Photo-illumination hardening of fullerene crystals

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We carried out the experiments of Vickers micro-hardness of C60 and C70 crystals. As results, we have found that the micro-hardness of C60 and C70 crystals increased after they are illuminated with white light. We call this phenomenon "Photo-illumination hardening". The results are as following,1. The hardness increases with illumination of time, and then approaches a constant value.2. The hardening depends on the specimen temperature, and disappears at temperatures higher than 400K.3. The hardness strongly depends on the wavelength of illumination of light. The magnitude of the hardness reaches a maximum at 700nm.4. The Raman peaks of C60 crystals, which were illuminated as a function of illumination time, shift from an intrinsic peak 1469cmィイD1-1ィエD1 to a photo-polymerized peak 1460 cmィイD1-1ィエD1 via 1465 cmィイD1-1ィエD1 and 1464 cmィイD1-1ィエD1.5. In case of C70 crystals, we also observed the same kinds of behaviors due to photo-illumination. although the illumination time needs more longer.From these results, we conclude the hardening is due to the photo-induced polymers of C60 or C70.