Anomalous TargetZDependence of Double to SingleK-Shell Vacancy Production in Cl-Beam X Rays

We have measured $X$ rays from 30-MeV Cl beams on various solid targets between carbon and copper. The ratio of double $K$-shell to single $K$-shell vacancy production in Cl exhibits a broad peak near $Z=20\mathrm{}$ (Ca). This behavior is not predicted by Coulomb ionization. This peaking is examined using a molecular-orbital mechanism. To explain the results both $K$-shell electrons must be promoted to the vacant orbitals of the Cl beam. This is the first evidence of double $K$-shell promotion.