Experimental Verification of the Chemical Sensitivity of Two-Site Double   Core-Hole States Formed by an X-ray FEL
  We have performed X-ray two-photon photoelectron spectroscopy (XTPPS) using the Linac Coherent Light Source (LCLS) X-ray free-electron laser (FEL) in order to study double core-hole (DCH) states of CO2, N2O and N2. The experiment verifies the theory behind the chemical sensitivity of two-site (ts) DCH states by comparing a set of small molecules with respect to the energy shift of the tsDCH state and by extracting the relevant parameters from this shift. 
P. Salen, P. van der Meulen, H. T. Schmidt, R. D. Thomas, M. Larsson,   R. Feifel, M. N. Piancastelli, L. Fang, B. Murphy, T...
physics.chem-ph