Chemistry Reference
In-Depth Information
methods, low-intensity X-rays are normally non-destructive to the sample and they
also provide comparably better resolution. Moreover, while by using electron
spectroscopy
in the UV-visible region - nowadays routinely employed to study
photoinduced and dynamic processes - one inevitably has to excite electrons,
X-rays do not interfere with electron transitions; thus they can be employed to
study electron excitations without interference with the observed phenomenon,
which is one of the most important assets of the technique discussed in this chapter.
From a practical viewpoint, X-rays are commonly available in either continuous
mode (e.g., from an X-ray tube) or pulsed mode (e.g., when produced by laser-
induced excitation of metals, or as synchrotron X-ray radiation obtained by slowing
very fast electrons), and they can be conveniently manipulated and easily detected.
In that respect, the Laue XRD method, which utilizes non-monochromatic X-rays
and for which some of the most important obstacles that were relevant in the past
have now been solved, holds great promise for further application to systems
spanning from small molecules to macromolecules [
2
,
3
]. More importantly,
X-rays interact with matter at very short (attosecond) timescales, and are thus a
convenient tool to probe a very wide range of processes which are slower than that
time-limit. Due to these properties, X-rays are the principle component of a wider
group of analytical methods for structural analysis of bulk materials and surfaces,
that, in addition to XRD methods (methods of single crystal and powder diffrac-
tion), include techniques that are based on scattering (small-angle X-ray scattering,
grazing-incidence X-ray scattering, and similar) and spectroscopy (X-ray absorp-
tion and emission spectroscopy), some of which are extensively elaborated on in the
other chapters of this volume. Moreover, by using X-rays, both surface and bulk
phenomena can be accessed, while the operating mode and conditions can be
adapted so as to provide spatially and time-averaged (or time-resolved) information
on structures of varying complexity. For instance, complex enzyme-catalyzed
processes can be conveniently “frozen” by physicochemical means at a certain
desired stage and directly studied [
4
]. While diffraction-based
steady-state
XRD
techniques are usually employed to investigate spatially averaged bulk structure,
imaging [
5
] or microscopic X-ray-based methods provide additional insights into
the spatial non-homogeneities of the material that are related to multiple phases,
defects, impurities, and similar imperfections of the structure. On the other hand,
the more recently developed
time-resolved
X-ray-based methods (some of which
are mentioned in the concluding section of this chapter), which are still rather
demanding in respect of instrumentation, can be used to study the structural
consequences of ultrafast photoinduced perturbations, while also analyzing the
temporal profile of the related processes.
1.2 The Basic Principles
At the forefront of recent developments in the field of X-ray-based
analytical methods is
X-ray photodiffraction
, usually referred to as
X-ray
