  Ir-based materials have drawn much attention due to the observation of
insulating phase believed to be driven by spin-orbit coupling while Ir 5$d$
states are expected to be weakly correlated due to their large orbital
extensions. IrO$_2$, a simple binary material, shows metallic ground state
which seems to deviate from the behavior of most other Ir-based materials and
varied predictions in these material class. We studied the electronic structure
of IrO$_2$ at different temperatures employing high resolution photoemission
spectroscopy with photon energies spanning from ultraviolet to hard $x$-ray
range. Experimental spectra exhibit a signature of enhancement of Ir-O
covalency in the bulk compared to the surface electronic structure. The
branching ratio of the spin-orbit split Ir core level peaks is found to be
larger than its atomic values and it enhances further in the bulk electronic
structure. Such deviation from the atomic description of the core level
spectroscopy manifests the enhancement of the orbital moment due to the solid
state effects. The valence band spectra could be captured well within the
density functional theory. The photon energy dependence of the features in the
valence band spectra and their comparison with the calculated results show
dominant Ir 5$d$ character of the features near the Fermi level; O 2$p$ peaks
appear at higher binding energies. Interestingly, the O 2$p$ contributions of
the feature at the Fermi level is significant and it enhances at low
temperatures. This reveals an orbital selective enhancement of the covalency
with cooling which is an evidence against purely spin-orbit coupling based
scenario proposed for these systems.
