الفهرس 
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Abstract
Document papers produced by digital printing methods, such as laser and inkjet printers, are used widely for different purposes. In forensic science, examination methods and scientific procedures are used to provide evidences about suspicious document papers. Nowadays, laser applications are widely used in industry, medicine, and communication. In this thesis, the role of laser application in counterfeiting and forgery for document papers is studied. Laser radiation was used to remove printed toner at different conditions and then different diagnostic techniques were employed to examine laser-treated document papers. Experiments were carried out using Q-switched Nd:YAG laser working at the second harmonic generation at wavelength of 532 nm. The full width at half maximum (FWHM) of the laser pulse was 7 ns and laser repetition rate was 10 Hz. Samples were performed at different laser fluencies of 1.02, 1.50, and 2.50 J/cm2 using plano-convex lens of f = 10 cm. The working distance was 11 cm from the focusing lens to avoid direct damaging of sample surface (i.e. out of focus). Laser beam was scanned over the printed toner on the sample surface using XYZ translation stage. Removing of toner and possible changes occurred on the white paper surface were investigated using optical microscope, scanning electron microscopy (SEM), energy dispersive electron spectroscopy (EDX), Fourier transform infrared (FTIR), and laser induced breakdown spectroscopy (LIBS). Residual toner and damaging in the cellulose fiber were clearly occurred after using laser fluence of 2.50 J/cm2 at different laser exposure, as seen by the optical microscope. SEM images of samples treated with laser at 1.02 and 1.50 J/cm2 showed slight impact on the thin surface layer after removing the toner. The residual toner could be related to the inhomogeneous of the toner density on the surface, which affected the absorption of the laser beam and thermal degradation of the coating layer of the paper surface and the cellulose fibers. EDX spectra show that carbon and calcium have been decreased after applying the laser beam. The FTIR spectrum of the white paper was compared with the ablated of the printed paper at various fluencies (1.02, 1.5, and 2.5 Jcm-2) in order to identify the bands resulting from the ablation process. The white paper showed a FTIR pattern of carbonyl groups, whose vibration modes were observed. The main function groups of FTIR spectra for white paper due to cellulose molecular vibration are OH stretching at peak 3200-3400 cm-1, CH stretching at 2900.41 cm-1, H2O bending at 1600-1650 cm-1, CCH - COH bending at 1300-1330 cm-1, the most pronounced changes in intensities. The observed for two bands at 1440 and 1330 cm-1. The 1440 cm-1 band, which is mainly due to plane bending vibrations of the groups H-C-H and O-H-C and 1330 cm-1 band due to C-O-H and H-C-C bending vibrations gains intensity with laser ablation at fluencies of 1.02, 1.5, 2.5 Jcm-2. Smaller changes in intensity are observed also for 1376 cm-1 band that behave similarly to the 1316 cm-1 band and are assigned to the bending vibrations of the groups C-O-H and H-C-C. At laser fluence of 2.5 Jcm-2 the oxidation of OH group was taken place. This is due to a highly localized photochemical degradation effect on the paper induced at high fluence (2.5 Jcm-2), in addition to the photochemical reactions. The oxidation of the OH groups causes the formation of ketones, aldehyde and carboxylic groups with a typical absorption band at 1730 cm-1. This effect is no longer occurred when the laser fluencies reduced to 1.02 and 1.5 Jcm-2 as this leads to prevent any component elements in paper from being ablated. By analyzing LIBS spectra, it showed that some elements present in some printers and no present in others as HP toner have element of Al, while disappeared in Samsung toner. We found that elements of Ni, P, K and Fe are present in the two toner samples. The results of LIBS show that iron, silicon, copper, calcium and ruthenium are the major elements percent present in toner because the first and the second used as a charge control and flow agents respectively. The LIBS results help us to differentiate between the toners brand. Finally, understanding laser interaction with toner and its effects on the cellulose fiber could suggest a procedure to investigate imitation and counterfeiting of document papers as future potential crimes.