  Topotactic reduction utilizing metal hydrides as reagents emerges as an
effective approach to achieve exceptionally low oxidization states of metal
ions and unconventional coordination networks. This method opens avenues to the
development of entirely new functional materials, with one notable example
being the infinite-layer nickelate superconductors. However, the reduction
effect on the atomic reconstruction and electronic structures -- crucial for
superconductivity -- remains largely unresolved. We design two sets of control
Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ thin films and implement secondary ion mass
spectroscopy to highlight the absence of reduction-induced hydrogen
intercalation. X-ray absorption spectroscopy shows a significant linear
dichroism with dominant Ni 3d$_{x2{-}y2}$ orbitals on superconducting samples,
indicating a Ni single-band nature of infinite-layer nickelates. Consistent
with the superconducting $T_c$, the Ni 3d orbitals asymmetry manifests a
dome-like reduction duration dependence. Our results unveil the critical role
of reduction in modulating the Ni-3d orbital polarization and its impact on the
superconducting properties.
