Mechanistic comparison of pulse laser induced phase separation of particulates from cellulose paper at 213 nm and 532 nm

Abstract

The laser-induced phase separation of charcoal particles on additive-free cotton linters cellulose paper was investigated by electron and optical microscopy, colorime- try, and diffuse reflectance FT-IR. The fibre bundles were vaporised in depth of several 10 µm above destruction flu- ence thresholds using visible 532 nm radiation. This is in contrast to mid-ultraviolet 213 nm radiation, where only the top fibre bundles were modified and partially evapo- rated. The colorimetric lightness results generally repre- sented the cleaning status, whereas the colorimetric yel- lowing data represented irreversible chemical and/or pho- tochemical changes. Charcoal-contaminated paper treated with visible and mid-ultraviolet radiation exhibited yellow- ing, whereas uncontaminated did not. This suggests that the electron-rich plasma generated by the evaporation of the par- ticles heats the adjacent substrate and also excludes oxygen. Mid-ultraviolet, in contrast to visible radiation, shows parti- cle removal always accompanied by paper destruction. IR spectroscopy results suggest cross-linking by ether bonds near the destruction threshold, but do not prove the forma- tion of oxidation products and double bonds as the basis of the yellowing. A "cleaning window" between the cleaning threshold (0.1 J/cm 2 ) and the paper destruction threshold (2.9 J/cm 2 ) with a pulse number of 2 is provided by visi- ble 532 nm laser treatment.