Single photon K^-2 and K^-1K^-1 double core ionization in C₂H_2n (n=1-3), CO, and N₂ as a potential new tool for chemical analysis

We have observed single photon double K-shell photoionization in the C ₂H_2n (n=1-3) hydrocarbon sequence and in N₂ and CO, using synchrotron radiation and electron coincidence spectroscopy. Our previous observations of the K^-2 process in these molecules are extended by the observations of a single photon double photoionization with one core hole created at each of the two neighboring atoms in the molecule (K ^-1K^-1 process). In the C₂H_2n sequence, the spectroscopy of K^-1K^-1 states is much more sensitive to the bond length than conventional electron spectroscopy for chemical analysis spectroscopy based on single K-shell ionization. The cross section variation for single photon K^-1K^-1 double core ionization in the C₂H_2n sequence and in the isoelectronic C₂H₂, N₂ and CO molecules validates a knock-out mechanism in which a primary ionized 1s photoelectron ejects another 1s electron of the neighbor atom. The specific Auger decay from such states is clearly observed in the CO case.