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It has been claimed that longitudinal field muon spin relaxation (LF-μSR) experiments on the organic semiconductor (OSC) tris-(8-hydroxyquinoline)aluminum(III) (Alq3) have measured electron hopping rates of ∼ 1 0 12 s − 1 , while density functional theory (DFT) calculations suggest that electron hopping between a muoniated radical and a neighboring molecule is energetically unfavorable and that the LF-μSR experiments were probing muoniated radicals with localized spin density. We have performed avoided level crossing muon spin resonance (ALC-μSR) and transverse field muon spin rotation (TF-μSR) measurements on Alq3 and 8-hydroxyquinoline (8hq), which is meant to model the muoniated radicals present in Alq3 when they are not in an OSC. These are supplemented by benchmarked DFT calculations. The ALC-μSR and TF-μSR spectra of 8hq and Alq3 are best explained by Mu adding to all six secondary carbons of the quinolate rings with roughly equal yields and localized spin density. There is no evidence in the TF-μSR spectrum of Alq3 for the formation of radicals with muon hyperfine coupling constants of 23 or 91 MHz as reported earlier by others. Our measurements support the view that there is localized spin density on the molecule to which Mu is covalently bound and the muon is not a passive probe in organic systems as it can be incorporated into radicals that have different electronic structures to the parent compounds. The muoniated radicals in Alq3 are more short-lived than in 8hq, which could be due to interactions with mobile electrons in the OSC, but with electron spin flip rates on the order of ∼ 1 0 7 s − 1 . [ABSTRACT FROM AUTHOR]

Abstract: Two types of muoniated spin probes were produced by the addition of muonium (Mu) to the phenyl ring of the rod-like liquid crystal 4-(trans-4-pentylcyclohexyl)benzonitrile (PCH5). Avoided level crossing muon spin resonance spec-troscopy was used to characterize the muoniated spin probes and to probe dynamics of PCH5 at the molecular level. The methylene proton hyperfine coupling constant (hfcc) of one of the muoniated spin probes shift in the nematic phase due to the dipolar hyperfine coupling, the ordering of the molecules along the applied magnetic field and fiuctuations about the local director. The amplitude of these fluctuations in PCH5 is determined from the temperature dependence of the methylene proton hfcc. [Copyright &y& Elsevier]

Muoniated spin probes were produced by the addition of muonium (Mu) to two rod-like liquid crystals: N-(4-methoxybenzylidene)-4′-n-butylaniline (MBBA) and cholesteryl nonanoate (CN). Avoided level crossing muon spin resonance spectroscopy was used to characterize the muoniated spin probes and to probe dynamics at the molecular level. In MBBA Mu adds predominantly to the carbon of the bridging imine group and the muon and methylene proton hyperfine coupling constants (hfccs) of the resulting radical shift in the nematic phase due to the dipolar hyperfine coupling, the ordering of the molecules along the applied magnetic field and fluctuations about the local director. The amplitude of these fluctuations in in the nematic phase of MBBA is determined from the temperature dependence of the methylene proton hfcc. Mu adds to the double bond of the steroidal ring system of CN and the temperature dependence of the Δ1line width provides information about the amplitude of the fluctuations about the local director in the chiral nematic phase and the slow isotropic reorientation in the isotropic phase. [ABSTRACT FROM AUTHOR]

Abstract: The high-field SR project at PSI has been triggered by the increasing demand on high magnetic fields (i.e. T) in the SR user community. As a first step towards the realisation of the high-field instrument we simulate its design using a program based on the geant4 package. This allows one to identify crucial parameters of the new instrument (i.e. distances, lengths, thicknesses, materials), and to optimise them. We demonstrate how the optimisation is performed, and what are the critical issues of the instrument. [Copyright &y& Elsevier]

The transfer rate of a probe molecule across the interfacial layer of a water-in-oil (w/o) microemulsion was investigated using a combination of transverse field muon spin rotation (TF-μ3SR), avoided level crossing muon spin resonance (ALC-μSR), and Monte Carlo simulations. Reverse microemulsions consist of nanometer-sized water droplets dispersed in an apolar solvent separated by a surfactant monolayer. Although the thermodynamic, static model of these systems has been well described, our understanding of their dynamics is currently incomplete. For example, what is the rate of solute transfer between the aqueous and apolar solvents, and how this is influenced by the structure of the interface? With an appropriate choice of system and probe molecule, μSR offers a unique opportunity to directly probe these interfacial transfer dynamics. Here, we have employed a well characterized w/o microemulsion stabilized by bis(2-ethylhexyl) sodium sulfosuccinate (Aerosol OT), with allyl alcohol (CH2＝CH-CH2-OH, AA) as the probe. Resonances due to both muoniated radicals, CMuH2-C*H-CH2-OH and C*H2-CHMu-CH2-OH, were observed with the former being the dominant species. All resonances displayed solvent dependence, with those in the microemulsion observed as a single resonance located at intermediate magnetic fields to those present in either of the pure solvents. Observation of a single resonance is strong evidence for interfacial transfer being in the fast exchange limit. Monte Carlo calculations of the ΔM = 0 ALC resonances are consistent with the experimental data, indicating exchange rates greater than 109 s-1, placing the rate of interfacial transfer at the diffusion limit. [ABSTRACT FROM AUTHOR]

Abstract: Three muoniated azacyclohexadienyl radical isomers were produced by the addition of muonium to pyridine in liquid pyridine and in high silica ZSM-5 zeolite () at 298K, and observed with avoided level crossing muon spin resonance. The hyperfine coupling constants of the azacyclohexadienyl radicals are similar in both environments, which indicates that the radicals do not interact strongly with the zeolite framework. The relative yield of the three radicals depends on the medium, with the yield of the para isomer being six times larger in ZSM-5. [Copyright &y& Elsevier]

He et al. studied the muoniated radicals formed by muonium (Mu) addition to the low-molecular-weight polymer poly(triarylamine) (PTAA) (He et al., 2015) [1] , an organic semiconductor, using avoided level crossing muon spin resonance (ALC-μSR) (McKenzie and Roduner, 2013; McKenzie, 2009) [2,3] . The goal of these experiments was to study the electron spin relaxation of a paramagnetic defect in PTAA using a local probe. Our main issue with this paper is the claim that parameters relating to the electronic structure of the muoniated radicals have been measured with greater precession than we believe is possible considering the quality of the ALC-μSR spectra presented. [ABSTRACT FROM AUTHOR]