ABSTRACT
VI. CONCLUSIONS We have shown that PDS can be a valuable tool for studying the absorption of polymers, especially in the wavelength region between 0.7 and 2.2 /tm where very few nondestructive techniques have comparable sensitivities. To further explore the potential of PDS as a spectroscopic tool, the resolution of the existing spectra should be improved. The instrumental resolution in a PDS experiment is defined by the slit width of the monochromator. For the spectra shown in Figs. 11 and 12, the instrumental resolution is of the order of 10 nm. On the other hand, the nar rowest peak in the data appears to be at least three times broader. Thus, it appears that the intrinsic linewidths of the overtone vibrations may have the major contri bution to the experimental linewidths. The intrinsic linewidth of a overtone vibra tion should be related to the lifetime of the vibrational excited state which should increase with energy due to the increase in the density of states [22]. Such results have indeed been obtained for the C -H overtones of benzene in the gas phase [27]. In Figs. 11 and 12, there are quite a few spectral peaks that appear to have some fine structures, but cannot be well resolved. For example, in the LDPE spectrum, the peak at 1210 nm has a shoulder at the short-wavelength region. Further im provement in the instrumental resolution (e.g., by using a more narrow-band light source such as a tunable laser) may help us to decide if the resolution in PDS is instrumental limited, or if the intrinsic resolution has already been achieved.
