  Exoplanets with cloud-free, haze-free atmospheres at the pressures probed by
transmission spectroscopy represent a valuable opportunity for detailed
atmospheric characterization and precise chemical abundance constraints. We
present the first optical to infrared (0.3-5 microns) transmission spectrum of
the hot Jupiter WASP-62b, measured with Hubble/STIS and Spitzer/IRAC. The
spectrum is characterized by a 5.1-sigma detection of Na I absorption at 0.59
microns, in which the pressure-broadened wings of the Na D-lines are observed
from space for the first time. A spectral feature at 0.4 microns is tentatively
attributed to SiH at 2.1-sigma confidence. Our retrieval analyses are
consistent with a cloud-free atmosphere without significant contamination from
stellar heterogeneities. We simulate James Webb Space Telescope (JWST)
observations, for a combination of instrument modes, to assess the atmospheric
characterization potential of WASP-62b. We demonstrate that JWST can
conclusively detect Na, H2O, FeH, and SiH within the scope of its Early Release
Science (ERS) program. As the only transiting giant planet currently known in
the JWST Continuous Viewing Zone, WASP-62b could prove a benchmark giant
exoplanet for detailed atmospheric characterization in the James Webb era.
