  The polarization dependence of the near edge x-ray absorption spectroscopy
(XANES) is an element specific probe to the real-space distribution of the
density of unoccupied states in solid-state materials. In this paper, we
present Fe and As $K$-edge experiments of Ba(Fe$_{1-x}$$M_{x}$)$_{2}$As$_{2}$
($M=$ Mn, Co and $x=0.0$ and $0.08$). The experiments reveal a strong
polarization dependence of the probed XANES spectra, which concerns mainly an
increase of the intensity of electronic transitions when the beam polarization
is set out of the sample's $ab$ crystallographic plane. The results show that
states with $p_{z}$-orbital character dominate the density of unoccupied states
close to the Fermi level. Partial substitution of Fe by Co is shown to decrease
the intensity anisotropy, suggesting that Co promotes electronic transfer
preferentially to states with $p_{z}$-orbital character. On the other hand, Mn
substitution causes the increase of the spectra $p_{z}$-orbital anisotropy,
which is proposed to take place by means of an enhanced local Fe $3d4p$ mixing,
unveiling the role of Fe $4p$ states in the localization of the Fe $3d$
orbitals. Moreover, by comparing our results to previous experiments, we
identify the relative mixing between Fe and the pnictide $4p_{x,y,z}$ orbitals
as a clear divide between the electronic properties of iron arsenides and
selenides. Our conclusions are supported by multiple-scattering theory
calculations of the XANES spectra and by quantum chemistry calculations of Fe
coordination electronic structure.
