The goal of a TR-SCXRD experiment is to monitor the time evolution of the
excited state during pulsed excitation by collecting full X-ray structures averaged
over suitably short time windows. The key benefit of conducting a TR-SCXRD
experiment, in comparison to more established TR spectroscopic measurements, is
its ability to provide full structural information at each time point. For monochromatic TR-SCXRD studies, this means that the 3D structures of all reactants, products
and short-lived intermediate species can be determined to better than atomic-scale
resolution. These TR snapshots can then be combined into 3D “molecular movies”,
providing a unique visualisation tool that allows us to watch the solid-state reaction
occurring in real time. It should be noted, however, that only transient species with
lifetimes longer than, or on the order of, the experiment time resolution will be
observed. Nonetheless, the insight provided by these experiments has the potential to
revolutionise our understanding of photochemical reaction mechanisms, which in
turn can inform the targeted design of new systems with enhanced functionality.
In this section, we explore some of the practicalities of TR-SCXRD experiments
to study molecular species with lifetimes in the microsecond to millisecond range,
including the choice of excitation and X-ray sources, X-ray detectors, sample
delivery and data processing, and we also highlight some recent work in this area
using linkage isomer crystals. Photocrystallographic experiments to study solid-state
species with sub-microsecond lifetimes are discussed in chapter “Time-Resolved
Single-Crystal X-Ray Crystallography”.
4.1 Pump-Probe Versus Pump-Multiprobe Measurements
TR experiments are generally designed around a pump-probe measurement
sequence. In the typical experiment illustrated in Fig. 15a, a regular repeating
cycle is set up during which the crystal is excited by a short light pulse (the pump)
at t ¼ 0 and is then allowed to return to the ground state during a decay period before
the next pulse arrives. A measurement (the probe) is synchronised to the pump pulse
such that data is collected after a fixed time delay Δt. The short pump pulse generates
a small excited-state population, and the following probe measures the population
over a short time period in line with the target time resolution.
For a TR-SCXRD experiment, the recorded diffraction intensity from the short
X-ray probe is typically weak, and, as a result, the pump-probe cycle must be
repeated many times to obtain sufficient signal-to-noise ratio. This process must
then be repeated with the crystal in many different orientations to collect complete
data for structure solution. In addition, each experiment records a single time delay,
and thus multiple experiments must be conducted at different Δt to build up a picture
of the behaviour across the whole cycle. Taken together, these requirements can lead
to overall experiment times totalling many hours or even days, depending on the
required time resolution, the diffraction power of the sample and its crystal
symmetry.
With the advent of electronically gated detectors with fast readout times, it is
possible to perform “pump-multiprobe” measurements, where multiple Δt are
224
L. E. Hatcher et al.
excited state during pulsed excitation by collecting full X-ray structures averaged
over suitably short time windows. The key benefit of conducting a TR-SCXRD
experiment, in comparison to more established TR spectroscopic measurements, is
its ability to provide full structural information at each time point. For monochromatic TR-SCXRD studies, this means that the 3D structures of all reactants, products
and short-lived intermediate species can be determined to better than atomic-scale
resolution. These TR snapshots can then be combined into 3D “molecular movies”,
providing a unique visualisation tool that allows us to watch the solid-state reaction
occurring in real time. It should be noted, however, that only transient species with
lifetimes longer than, or on the order of, the experiment time resolution will be
observed. Nonetheless, the insight provided by these experiments has the potential to
revolutionise our understanding of photochemical reaction mechanisms, which in
turn can inform the targeted design of new systems with enhanced functionality.
In this section, we explore some of the practicalities of TR-SCXRD experiments
to study molecular species with lifetimes in the microsecond to millisecond range,
including the choice of excitation and X-ray sources, X-ray detectors, sample
delivery and data processing, and we also highlight some recent work in this area
using linkage isomer crystals. Photocrystallographic experiments to study solid-state
species with sub-microsecond lifetimes are discussed in chapter “Time-Resolved
Single-Crystal X-Ray Crystallography”.
4.1 Pump-Probe Versus Pump-Multiprobe Measurements
TR experiments are generally designed around a pump-probe measurement
sequence. In the typical experiment illustrated in Fig. 15a, a regular repeating
cycle is set up during which the crystal is excited by a short light pulse (the pump)
at t ¼ 0 and is then allowed to return to the ground state during a decay period before
the next pulse arrives. A measurement (the probe) is synchronised to the pump pulse
such that data is collected after a fixed time delay Δt. The short pump pulse generates
a small excited-state population, and the following probe measures the population
over a short time period in line with the target time resolution.
For a TR-SCXRD experiment, the recorded diffraction intensity from the short
X-ray probe is typically weak, and, as a result, the pump-probe cycle must be
repeated many times to obtain sufficient signal-to-noise ratio. This process must
then be repeated with the crystal in many different orientations to collect complete
data for structure solution. In addition, each experiment records a single time delay,
and thus multiple experiments must be conducted at different Δt to build up a picture
of the behaviour across the whole cycle. Taken together, these requirements can lead
to overall experiment times totalling many hours or even days, depending on the
required time resolution, the diffraction power of the sample and its crystal
symmetry.
With the advent of electronically gated detectors with fast readout times, it is
possible to perform “pump-multiprobe” measurements, where multiple Δt are
224
L. E. Hatcher et al.