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Vibronic Features in Polymer NEXAFS
Researchers conducting
polymer analyses with the Scanning Transmission X-Ray Microscope (STXM)
on Beamline 7.0.1 have observed a phenomenon not previously considered relevant
to the spectra of polymers. Their work has shown that vibronic effects--the
combination of electronic and vibrational changes--have a significant effect
on the near-edge x-ray absorption fine structure (NEXAFS) spectra of polystyrene.
The observation of vibronic features in this polymer suggests that the phenomenon
affects spectra of other large, complex molecules. This information will
be crucial in interpreting future spectra as finer resolutions make smaller
spectral features observable.
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. |
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Molecular
Wiggles Merit Attention
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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.
![comparative spectra of H-PS and D-PS](/pics/polystyrene_spectra2.gif)
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).
ALSNews Vol.
168, January 17, 2001 |
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