Wavelength dependence of the photoinduced indentation hardening in C60 crystals

The photoinduced indentation hardening of C60 crystals was investigated in ultraviolet-visible light. The hardening strongly depended on the wavelength of the illumination light: the magnitude increased with increasing wavelength, and then reached a maximum at 700 nm. This indicated that red light is more effective than ultraviolet light in the hardening. The wavelength dependence of the hardening is discussed in terms of an interplay between the effective depth of the photoinduced polymerization and the penetration depth of the indenter.