable
flux. Additionally, the first HST orbit can be fully utilized in the
time-series analysis. We present detailed extraction and reduction methods for
use by future investigations with this mode, testing multiple techniques. We
find the results fully consistent with STIS measurements of HAT-P-41b from
310-800 nm, with the G280 results representing a more observationally efficient
and precise spectrum. We fit HAT-P-41b's transmission spectrum with a forward
model at Teq=2091K, high metallicity, and significant scattering and cloud
opacity. With these first of their kind observations, we demonstrate that
WFC3/UVIS G280 is a powerful new tool to obtain UV-optical spectra of exoplanet
atmospheres, adding to the UV legacy of Hubble and complementing future
observations with the James Webb Space Telescope.
