  Mott insulators with spin-orbit entangled j=1/2 moments host intriguing
magnetic properties. The j=1/2 wave function requires cubic symmetry, while a
noncubic crystal field mixes j=1/2 and 3/2 character. Spectroscopic studies of
$5d^5$ iridates typically claim noncubic symmetry, e.g., based on a splitting
of the excited j=3/2 quartet. A sizable splitting is particularly puzzling in
antifluorite-type K$_2$IrCl$_6$, a frustrated fcc quantum magnet with global
cubic symmetry. It raises the fundamental question about the stability of j=1/2
moments against magneto-elastic coupling. Combining resonant inelastic x-ray
scattering with optical spectroscopy, we demonstrate that the multi-peak line
shape in K$_2$IrCl$_6$ reflects a vibronic character of the j=3/2 states rather
than a noncubic crystal field. The quasimolecular crystal structure with well
separated IrCl$_6$ octahedra explains the existence of well-defined sidebands
that are usually smeared out in solids. Our results highlight the
spin-orbital-lattice entangled character of cubic K$_2$IrCl$_6$ with ideal
j=1/2 moments.
