powered and driven by inexpensive electronics. They can also be flexibly built into
custom arrays to fit the diffraction setup, allowing irradiation of multiple sides of the
crystal. This is a distinct benefit over point laser sources, as irradiation at multiple
crystal faces may enhance the achievable photoconversion level. Paired with a
comparatively inexpensive function generator, LEDs can also be pulsed down to
microseconds or faster. Using LEDs in this way provides fine control over the pumpprobe cycle time, including the ability to vary the total cycle time and the portion of
the time for which the LEDs are active. Where the target time resolution allows,
lengthening the LED pump-pulse (c.f. Fig. 15) may be beneficial for some systems
as this helps to spread out the energy load on the crystal in time, potentially
circumventing some of the heating/damage and absorption issues outlined above
and, in principle, accessing higher conversion levels.
4.3 X-Ray Sources
The X-ray source and detector primarily determine how much signal can be accumulated during each pump-probe (or pump-multiprobe) cycle. A high-flux X-ray
source and efficient detector will allow the same signal-to-noise ratio to be obtained
in fewer cycles and a shorter overall experiment duration than with a lower-flux
source or less efficient detector. As well as the absolute time saving, a shorter
experiment minimises the total exposure of the sample to the excitation source,
which is important for systems where excitation causes progressive crystal damage
or bleaching.
Laboratory X-ray instruments are based on the Coolidge hot-cathode X-ray tube.
A tungsten filament cathode is heated to glowing temperature under vacuum to
produces free electrons by thermionic emission. The electrons are accelerated at very
high voltage and directed by an electrostatic lens to impact a metal anode. On
impacting the anode, the majority of the electron kinetic energy is dissipated as
heat, but a small percentage is converted X-ray photons. Electrons deflected by
atomic nuclei produce a continuous spectrum of polychromatic X-rays through the
bremsstrahlung (“braking radiation”) effect. Provided the kinetic energy is greater
than the binding energy, some impacts can eject core electrons to produce vacant
core holes. When these are filled by relaxation of valence electrons, the excess
energy is emitted as X-ray photons with well-defined energies. By filtering the
emission spectrum to remove the bremsstrahlung radiation using monochromator
crystals, monochromatic X-rays at these atomic wavelengths are obtained.
Typical X-ray tubes for crystallography operate at 50 kV and use Cu or
Mo anodes for which the strongest K α emission lines are at 8.046 and
17.480 keV (1.54 and 0.71 Å, respectively). The improved availability of Ag has
also led to sources using Ag K α radiation becoming more common in the laboratory
(22.163 keV, 0.56 Å). The X-ray flux produced by the tube depends on the power
Watching Photochemistry Happen: Recent Developments in Dynamic Single-Crystal. . .
227
custom arrays to fit the diffraction setup, allowing irradiation of multiple sides of the
crystal. This is a distinct benefit over point laser sources, as irradiation at multiple
crystal faces may enhance the achievable photoconversion level. Paired with a
comparatively inexpensive function generator, LEDs can also be pulsed down to
microseconds or faster. Using LEDs in this way provides fine control over the pumpprobe cycle time, including the ability to vary the total cycle time and the portion of
the time for which the LEDs are active. Where the target time resolution allows,
lengthening the LED pump-pulse (c.f. Fig. 15) may be beneficial for some systems
as this helps to spread out the energy load on the crystal in time, potentially
circumventing some of the heating/damage and absorption issues outlined above
and, in principle, accessing higher conversion levels.
4.3 X-Ray Sources
The X-ray source and detector primarily determine how much signal can be accumulated during each pump-probe (or pump-multiprobe) cycle. A high-flux X-ray
source and efficient detector will allow the same signal-to-noise ratio to be obtained
in fewer cycles and a shorter overall experiment duration than with a lower-flux
source or less efficient detector. As well as the absolute time saving, a shorter
experiment minimises the total exposure of the sample to the excitation source,
which is important for systems where excitation causes progressive crystal damage
or bleaching.
Laboratory X-ray instruments are based on the Coolidge hot-cathode X-ray tube.
A tungsten filament cathode is heated to glowing temperature under vacuum to
produces free electrons by thermionic emission. The electrons are accelerated at very
high voltage and directed by an electrostatic lens to impact a metal anode. On
impacting the anode, the majority of the electron kinetic energy is dissipated as
heat, but a small percentage is converted X-ray photons. Electrons deflected by
atomic nuclei produce a continuous spectrum of polychromatic X-rays through the
bremsstrahlung (“braking radiation”) effect. Provided the kinetic energy is greater
than the binding energy, some impacts can eject core electrons to produce vacant
core holes. When these are filled by relaxation of valence electrons, the excess
energy is emitted as X-ray photons with well-defined energies. By filtering the
emission spectrum to remove the bremsstrahlung radiation using monochromator
crystals, monochromatic X-rays at these atomic wavelengths are obtained.
Typical X-ray tubes for crystallography operate at 50 kV and use Cu or
Mo anodes for which the strongest K α emission lines are at 8.046 and
17.480 keV (1.54 and 0.71 Å, respectively). The improved availability of Ag has
also led to sources using Ag K α radiation becoming more common in the laboratory
(22.163 keV, 0.56 Å). The X-ray flux produced by the tube depends on the power
Watching Photochemistry Happen: Recent Developments in Dynamic Single-Crystal. . .
227