irradiation sources such as low-power continuous-wave lasers or LEDs are common.
Of these, LED setups are an attractive choice as they allow for flexible array design,
a choice of a wide variety of LED wavelengths and form factors and are comparatively low cost.
An example of an LED setup is shown in Fig. 5 [53]. In this setup the LEDs are
mounted in a supporting ring attached to the end of the diffractometer cryostream.
The ring positions six LEDs with the centre of the emission cone directed to the
sample position and incorporates gaps for the incident and diffracted X-ray beams.
Other LED device designs for in situ SCXRD incorporate fibre optics and focussing
mirrors to achieve the desired illumination arrangement [54]. As well as being highly
practical and easy to implement, LED arrays also spread the radiation load uniformly
over time and provide better spatial coverage across the crystal surface, avoiding
potential problems with laser beam damage, unwanted sample heating and uneven
illumination.
3.2 Static Ground and Excited States (Photostationary
Structures)
Where the excited state has a lifetime of hours or greater at low temperature, standard
SCXRD experiments can be conducted to establish the ground- and excited-state
structures.
A crystal is mounted and cooled in the dark, allowing a structure of the ground
state to be obtained with minimal contamination from the excited state. The crystal is
then illuminated for a set amount of time, which typically ranges from minutes to
hours depending on the system. At low temperatures, the excited state is kinetically
trapped, and a substantial steady-state population accumulates during irradiation.
This typically happens at temperatures below 150 K, although the critical temperature varies between systems. A second X-ray dataset is then collected to obtain the
structure of the excited state. To confirm that a maximum excitation level has been
reached, a second period of illumination is usually performed and inspected for
further conversion to the excited-state isomer. Once the maximum photostationary
population has been obtained, the crystal is held in the dark, and a series of variabletemperature data collections are completed to investigate the temperature range over
which the metastable state decays.
Figure 4 in the previous section shows as an example of the structures of the
ground-state nitro (η
1 -NO 2 ) and 100% excited-state endo-nitrito (η
1 -ONO) isomers
of the [Pd(Bu 4 dien)(NO 2 )]BPh 4 nitrite linkage isomer complex.
210
L. E. Hatcher et al.
Of these, LED setups are an attractive choice as they allow for flexible array design,
a choice of a wide variety of LED wavelengths and form factors and are comparatively low cost.
An example of an LED setup is shown in Fig. 5 [53]. In this setup the LEDs are
mounted in a supporting ring attached to the end of the diffractometer cryostream.
The ring positions six LEDs with the centre of the emission cone directed to the
sample position and incorporates gaps for the incident and diffracted X-ray beams.
Other LED device designs for in situ SCXRD incorporate fibre optics and focussing
mirrors to achieve the desired illumination arrangement [54]. As well as being highly
practical and easy to implement, LED arrays also spread the radiation load uniformly
over time and provide better spatial coverage across the crystal surface, avoiding
potential problems with laser beam damage, unwanted sample heating and uneven
illumination.
3.2 Static Ground and Excited States (Photostationary
Structures)
Where the excited state has a lifetime of hours or greater at low temperature, standard
SCXRD experiments can be conducted to establish the ground- and excited-state
structures.
A crystal is mounted and cooled in the dark, allowing a structure of the ground
state to be obtained with minimal contamination from the excited state. The crystal is
then illuminated for a set amount of time, which typically ranges from minutes to
hours depending on the system. At low temperatures, the excited state is kinetically
trapped, and a substantial steady-state population accumulates during irradiation.
This typically happens at temperatures below 150 K, although the critical temperature varies between systems. A second X-ray dataset is then collected to obtain the
structure of the excited state. To confirm that a maximum excitation level has been
reached, a second period of illumination is usually performed and inspected for
further conversion to the excited-state isomer. Once the maximum photostationary
population has been obtained, the crystal is held in the dark, and a series of variabletemperature data collections are completed to investigate the temperature range over
which the metastable state decays.
Figure 4 in the previous section shows as an example of the structures of the
ground-state nitro (η
1 -NO 2 ) and 100% excited-state endo-nitrito (η
1 -ONO) isomers
of the [Pd(Bu 4 dien)(NO 2 )]BPh 4 nitrite linkage isomer complex.
210
L. E. Hatcher et al.