X-ray photoelectron spectroscopy of air-exposed C₆₀ films: Origin of the O1 s core peak

X-ray photoelectron spectra of C₆₀ films exposed to air of ca. 20% and 60% humidity at 23°C for 1 h are presented. It was found that the O1 s core peak appeared at a binding energy of 533.5 eV for the film exposed to air of ca. 60% humidity, while no peak corresponding to the O1 s photoelectrons was observed for the film exposed to air of ca. 20% humidity. The binding energy of 533.5 eV for the O1 s core peak is close to that of about 533 eV for the O1 s core peak of solid H₂O, while it is different from that of 537 eV for the O1 s core peak of solid O₂ on C₆₀ film. These facts indicate that the O1 s peak observed for the air-exposed C₆₀ film was due to H₂O molecules adsorbed on the film.