  Massive Population III stars from 140 - 260 solar masses ended their lives as
pair-instability supernovae (PISNe), the most energetic thermonuclear
explosions in the universe. Detection of these explosions could directly
constrain the primordial IMF for the first time, which is key to the formation
of the first galaxies, early cosmological reionization, and the chemical
enrichment of the primeval IGM. We present radiation hydrodynamical
calculations of Pop III PISN light curves and spectra performed with the RAGE
code. We find that the initial radiation pulse due to shock breakout from the
surface of the star, although attenuated by the Lyman-alpha forest, will still
be visible by JWST at z ~ 10 - 15, and possibly out to z ~ 20 with strong
gravitational lensing. We have also studied metal mixing at early stages of the
explosion prior to breakout from the surface of the star with the CASTRO AMR
code and find vigorous mixing in primordial core-collapse explosions but very
little in PISNe. This implies that the key to determining progenitor masses of
the first cosmic explosions is early spectroscopy just after shock breakout,
and that multidimensional mixing is crucial to accurate low-mass Pop III SNe
light curves and spectra.
