applied from the simplest form which is based on the fractional change of each of the
reflections on exposure to light to more sophisticated weighted least-squares scaling
[31, 37].
Once the Laue data has been collected and the data set corrected, as with
monochromatic photocrystallographic experiments, it is usual to compute a Fourier
photodifference map to evaluate the structural changes that occur upon photoexcitation [38]. When using the RATIO method, the photodifference Fourier map is
simply based on the difference between the observed laser-on and laser-off structure
factors [29, 39]. These calculations are based on the fact that the crystal does not
change phase (crystal system or space group) upon excitation which is generally true
because percentage conversions from ground to excited state structures upon excitation are low for the short-lived species generated.
In an alternative approach to the pump-probe experiments, rather than
synchronising the laser pump pulses with X-ray probe pulses generated by use of
a mechanical chopper, new detector technology has allowed the X-ray detector to be
synchronised with the laser pulse so that the detector only records X-ray intensity
while the laser is on (or records after a designated time delay). These new detectors
have fast read-out times, have the ability to internally stack series of recorded images
and, most importantly, provide very fast and reliable “gating” affording fine control
of when the detector is recording or not. With the efficiency of these detectors, some
of the limitations of the X-ray flux can be minimised, and it is possible to consider
the use of laboratory X-ray sources instead of synchrotrons for some longer lifetime
photocrystallographic experiments. The use of a gated hybrid pixel detector,
mounted on a conventional laboratory X-ray diffractometer, has been proven in an
analysis of the photoinduced linkage isomerism of sodium nitroprusside [26]. The
light-induced intensity variation between ground and excited states was detected at
the 1% level, caused by the photoswitching of the nitrosyl group, and this change
could be detected in a 6 microsecond window. The experimental approach is
illustrated in Fig. 5. In the experiment a continuous X-ray beam impinges on the
sample, and during the pump-probe cycle, with the structure continuously changing,
the scattered diffraction pattern is sampled by the gated detector. The X-ray signal is
only detected during a short adjustable window X(t). The laser pulse serves as a
trigger for the gating of the detector and synchronisation with a tuneable time delay
Δt. The maximum time resolution possible is also dependent on the electronic
response time of the X-ray detector.
With the gated hybrid pixel detector, the photon counting statistics determine
the quality of the data, and there is no dark current or read-out noise as with a
conventional CCD detector. The signal acquisition is defined by a tuneable measurement time window (X(t) in Fig. 5) whose temporal width is only limited by the
detector response time, which is the order of 100–200 ns. Also, with the pixel
detector, it is possible to have a number of simultaneous measurement windows,
with different delay times that can be acquired by the detector at the same time.
This means that multiple time-resolved experiments could be carried out at the
same time.
Time-Resolved Single-Crystal X-Ray Crystallography
249
reflections on exposure to light to more sophisticated weighted least-squares scaling
[31, 37].
Once the Laue data has been collected and the data set corrected, as with
monochromatic photocrystallographic experiments, it is usual to compute a Fourier
photodifference map to evaluate the structural changes that occur upon photoexcitation [38]. When using the RATIO method, the photodifference Fourier map is
simply based on the difference between the observed laser-on and laser-off structure
factors [29, 39]. These calculations are based on the fact that the crystal does not
change phase (crystal system or space group) upon excitation which is generally true
because percentage conversions from ground to excited state structures upon excitation are low for the short-lived species generated.
In an alternative approach to the pump-probe experiments, rather than
synchronising the laser pump pulses with X-ray probe pulses generated by use of
a mechanical chopper, new detector technology has allowed the X-ray detector to be
synchronised with the laser pulse so that the detector only records X-ray intensity
while the laser is on (or records after a designated time delay). These new detectors
have fast read-out times, have the ability to internally stack series of recorded images
and, most importantly, provide very fast and reliable “gating” affording fine control
of when the detector is recording or not. With the efficiency of these detectors, some
of the limitations of the X-ray flux can be minimised, and it is possible to consider
the use of laboratory X-ray sources instead of synchrotrons for some longer lifetime
photocrystallographic experiments. The use of a gated hybrid pixel detector,
mounted on a conventional laboratory X-ray diffractometer, has been proven in an
analysis of the photoinduced linkage isomerism of sodium nitroprusside [26]. The
light-induced intensity variation between ground and excited states was detected at
the 1% level, caused by the photoswitching of the nitrosyl group, and this change
could be detected in a 6 microsecond window. The experimental approach is
illustrated in Fig. 5. In the experiment a continuous X-ray beam impinges on the
sample, and during the pump-probe cycle, with the structure continuously changing,
the scattered diffraction pattern is sampled by the gated detector. The X-ray signal is
only detected during a short adjustable window X(t). The laser pulse serves as a
trigger for the gating of the detector and synchronisation with a tuneable time delay
Δt. The maximum time resolution possible is also dependent on the electronic
response time of the X-ray detector.
With the gated hybrid pixel detector, the photon counting statistics determine
the quality of the data, and there is no dark current or read-out noise as with a
conventional CCD detector. The signal acquisition is defined by a tuneable measurement time window (X(t) in Fig. 5) whose temporal width is only limited by the
detector response time, which is the order of 100–200 ns. Also, with the pixel
detector, it is possible to have a number of simultaneous measurement windows,
with different delay times that can be acquired by the detector at the same time.
This means that multiple time-resolved experiments could be carried out at the
same time.
Time-Resolved Single-Crystal X-Ray Crystallography
249