When the muon binds to a molecule with at least one spin-unpaired electron the interactions with the electron spin become dominant.
This is one specific type of paramagnetic species that the muon may form, the muonic adduct radical, quite typical of muons implanted in molecular compounds containing unsaturated organic molecules, i.e. double or triple bonds. In these cases the muon actually modifies greatly its surrounding by forming a unique molecule, very different from the others. The state thus formed is however a very sensitive, although different, probe in itself, yielding a wealth of valuable information, via the different spectroscopic signatures that it provides.
We shall start describing the simplest of these states, the muonium atom.
The muonium and radical states are directly observable. Let's start with a short experimental summary: muonium may be found in vacuum (evaporated from suitable hot spongy surfaces) and inside matter in a few cases (e.g. in quartz, inside the fullerene cage, in a slightly distorted form in silicon).