Photoemission of Cooper Pairs from Aromatic Hydrocarbons*

We have measured the ratio of doubly to singly charged molecular parent ions of benzene, naphthalene, anthracene, coronene, pyrrole, and furan over a wide range of photon energies. About 40 eV above the double-ionization threshold, the first four 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.\footnote{R.\ Wehlitz {\it et al.} Phys.\ Rev.\ Lett. \textbf{109}, 193001 (2012)} 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. Interestingly, coronene also exhibits a hump that corresponds to a de Broglie wavelength of twice the C--C distance. Pyrrole and furan, on the other hand, do not show any hump in the ratio probably due to their pentagonal structure. Photoelectron measurements indicate the break-up of the emitted Cooper pair in support of our interpretation.