The saturation number of the $[2+2]$ four-membered cross-link between adjacent ${\mathrm{C}}_{60}$ molecules in a photopolymerized ${\mathrm{C}}_{60}$ film has been determined using the intensity of the C $1s$ shake-up satellite, which originates from the $\pi -{\pi }^{*}$ transition of $\pi$ electrons of ${\mathrm{C}}_{60}$, by x-ray photoelectron spectroscopy. It was found that six $[2+2]$ rings were formed on a ${\mathrm{C}}_{60}$ molecule when polymerization was saturated. A possible configuration of the ${\mathrm{C}}_{60}$ polymer was postulated on the basis of the saturation number and the structure of a pristine ${\mathrm{C}}_{60}$ film.

• Received 23 June 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.2987