We report the discovery of the formation of an electron Cooper pair approximately 40 eV above the double-ionization threshold in benzene, naphthalene, anthracene, and coronene after absorption of a single photon. We have measured the ratios of doubly to singly charged parent ions of the above mentioned molecules as well as pyrrole and furan by using monochromatized synchrotron radiation up to 100 eV above the corresponding thresholds. We also recorded photoelectron spectra of benzene and naphthalene at selected energies. The electron-pair formation is based on the specific structure of the molecules and does not exist for pyrrole and furan.

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We have measured the ratio of doubly to singly charged parent ions of benzene, naphthalene, anthracene, and pentacene using monochromatized synchrotron radiation up to 30 eV above the corresponding threshold. Our measurements show a striking similarity between the ratio of doubly charged to all parent ions and the ratio for helium. Moreover, the magnitudes of the ratios for these molecules scale linearly with their lengths with an amazing accuracy. A high ratio, i.e., a high relative double-photoionization probability, makes a molecule an important source of low-energy electrons that can promote radiation damage of biomolecules [B. Boudaïffa et al., Science 287, 1658 (2000)].

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