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Photoemission data taken with hard X-ray radiation on cleaved single crystalsof the barium parent compound of the MFe$_2$As$_2$ pnictide high temperaturesuperconductor family are presented. Making use of the increasedbulk-sensitivity upon hard X-ray excitation, and comparing the results to datataken at conventional VUV photoemission excitation energies, it is shown thatthe BaFe$_2$As$_2$ cleavage surface provides an electrostatic environment thatis slightly different to the bulk, most likely in the form of a modifiedMadelung potential. However, as the data argue against a different surfacedoping level, and the surface-related features in the spectra are by no meansas dominating as seen in systems such as YBa$_2$Cu$_3$O$_x$, we can concludethat the itinerant, near-E$_F$ electronic states are almost unaffected by theexistence of the cleavage surface. Furthermore, exploiting the strong changesin photoionisation cross section between the Fe and As states across the widephoton energy range employed, it is shown that the degree of energetic overlapbetween the iron 3d and arsenic 4p valence bands is particularly small at theFermi level, which can only mean a very low degree of hybridization between theFe 3d and As 4p states near and at E$_F$. Consequently, the itinerancy of thecharge carriers in this group of materials involves mainly the Fe 3d - Fe 3doverlap integrals with at best a minor role for the Fe 3d - As 4p hoppingparameters, and that the states which support superconductivity upon doping areessentially of Fe 3d character.

physical review. b, condensed matter and materials physics

A high resolution, hard X-ray photoemission investigation of  BaFe$_2$As$_2$: moderate influence of the surface and evidence for a low  degree of Fe 3d - As 4p hybridization of the near-E$_F$ electronic states