Cooper Pair Formation in Acenes

We have measured the ratio of doubly to singly charged molecular parent ions of benzene, naphthalene, anthracene, and pyrrole over a wide range of photon energies. About 40 eV above the double-ionization threshold, the first three of the above molecules exhibit a hump of very similar shape and magnitude in the double-to-single photoionization ratio, which we attribute to the formation and emission of an electron Cooper pair from a free molecule. Our results suggest that the de Broglie wave of this highly correlated pair of electrons forms a closed loop in the system of overlapping $\pi$ bonds with a wavelength that matches the distance between neighboring carbon atoms. Pyrrole with its pentagonal structure does not allow the formation of a closed de Broglie wave and, thus, does not exhibit a hump in the ratio. Photoelectron measurements indicate the break-up of the emitted Cooper pair by two electron peaks sitting on top of the mainly U-shaped double-ionization continuum in support of our interpretation.