6
K. D. M. Harris et al.
properties of the bonding environment of the X-ray absorbing atom, measurement
of X-ray birefringence has the potential to yield information on the orientational
properties of individual molecules and/or bonds within an anisotropic material.
X-ray birefringence is significant only when the energy of the incident linearly
polarized X-ray beam is close to an X-ray absorption edge of an element in the
material. As such, the technique is sensitive to the orientational properties of the local
bonding environment of the X-ray absorbing element. Our early applications of the
XBI technique focused on materials containing brominated organic molecules, using
incident linearly polarized X-rays with energy tuned to the Br K-edge. In this case,
it was shown [8] that X-ray birefringence depends specifically on the orientations of
C–Br bonds in the material. The strong dependence on the orientation of the C–Br
bonds arises because the incident X-ray beam, with energy corresponding to the Br
K-edge, can promote a core (1s) electron on the Br atom to the σ* anti-bonding
orbital associated with the C–Br bond. Given the directional characteristics of the
vacant σ* anti-bonding orbital, the probability of occurrence of this process depends
strongly on the orientational relationship between the C–Br bond and the direction of
linear polarization of the incident X-ray beam. We note that the phenomenon of Xray birefringence is “parity even”, and thus anti-parallel C–Br bond directions (i.e.,
C–Br and Br–C) within a material exhibit identical behavior (consequently, X-ray
birefringence is observed for centrosymmetric materials).
The capability of X-ray birefringence measurements to yield insights into molecular orientational properties was first demonstrated from studies of a model material
with known bond orientations [8], and this capability was then exploited to determine
changes in molecular orientational distributions associated with an order–disorder
phase transition in the solid state [9]. However, these early X-ray birefringence studies
used a narrowly focused incident X-ray beam and did not provide spatially resolved
mapping of X-ray birefringence across the material. Subsequently, an experimental
setup (Fig. 1.2) was proposed [19] to allow X-ray birefringence data to be recorded
in “imaging mode”, using a large-area linearly polarized incident X-ray beam and
recording the X-ray intensity in a spatially resolved manner using an area detector.
With this experimental setup, X-rays transmitted through different parts of the sample
impinge on different pixels of the detector, allowing the X-ray birefringence of the
sample to be mapped in a spatially resolved manner. This development represented
the first report [19] of the X-ray birefringence imaging (XBI) technique.
While early XBI experiments focused on studies of brominated materials using
linearly polarized X-rays tuned to the Br K-edge, the application of XBI has also been
extended to study other X-ray absorption edges, allowing the local bonding environment of other types of element in materials to be probed. However, in the overview
presented in this chapter, we focus on XBI studies at the Br K-edge, presenting examples of the application of the technique to determine the orientational properties of
C–Br bonds in a range of organic materials.
K. D. M. Harris et al.
properties of the bonding environment of the X-ray absorbing atom, measurement
of X-ray birefringence has the potential to yield information on the orientational
properties of individual molecules and/or bonds within an anisotropic material.
X-ray birefringence is significant only when the energy of the incident linearly
polarized X-ray beam is close to an X-ray absorption edge of an element in the
material. As such, the technique is sensitive to the orientational properties of the local
bonding environment of the X-ray absorbing element. Our early applications of the
XBI technique focused on materials containing brominated organic molecules, using
incident linearly polarized X-rays with energy tuned to the Br K-edge. In this case,
it was shown [8] that X-ray birefringence depends specifically on the orientations of
C–Br bonds in the material. The strong dependence on the orientation of the C–Br
bonds arises because the incident X-ray beam, with energy corresponding to the Br
K-edge, can promote a core (1s) electron on the Br atom to the σ* anti-bonding
orbital associated with the C–Br bond. Given the directional characteristics of the
vacant σ* anti-bonding orbital, the probability of occurrence of this process depends
strongly on the orientational relationship between the C–Br bond and the direction of
linear polarization of the incident X-ray beam. We note that the phenomenon of Xray birefringence is “parity even”, and thus anti-parallel C–Br bond directions (i.e.,
C–Br and Br–C) within a material exhibit identical behavior (consequently, X-ray
birefringence is observed for centrosymmetric materials).
The capability of X-ray birefringence measurements to yield insights into molecular orientational properties was first demonstrated from studies of a model material
with known bond orientations [8], and this capability was then exploited to determine
changes in molecular orientational distributions associated with an order–disorder
phase transition in the solid state [9]. However, these early X-ray birefringence studies
used a narrowly focused incident X-ray beam and did not provide spatially resolved
mapping of X-ray birefringence across the material. Subsequently, an experimental
setup (Fig. 1.2) was proposed [19] to allow X-ray birefringence data to be recorded
in “imaging mode”, using a large-area linearly polarized incident X-ray beam and
recording the X-ray intensity in a spatially resolved manner using an area detector.
With this experimental setup, X-rays transmitted through different parts of the sample
impinge on different pixels of the detector, allowing the X-ray birefringence of the
sample to be mapped in a spatially resolved manner. This development represented
the first report [19] of the X-ray birefringence imaging (XBI) technique.
While early XBI experiments focused on studies of brominated materials using
linearly polarized X-rays tuned to the Br K-edge, the application of XBI has also been
extended to study other X-ray absorption edges, allowing the local bonding environment of other types of element in materials to be probed. However, in the overview
presented in this chapter, we focus on XBI studies at the Br K-edge, presenting examples of the application of the technique to determine the orientational properties of
C–Br bonds in a range of organic materials.