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# ParamagneticSpecies

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).

 The plot on the right shows the energy levels of muonium, as will be deduced in the next page. The easy way to detect these states is by looking at the so called triplet transitions, in a low transverse field. They are the two nearly degenerate transitions shown by red double arrows on the left. The name triplet derives from the fact that the upper degenerate state at zero field is the spin triplet of electron and muon, whereas the lower state is the singlet. The spin states at high field are shown by the labels ($\pm$ stand for spin up and down). In the high field condition the additional interactions with other nuclei are decoupled so that a diagram of this kind can be used also for more the complex radicals (at least in the liquid state). The transitions that become detectable are the two shown by the blue double arrows , which represent a spectroscopical signature of each radical.