Once implanted inside a material, muons interact with their local atomic environment. This interaction can be particularly strong when an energy level in the muon system matches one within the environment. The muons and their surroundings are then put on ‘speaking terms’, and this can strongly affect the muons’ behaviour. Such resonances – called level crossing resonances, LCR, (or, sometimes, ‘avoided level crossing resonances’) – between the muons and their environment can be produced by changing the applied magnetic field in a muon experiment. The resonances can be detected by observing the muon polarisation – they are seen as a dip in the polarisation as the applied field is changed. Observation of such resonances gives us additional information about the muons’ atomic environment.
Muon level crossing resonance, LCR, can be used to:
• determine free radical structures by measurement of muon hyperfine coupling constants
• investigate the reactions, molecular dynamics and local environment of free radicals
• study spin dynamics in magnetic systems
• determine muon sites, for example in semiconductors.
The new high-field muon instrument at ISIS, HiFi, is particularly suited to muon LCR studies, as its applied magnetic field range (up to 5T) covers the majority of level crossing resonances found in most systems. For example, in molecular systems, resonances typically occur in the 1-3T field range.