Hard X-ray core-level photoemission of

We study the Mott-Hubbard metal-insulator transition in single-crystal $\chem{V_2O_3}$ using hard X-ray ($\tx{HX}:h\nu=5940$$\un{eV}$) core-level photoemission spectroscopy (PES). HX-PES enables a bulk probe of the electronic structure in the paramagnetic metallic and antiferromagnetic insulating phases. Metallic $\chem{V_2O_3}$ shows additional features at low binding energy to the main peak of V $1s$, $2p$, and $3s$ core levels, which vanish in the insulating phase. The low-binding-energy features in the metallic phase are also observed using soft X-rays ($\tx{SX}:h\nu=1450$$\un{eV}$), but with a reduced relative intensity compared to the HX-PES spectrum. A model cluster calculation including core hole screening by coherent states at the Fermi level, reproduces the low-binding-energy features in the metallic phase. The O $1s$ core level also shows spectral changes: the asymmetric line shape in the metallic phase transforms to a symmetric line shape in the insulating phase, consistent with the metal-insulator transition. The results indicate screening by coherent states is more effective in the bulk compared to the surface. HX-PES is, thus, a reliable probe of the electronic-structure changes across the metal-insulator transition in strongly correlated electron systems