We have observed single photon double K-shell photoionization in the C 2H2n (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 C2H2n 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 C2H2n 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.
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