The HiFi laser, shooting into a power meter.
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This week the new laser system has been installed on the HiFI Spectrometer, expanding the scope of science possible at the Muon facility. The upgrade is part of a project supported by the European Research Council and led by Dr Alan Drew, Queen Mary University.
Dr Alan Drew and his team from Queen Mary's University were the first users of the HIFI instrument, back in 2009, where they investigated a class of semiconductors which are organic and have interesting electronic or magnetic properties and possible applications in spintronic devices. Since then Alan was awarded a €1.5M European Research Council (ERC) grant to upgrade the HIFI spectrometer.
Alan Drew, explains, “Using the new laser system we will be able to perform laser excited muon spectroscopy on materials using HIFI. The introduction of this laser system on HIFI will increase the magnetic field size available expanding the science that we can do, specifically the wealth of science about electron dynamics in soft matter systems”
Alan and his team plan to use a technique called avoided level crossing spectroscopy to track how electrons move in different systems. This is a relatively new technique where the laser acts as a ‘pump’ of electronic excitations, and the muon acts as a probe of their time dependence. The muon or, the muonium in this case, attaches itself to different parts of the molecule. The position where it bonds gives a certain resonance at a different magnetic field meaning one end of a molecule may be a give a different resonance compared to the middle. By changing the magnetic field it’s possible to tune to one particular portion of the molecule and then fire a laser at the molecule, creating an electronic excitation. The excitation can then be tracked through the molecule from a donor to an acceptor state, or as it moves off the molecule to another molecule or just as it relaxes back down to the ground state. The wave function of that excited state is somewhat localised to certain portions in the molecule and this position is what can change as a function of time.
The HiFI Laser System
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“The idea is to try and track the movement of electrons through a material as a function of time.” Explains Alan, “But why is this important? Well, we’ve all heard of photosynthesis - light hits a molecule, which excites a portion of the molecule and this excited state moves through the molecules of an acceptor path enabling a chemical reaction. But how does it move? What path does it take? A short cut or the long way around? Similarly if you photo excite a protein but then make one small change to its amino acid chain; it drastically changes the electron transfer properties. But why? In the same way, solar cells involve the process of electron transfer. How do these work? Measuring this excited state at different parts of the molecule in relation to time, means we can track the route the electrons take.”
“Photomagnets are another exciting possibility for study. Here, you have a system with no or very low net spin, but by creating an excited state you can end up creating some net spin, making the molecule magnetic. In some of these molecules the excited states last for milliseconds, and in others days. This depends on the molecules and the subtleties in their conformation. The new laser system on HIFI will mean this unique area of science can be accessed for the first time.”
Research date: December 2013
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