Muon spin spectroscopy has shown itself to be a very powerful probe of material properties, with Europe leading this research due to it operating half of the available muon sources in the world, but to date there has been very little work done on spectroscopy of excited states. In the first instance, this proposal will design and build an upgrade to an existing spectrometer (HiFi at ISIS) that will comprise a high-power tunable laser to provide the electronic excitation. This will be followed by a study of the physics of excited state muon spectroscopy, an entirely unexplored area of the technique. The fundamental mechanisms of charge carrier transport in organic semiconductors will then be investigated, and the ground-work for directly measuring the recombination zone in organic LEDs will be done. Perhaps most importantly, the fundamental physics of electron transfer in peptides will be performed - which is responsible for many biological processes and not well understood. The muon technique has recently been shown by the applicant to offer both spatial and temporal information on the electron’s progress through the molecule - this is the only technique that can measure the electron’s wavefunction at different locations on the molecule as a function of time after the excitation is formed.