  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.
