The positive muon is observable through its magnetic moment and the interactions of its nuclear spin with any unpaired electrons in its immediate environment, as described in chapter 3. The purpose of the present chapter is to describe the use and scope of these data for studies in chemistry. There are three distinct types of chemical associations of μ+ which are readily detectable by μSR: diamagnetic states (for convenience collectively given the subscript, D), free muonium atoms (Mu or subscript, M), and Mu-substituted free radicals (subscript, R). Each has its characteristic, field-dependent, precession frequency, and in practice these are so far apart that the three states can be studied separately at different fields.

All μSR histograms are accumulated from one-at-a-time events (except in pulsed mode), so that a vast number of isolated events are summed together and it is assumed this is equivalent to the time distribution that a large instantaneous ensemble would have. There is always the overall exponential fall-off (exp(—t/τ)) in the number of muons remaining with the progress of time, because of the natural decay of the muons by which one observes their spins. But this decay is of no interest, so the computer is made to remove it at an early stage in the analysis of data. The decay that is mainly of interest in chemistry is represented by λ in Eq. [3.10].