X-ray absorption studies of two-electron-one-photon excitations in krypton

X-ray photoabsorption measurements of simultaneous two-electron transition in krypton were carried out using synchrotron radiation. A full spectrum involving all $1s+nl$ electrons was detected. Nonrelativistic Hartree-Fock and, in particular, relativistic, Dirac-Fock energy-level calculations are in good agreement with the data, as are those derived from the optical data of Rb in the $z+1$ approximation. The cross sections relative to that of the single 1s ionization are found to decrease with shell number, from 2.2% for 1s4p to 0.058% for 1s2s i.e., slower than predicted theoretically. No indications are found for a preferred Coster-Kronig decay mode over the radiative one for the 1sns levels.