The group from North Carolina State University and the State University of New York, Stony Brook, came to the ALS after preliminary work at the National Synchrotron Light Source's Beamline X1A. The ALS STXM offered a high photon flux with a resolving power of 5700, enabling them to study subtle differences in the fine structures of polystyrene spectra. In order to isolate the vibronic effect from the chemical shifts known to affect the shapes of NEXAFS peaks, they collected spectra from regular protonated polystyrene (H-PS) and polystyrene in which the hydrogen atoms were replaced by deuterium (D-PS). In D-PS, the deuterium atoms that replace the hydrogen atoms differ from the hydrogen atoms in mass only. The mass difference leads to a dramatic change in the C-H stretch energies without introducing a difference in chemical state. These stretches translate to vibronic features in the NEXAFS spectra. Thus, any real differences between the spectra for H-PS and D-PS could be attributed to vibronic effects.

A clever sample setup took advantage of STXM's spatial resolution to provide a control for the inevitable differences between sets of data taken in separate sessions. The scientists mounted two samples, one of H-PS and one of D-PS, on the same sample mount less than one micron apart. They then captured a series of x-ray transmission images of the samples, scanning across a typical range of energies. From this they could measure x-ray transmission and incident photon flux (needed to normalize the data) for both samples at each energy in the scan. By taking the data for both samples concurrently, they minimized low-frequency noise, mechanical drift, and electron beam decay factors and exactly matched the relative energy scales for the two samples.

Samples of deuterated polystyrene (D-PS, left) and protonated hydrogenated polystyrene (H-PS, right) mounted next to each other, as imaged in the scanning transmission x-ray microscope. The black areas are a supporting grid, and the light gray is the area between samples.

Molecular Wiggles Merit Attention

The amount of x-ray light a polymer absorbs at different wavelengths, or energies, reveals surprising detail about the chemistry of the material. In fact, the shape of a plot of these absorptions (an x-ray absorption spectrum) is so characteristic that it is often referred to as a chemical fingerprint. Near-edge x-ray absorption fine structure (NEXAFS) spectra show the fine details near the lowest x-ray energies the material can appreciably absorb. Recent advances in resolution have improved the sensitivity of NEXAFS to the chemical states of atoms in a polymer, and now comes evidence that these improvements are also bringing another kind of effect into view--vibronic features, caused by vibrations within the polymer molecule.

Vibronic effects have been known to play a significant role in shaping the spectra of relatively small molecules, but until now the effect was considered irrelevant for larger molecules, such as complex polymers. Yet, this study on polystyrene shows an unambiguous difference between the spectra of two high-molecular-weight polymers whose only practical difference is the vibration of their atoms. Clearly, vibronic features do significantly affect NEXAFS spectra in larger molecules. And as spectroscopists achieve still higher resolutions, they can now expect the study of vibronic effects to yield new insights into polymer chemistry.

The resulting carbon 1s spectra for H-PS and D-PS clearly differed. Moreover, the differences between them corresponded well to the vibronic shift seen in previous studies of benzene, which consists of a ring structure that is analogous to that in the repeating unit of polystyrene. Correlations were also seen with vibronic effects in calculated spectra for toluene (an even closer analog to polystyrene's repeating structure). This, together with the much wider peak width seen for the polystyrene spectrum than predicted by chemical shift alone, suggests that vibronic effects have a notable effect on the C 1s --> 1p* transition in all aromatic polymers.

Comparison of high-resolution C 1s NEXAFS spectra from H-PS and D-PS, showing a clear shape difference attributable to vibronic effects. You can also see this as an MPEG (512K) or QuckTime (513K) movie.

This study is the first to report vibronic effects in the carbon 1s NEXAFS spectrum of a high-molecular-weight polymer. Though current theories of polymer spectroscopy hold that only changes in chemical state are relevant to the spectra of such large molecules, this work has shown clearly that vibronic effects must be considered in these analyses.

Research conducted by S.G. Urquhart and H. Ade (North Carolina State University) and M. Rafailovich, J.S. Sokolov, and Y. Zhang (State University of New York, Stony Brook).

Research funding: Office of Basic Energy Sciences (BES) and Office of Biological and Environmental Research, U.S. Department of Energy; National Science Foundation. Operation of the ALS is supported by BES.

Publication about this research: S.G. Urquhart, H.Ade, M. Rafailovich, J.S. Sokolov, and Y. Zhang, "Chemical and vibronic effects in the high-resolution near-edge x-ray absorption fine structure spectra of polystyrene isotopomers," Chem. Phys. Lett. 322, 412 (2000).