8 Muon spectroscopy

Abstract

The elusiveness of reactive free radicals poses great challenges for their investigation per se in most processes in which they have an intermediary role; this is true particularly in heterogeneous media. There are, however, methods available for the detection of radicals with enormous sensitivity and specificity which are given the collective acronym mSR (MuSR). All the MuSR methods rely upon the addition of the light hydrogen atom 'muonium' -a hydrogen atom with a radioactive positive muon as its nucleus, having a mass 1 9 that of a protium atom -to an organic substrate. In addition to its property of radioactive decay, the muon also has spin, and therefore experiments akin to those of conventional magnetic resonance experiments may be undertaken. Among the suite of methods thus encompassed is transverse-field MuSR (TF-MuSR), which is equivalent to ENDOR and yields the nucleus (muon) coupling directly, while the avoided level crossing (ALC) approach yields the magnitudes of the couplings from other nuclei present in a given radical, and also their relative signs (like TRIPLE does). Although these are resonance experiments, they do not require the application of external radiofrequency fields; however, radiofrequency experiments are possible, in which the analogies with more conventional magnetic resonance experiments are more direct. However, the experiments which are rendered possible by muons are unique, and have no practicable counterpart in more conventional techniques. In particular, the feature of single-particle-counting methods being used in combination with nuclear spins which are 100% polarised (i.e. vastly in excess of the Boltzmann factor!) permits unparalleled sensitivity regarding reactive radicals. Through the incisiveness of these methods, knowledge in the field of free radical chemistry has been gained that would be unachievable using the more established methods of EPR and its relatives.In consequence of these benefits, although my own background is in EPR (a topic which I have reviewed previously in this series), 1-3 my research group has become increasingly involved in the techniques of MuSR, especially in regard to determining the behaviour of radicals in catalytic systems and in heterogeneous environmental processes, and for the measurement of free radical reaction rates pertinent to biological membrane damage and repair. These are all topics whose investigation we could not have attempted without muons.

Exploiting Large‐Pore Metal–Organic Frameworks for Separations through Entropic Molecular Mechanisms

“…Nanoporous materialssuch as zeolites, [1,2] metal-organic frameworks (MOFs), [3][4][5][6][7] andz eolitic imidazolate frameworks (ZIFs) [8] offer great potential as energy-efficient alternatives to conventional separation processes like distillation, absorption, and extraction.B esides being industrially relevant, they are scientifically interesting and allow us to gain an understanding of the fundamentals of the separation mechanisms governing adsorbed molecules in confinement. Separation in nanoporous materials relies on adsorption [9] and diffusion, [10] and can be achievedt hrough size/shapee xclusion (steric separation), differences in adsorbate-adsorbent interactions and adsorbate packing (thermodynamic equilibrium effects), as well as differences in the diffusion rate of adsorbatesw ithin the adsorbent channels.…”

Hybrid Silicon Nanostructures with Conductive Ligands and Their Microscopic Conductivity

“…The quantity that is measured is the forward-backward asymmetry of the muon spin in an applied field, which provides a way of measuring the levelcrossing resonances that in turn provide information on the local hyperfine fields felt by the muon. 7 The technique is best explained using a simple energy diagram of a muon interacting with an electron, 9 called a Breit-Rabi diagram, illustrated in Fig. 1.…”

“…The pions thus produced decay rapidly (lifetime $2 ns) to give muons. 6,7 The pions that decay to muons close to the surface of the target may be focussed into a muon beam. This positive muon is used as a probe of structure and dynamics in spectroscopy when implanted into a molecule with nearly 100% spin polarization.…”

Molecular motion in some radiolabelled dipeptides: a muon spin relaxation study

“…Muonium (Mu), a radioactive hydrogen atom with a positive muon as its nucleus, and a mass one ninth that of a protium atom, may be used as a probe of molecular motion in a range of materials. 1 The strategy employed involves the addition of muonium to molecules containing unsaturated groups, 1 -3 thus converting them to free radicals, e.g. Eqns.…”

“…(3), where is the motional correlation time, ω is the frequency of the dominant spectral transition in the muon-electron coupled system, and υA is the variation in the muon hyperfine coupling constant during the motional event. The activation terms 1 1 and E a are obtained by fitting the values measured at particular temperatures (T) from each data set to Eqn. (4).…”