Proton irradiation of thin films of C60molecules

Vacuum-sublimed films of C60 molecules with thicknesses of 180–300 nm were irradiated with 100- or 200-keV protons at room temperature. The effects of fluences of 3.3 × 1014 to 6.1 × 1016 H/cm2 have been examined in detail using Fourier transform infrared (FTIR) spectrometry, glancing-angle X-ray diffraction (GXRD) and low-energy X-ray fluorescence (LXRF). All three techniques indicated that the major changes occurred for fluences greater than 1 × 1015 H/cm2. Intensities of the C60 FTIR peaks decreased exponentially with fluence without significant peak shifts or peak width changes. This suggests that each C60 molecule was destroyed in a catastrophic event, rather than a gradual process. Because essentially all the energy deposited in the films by the protons was in the form of electronic excitation and because solid C60 is an electrically insulating material, a Coulombic, or ionic, explosion at each C60 molecule is proposed to explain the observations. For the larger fluences, the presence of broad FTIR bands near 740 and 1420 cm−1 and the absence of sharp diffraction peaks in GXRD were taken as evidence that the C60 films were transformed into a form of amorphous carbon.