We have observed single photon double $K$-shell photoionization in the ${\mathrm{C}}_2{\mathrm{H}}_{2n}$ ($n=1–3$) hydrocarbon sequence and in ${\mathrm{N}}_2$ 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 ($K^{-1}{K}^{-1}$ process). In the ${\mathrm{C}}_2{\mathrm{H}}_{2n}$ sequence, the spectroscopy of $K^{-1}{K}^{-1}$ 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 $K^{-1}{K}^{-1}$ double core ionization in the ${\mathrm{C}}_2{\mathrm{H}}_{2n}$ sequence and in the isoelectronic ${\mathrm{C}}_2{\mathrm{H}}_2$, ${\mathrm{N}}_2$ 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.

• Received 14 November 2012

DOI:https://doi.org/10.1103/PhysRevLett.110.163001