Chemistry Reference
In-Depth Information
2D detector
undulator
laser
pulses
storage ring of
synchrotron
e
-
X-ray
pulses
Pulsed
X-ray beam
single
crystal
sample
Fig. 4 Schematic representation of the principles of pump-probe time-resolved X-ray photodif-
fraction
to increase its concentration to a sufficient amount, and the structure of the partially
reacted crystal is determined. The concentration of the species produced in such an
experiment which is considered sufficient for a reliable structure determination
depends on the particular case, and also relates to factors that include the magnitude
of the expected structural change, the diffraction resolution/ability of the sample,
and eventual deterioration of crystallinity during excitation. For organic crystals, as
a very arbitrary value, approximate conversions of
10% are commonly considered
sufficient to obtain a reliable structural signature, whereas yields down to 2-5% are
also significant for metal-organic or inorganic samples. It should be noted, how-
ever, that the estimated values of conversion yields are inherently biased by the
method of their measurement (in particular, they could largely depend on the details
of the refinement procedure). As there is no method for straightforward and
completely accurate detection of yield in single crystals, yields can occasionally
be very different when they are obtained by different methods. Empirically, the best
estimates are obtained when two or more (diffraction and spectroscopic) methods
based on fundamentally different signals from the product species are used to
estimate the photoyield.
In the absence of undesired phase transitions, in order to decrease decay of the
product and thermal effects, which would otherwise cause additional smearing of
electron density due to atomic oscillations, X-ray photodiffraction methods are
usually performed at low temperatures or even under cryogenic conditions.
Although in ordered crystals temperature effects on atomic positions can be deter-
mined precisely, in disordered crystals, and especially when occupancies of the two
components are very different, when component structures have very similar
conformations or in cases of multiply disordered structures, increased thermal
motion adds up to burden structural complexity. Low temperatures also contribute
to decreased heating effects, and they could also prevent decreased diffraction
ability due to partial melting of the sample, desolvation, or undesired side reactions
(e.g., diffusion-controlled recombination in the case of radical reactions). On rare
occasions, however, application of low temperature may be unsuitable and coun-
terproductive, for example, if the studied reactions/processes have qualitatively
different product(s) as outcome at low temperature relative to room temperature, or
when reactants or products undergo thermal phase transitions by cooling. Selective
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