Ac. Cefalas et al., Efficient removal of foxing from a medieval Ptolemaic map using a molecular fluorine laser at 157 nm, APPL PHYS A, 73(5), 2001, pp. 571-578
In this communication, we have used a molecular fluorine laser at 157 nm to
remove foxing marks, for the first time, from a medieval Ptolemaic map pub
lished in 1580 AD, By applying laser-ablation experimental techniques, the
absorption coefficient and the threshold-energy fluence of the paper manusc
ript were determined at 248, 193 and 157 nm. The absorption coefficient at
157 nm was found to be one order of magnitude higher than at 248 nm. There
was no evidence of oxidising of the paper substrate following laser treatme
nt of foxing areas at 157 nm (yellowish colour). In order to investigate th
e basic photo-chemical mechanism of the photo-dissociation dynamics of the
paper's cellulose fibres under vacuum ultraviolet irradiation, we applied m
ass-spectroscopic techniques. Mass spectroscopy reveals that there is break
ing of the molecular bonds of the cellulose, even at low laser energy at 15
7 nm. The cellulose monomer is disintegrated into small photo-fragments. wh
ich fly apart with supersonic speed. There were no photo-fragments observed
for m/e higher than 32 amu (atomic mass units of m/e). Fragments with two
carbon atoms have a relatively higher probability to be dissociated from th
e parent cellulose molecule than heavier ones. These experimental findings
suggest that the bound potential excited electronic states of the parent mo
lecule correlate with dissociative excited electronic states of the molecul
ar photo-fragments at 7.8 eV (lambda = 157 nm), in agreement with previous
results for various organic polymers of similar chemical structure.