Effect of specimen size on photoplastic effects in anthracene crystals

The photoplastic effect (PPE) in anthracene crystals, which is a photoinduced hardening, has been studied by shear deformation. The PPE is produced at wavelengths ranging from less than 300 to 440 nm, with a maximum effect at 430 nm. The PPE is a bulk effect; it can be produced by light that has travelled more than 2 mm into a crystal. When PPE occurs with short wavelength (≤400 nm) small attenuation length (∼10−4 cm) illumination, it is not produced directly by the intrinsic light absorption, but rather by longer wavelength fluorescence radiation from the near-surface region. The photoproducts that interact with dislocations to increase the flow stress are most likely physical dimers near defect sites.