measured during a single experiment as shown in Fig. 15b. The process is conceptually similar to the traditional pump-probe experiment except that all the Δt are
collected in one series after arrival of the excitation pulse. This removes the need for
multiple pump-probe experiments to access different time delays and thus dramatically shortens the overall experiment duration.
4.2 Excitation Sources
Historically, one of the major goals of pump-probe experiments has been to achieve
the shortest possible time resolution. As a result, experiments have generally used
pulsed lasers to provide intense, focussed light pulses with pulse widths in the
nanosecond to picosecond range. For experiments with target time resolutions in
this range, lasers remain the light source of choice.
Intense laser pulses can however present difficulties that should be considered
when planning an experiment. Photocrystallography experiments intentionally use
the smallest crystals possible, within the limit of achieving sufficient diffraction
intensity from the chosen X-ray setup, to maximise the penetration depth of the
pump light through the crystal. As a consequence, a high-power laser pump with a
short pulse duration will likely deliver many more photons per pulse than the number
of molecular absorbers present in the sample volume. This increases the likelihood
of non-linear responses such as multiphoton absorption processes that may be both
unexpected and undesirable [66]. Intense light pulses may also lead to
photobleaching, so that only a fraction of the photons induce photoconversion and
the remaining photons are either not absorbed or, worse, produce laser heating and
accelerate the decay of the excited state and/or damage the crystals.
Laser sources also bring with them several practical issues. The requirement for
careful and often time-consuming alignment, together with the stringent safety
protocols that need to be put in place to mitigate risks to users, adds to the
complexity of the experiment. The cost of both the laser and of the optics needed
to deliver the beam to the sample is generally considerable. Finally, some pulsed
lasers operate at a fixed repetition rate, which somewhat reduces flexibility. If the
timescale of the process being studied is much faster than the time between repetitions, considerable “dead time” is introduced between measurements, whereas if a
system does not relax quickly enough between pulses, it may simply not be possible
to study it with the setup. Newer, variable repetition rate lasers circumvent this issue,
but are considerably more expensive.
If a high intensity and short pulse duration are not required, it may be worthwhile considering other excitation sources. As outlined in Sect. 3, some
photocrystallographic experiments, particularly on laboratory equipment, are
performed using LEDs. LEDs are cheap and readily available in a range of visible
and near-UV wavelengths, can reach a reasonably high power output and can be
226
L. E. Hatcher et al.
collected in one series after arrival of the excitation pulse. This removes the need for
multiple pump-probe experiments to access different time delays and thus dramatically shortens the overall experiment duration.
4.2 Excitation Sources
Historically, one of the major goals of pump-probe experiments has been to achieve
the shortest possible time resolution. As a result, experiments have generally used
pulsed lasers to provide intense, focussed light pulses with pulse widths in the
nanosecond to picosecond range. For experiments with target time resolutions in
this range, lasers remain the light source of choice.
Intense laser pulses can however present difficulties that should be considered
when planning an experiment. Photocrystallography experiments intentionally use
the smallest crystals possible, within the limit of achieving sufficient diffraction
intensity from the chosen X-ray setup, to maximise the penetration depth of the
pump light through the crystal. As a consequence, a high-power laser pump with a
short pulse duration will likely deliver many more photons per pulse than the number
of molecular absorbers present in the sample volume. This increases the likelihood
of non-linear responses such as multiphoton absorption processes that may be both
unexpected and undesirable [66]. Intense light pulses may also lead to
photobleaching, so that only a fraction of the photons induce photoconversion and
the remaining photons are either not absorbed or, worse, produce laser heating and
accelerate the decay of the excited state and/or damage the crystals.
Laser sources also bring with them several practical issues. The requirement for
careful and often time-consuming alignment, together with the stringent safety
protocols that need to be put in place to mitigate risks to users, adds to the
complexity of the experiment. The cost of both the laser and of the optics needed
to deliver the beam to the sample is generally considerable. Finally, some pulsed
lasers operate at a fixed repetition rate, which somewhat reduces flexibility. If the
timescale of the process being studied is much faster than the time between repetitions, considerable “dead time” is introduced between measurements, whereas if a
system does not relax quickly enough between pulses, it may simply not be possible
to study it with the setup. Newer, variable repetition rate lasers circumvent this issue,
but are considerably more expensive.
If a high intensity and short pulse duration are not required, it may be worthwhile considering other excitation sources. As outlined in Sect. 3, some
photocrystallographic experiments, particularly on laboratory equipment, are
performed using LEDs. LEDs are cheap and readily available in a range of visible
and near-UV wavelengths, can reach a reasonably high power output and can be
226
L. E. Hatcher et al.