Single PhotonK−2andK−1K−1Double Core Ionization inC2H2n(n=1–3), CO, andN2as a Potential New Tool for Chemical Analysis

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 ${K}^{\ensuremath{-}2}$ 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 (${K}^{\ensuremath{-}1}{K}^{\ensuremath{-}1}$ process). In the ${\mathrm{C}}_{2}{\mathrm{H}}_{2n}$ sequence, the spectroscopy of ${K}^{\ensuremath{-}1}{K}^{\ensuremath{-}1}$ 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 ${K}^{\ensuremath{-}1}{K}^{\ensuremath{-}1}$ 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.