Chemistry Reference
In-Depth Information
photocrystallography
.
1
This evolving analytical technique is related to application
of XRD diffraction methods to resolve photophysical or photochemical problems.
In its most general definition, X-ray photodiffraction utilizes XRD on samples that
are excited by light, aimed at studying the minor changes in their structure related to
photoexcitation - either during the process, immediately after excitation, or after
the system has relaxed to a ground state. In practice, a sample in single crystal or
powder state is excited by UV or visible light, and diffraction data are collected
before, during, and/or after excitation (Fig.
1
). Depending on the process (reversible
or irreversible) and the stability of the photoinduced species (unstable, metastable,
or stable), the duration and the timing of the “excitation” beam (UV or visible light)
and the “analytical” beam (X-rays) is adjusted so as to collect a sufficient amount of
data which would provide meaningful information on the structure before and
during/after photoexcitation. With ordinary experimental setups, typical times of
data collection are on the order of hours, while with special experimental setups and
in time-resolved mode they can be in the millisecond, microsecond, or picosecond
range (currently, down to a resolution of several tens to several hundreds of
picoseconds). When the timescale of the experiment is ultrafast, the excitation and
analytical beams are pulsed and they are usually termed the “pump” and “probe.”
In principle, X-ray photodiffraction can be used in
steady-state mode
(Figs.
2
and
3
)orin
time-resolved mode
(Fig.
4
) by application on either single crystals
(Fig.
2
) or microcrystals (Fig.
3
). In the course of steady-state photodiffraction, the
sample is excited ex situ (the species is created before actual structure analysis) or
in situ (the species is being created simultaneously with data collection for structure
determination) by using either a continuous-wave source or pulsed light source (or a
combination of several sources, some of which can be used for creation and other
for intentional decay of the desired species). The structural changes are probed by
using X-rays from a laboratory or synchrotron source. For instance, in the simplest
Fig. 1 Schematic of a setup
for in situ steady-state X-ray
photodiffraction using
continuous wave and pulsed
excitation sources
1
Because the term “diffraction” describes the actual physical phenomenon, it is more general in
meaning, and it is also frequently used by physicists and biologists, “X-ray photodiffraction”
appears to be a more appropriate term than “photocrystallography” and thus it will be used
throughout this article.
