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Carbonyl core (C 1s, O 1s) f π* CdO transitions are distinctive in the near-edge X-ray absorption fine structure (NEXAFS) spectra of species containing carbonyl groups. These features are used for the chemical microanalysis of organic materials using X-ray microscopy. We have explored the chemical sensitivity of these features in C 1s and O 1s NEXAFS spectra for a series of polymers containing the carbonyl group in a range of different bonding environments. Ab initio calculations are used to explain the origin of the observed trends and to explore the effect that orbital interactions have on the energy of these core (C 1s, O 1s) f π* CdO features. The differences between the experimental and the calculated carbonyl core (C 1s, O 1s) f π* CdO transition energies are systematic and can be used to develop a semiempirical method for predicting the absolute (experimental) transition energies from the calculated transition energies. This relationship is applied to a large body of calculated transition energy data to prepare correlation diagrams for the carbonyl (C 1s, O 1s) f π* CdO transitions. These correlation diagrams will be useful for the analytical application of NEXAFS spectroscopy to organic materials.

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The C 1s and O 1s X-ray absorption spectra of poly(ethylene terephthalate) (PET) have been recorded using transmission, fluorescence, and electron yield detection. The corresponding electron energy loss spectra (EELS) have been recorded in a scanning transmission electron microscope. These results are compared to the C 1s and O 1s spectra of gas phase 1,4-dimethyl terephthalate (the monomer of PET) recorded using EELS. The comparison of monomer and polymer materials in different phases and with different techniques has aided the understanding of the relative strengths and limitations of each technique as well as assisting the spectral interpretation. Good agreement is found in the overall shape and the energies of the spectral features. Relatively minor differences in intensities can be understood in terms of the properties of the individual spectroscopic techniques. The critical dose for radiation damage by 100 keV electrons incident on PET at 100 K is found to be (1.45 ( 0.15) × 10 3 eV nm -3 . In contrast, the critical dose for radiation damage by 302 eV X-rays incident on PET at 300 K is (1.2 ( 0.6) × 10 4 eV nm -3 . A figure of merit involving the product of critical energy dose and spectral efficiency (as expressed by the appropriate G value) is developed. This indicates that, for near-edge studies involving a 20 eV spectral width, there is ∼500-fold advantage of X-ray absorption studies on room temperature PET relative to electron energy loss studies of cooled PET.

SYNOPSISCore excitation spectra of selected small molecule analogue species have been acquired to assist interpretation of the core excitation spectra of model methylenediphenyldiisocyanate (MDI) polyurethane polymers. Oscillator strength spectra for C Is and 0 Is core excitation of diethyl ether and diisopropyl ether; C Is, N Is, and 0 IS core excitation of urea, Nphenyl urea, N,N-diphenyl urea, ethyl carbamate, N-phenyl carbamate, N-phenyl N-methyl carbamate, and benzyl carbamate have been derived from gas phase electron energy loss spectra (EELS). Extended Huckel Molecular Orbital (EHMO) calculations are used to assist assignment and to interpret the effect of *-electron delocalization on the gas phase spectra. Functional group identification by core excitation is explored for the purpose of using core excitation spectra for microanalysis of polyurethane polymers. 0 1995 John Wiley & Sons, Inc.Keywords: polyurethanes core excitation spectroscopy NEXAFS EELS EHMO molecular models of polymer subunits I NTRO DU CTlO NPart I of this series of articles' explored the use of core excitation spectroscopy for compositional analysis of polyurethane polymers. In particular, transitions characteristic of various functional groups were identified in order to gain insight into how spatially resolved core excitation spectroscopy could be used to identify segregated species in polyurethane foams. Compositional information is required to provide an improved understanding of phase separation processes in polyurethanes and to assist in the correlation of phase separation effects with ultimate polymer performance. Core excitation spectroscopy' is well suited to this sort of problem since the spectral features are very sensitive to local chemical structure and since very high spatial res-

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