There are many interesting chemical compounds whose structure and properties change when they are illuminated. It's useful to understand the difference in the structure and behaviour of the compound both in the light and dark to develop a full characterisation of the material.
Inelastic neutron scattering (INS) is a useful spectroscopic technique, as it allows the measurement of vibrational modes without any restrictions from the usual optical selection rules seen for IR and Raman spectroscopies. However, in these experiments, the sample is sealed and often needs to be kept at low temperatures, making it only possible to measure the sample in the dark.
ISIS scientist Hamish Cavaye has a background in light-sensitive materials, and wondered if it would be possible to create an environment where a sample could be illuminated whilst it is in the neutron beam. Alongside Maksim Schastny from the ISIS Electrical and Electronic User Support Group, he developed a custom-made cell that includes an LED that illuminates the sample in situ.
“The first challenge was to find out if the LED would survive the low temperatures" explains Maksim; “usually they are rated to -20°C, but we wanted to go to below -200°C! Through testing them in liquid nitrogen, we were able to find an LED that was still functional, and incorporate that into an aluminium sample holder with a 3D-printed bracket. I wanted to keep the design simple, and not over engineer it."
Once Maksim had built the cell, Hamish was able to run an experiment on Tosca and measure the change in structure of the photochromic material CMTE. “The wavelength of the light from the LED was different under the cold conditions" explains Hamish; “but it was still within the range needed for the change in CMTE to take place."
The results of the experiment confirmed that the structural change had occurred when the sample was illuminated, and the INS showed that the vibrational modes were affected by this change, indicative of the formation of a bridging C-C bond.
“We are already looking at how this new sample environment can be used in other experiments, including studying light-sensitive proteins using quasi-elastic neutron scattering," says Hamish. “I hope it will be part of the user programme soon, and that we can adapt it to suit their experimental requirements."
As an electronics technician that joined ISIS through the apprenticeship scheme, this is Maksim's first academic paper authorship; “It was really nice to see the development of the equipment all the way through to the experiment itself. It was also interesting to work on something small alongside my other work on much larger pieces of kit, such as the collimator selector assembly for one of the instruments that will be going to the European Spallation Source."
The full paper can be found at DOI: 10.1039/D1CP03716A
To find out more about the ISIS electrical and electronic user support group, read our team case study.