Radio-frequency muon spin resonance studies of endohedral and exohedral muonium adducts of fullerenes

The radio-frequency muon spin resonance technique (RF-μSR) is described, with examples drawn from muon studies of fullerences. Two distinct species can be detected by RF-μSR when solid C₆₀ is irradiated with positive muons. Endohedral muonium (Mu@C₆₀) is characterized by a muon hyperfine constant (A _μ) close to the vacuum value. A remarkable feature of the RF-μSR spectrum is the double quantum transition, which appears when the allowed transitions are saturated. The exohedral muonium adduct (C₆₀Mu) is also detected, and has a much smaller value ofA _μ typical of a carbon-centred organic radical. It has been studied by RF-μSR in dilute solution, which is not possible for transverse field muon spin rotation (TF-μSR). There is a significant difference inA _μ of C₆₀Mu in the solid and in solution, a result of great import to the analysis of avoided-level crossing experiments on¹³C₆₀Mu.     

Intramolecular motion and isotope effects in muonium‐substituted chloroalkyl radicals

Muon irradiation of pure liquid 3‐chloropropene, CH₂=CH-CH₂Cl, yields a primary radical, \dot\mboxCH₂-CHMu-CH₂Cl, and a secondary radical, MuCH₂-\rm\dot\mboxCH-CH₂Cl. 2‐methyl‐3‐chloropropene yields only the tertiary radical, MuCH₂-\rm\dot\mboxC(CH₃)-CH₂Cl. These three chloroalkyl radicals have been characterized by μSR and μLCR, and the hyperfine coupling constants (hfcs) have been determined over a range of temperatures, either in the pure liquid precursor or in concentrated solution. The temperature variation of the hfcs has been analyzed to obtain estimates of the barrier to internal rotation about the C_\alpha-C_\beta axis for various alkyl groups, and also their minimum energy conformations, i.e. their orientations with respect to the axis of the 2p_z orbital of the unpaired electron. The tertiary radical is particularly interesting because all three methyl‐like groups, -CH₃,-CH₂Cl and -CH₂Mu, are represented. The results can be compared to electron spin resonance data for analogous radicals, to provide information on the effects of Mu substitution for H. This revised version was published online in August 2006 with corrections to the Cover Date.     

Spin relaxation studies of the muonium substituted ethyl radical in the gas phase

This short communication draws attention to the power of μSR and related measurements in providing an unusually complete characterisation of muonium substituted organic radicals in the gas phase. Spectroscopic information is available from muon spin rotation and muon level crossing resonance, giving all the nuclear hyperfine coupling constants, just as in the liquid phase. In addition, measurements of the relaxation time of the muon Zeeman energy become possible; these are potentially informative on the molecular collision dynamics. Demonstration results are presented in summary for the muonium substituted ethyl radical, ĊH₂CH₂Mu, in ethene gas.     

Longitudinal field repolarization studies of reorientational dynamics in the fullerenes C60 and C70

LFμSR studies on the pure fullerenes C₆₀ and C₇₀ show temperature dependent features associated with the reorientational behaviour of the fullerenyl radicals MuC₆₀ and MuC₇₀ in the crystalline host substrate. If not excluded from the experimental setup, molecular oxygen undergoes spin exchange reactions with the radicals. The data for C₆₀ correspond to a situation effectively static at 150 K and fully averaged to isotropy at 300 K, while for C₇₀ the data conspicuously do not conform to the predictions of the pseudostatic model employed in ALC‐μSR studies. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of radiation dose with low dose rate on degradation of propylene–ethylene copolymer

Medical grade propylene–ethylene (P–E) copolymer was irradiated by gamma rays. The radicals generated in the irradiated P–E copolymer were identified by using electron spin resonance (ESR) technique and the structural changes in the polymer were monitored with Fourier transform infrared spectroscopy (FTIR). The ESR spectra were analysed with computer simulations. The ESR studies show the formation of macro (～CH₂–?H–CH₂～), peroxy (POO˙), methyl and acyl (R–?=O) radicals and the asymmetric doublet, characteristic of peroxy radicals in the case of the sample irradiated at low dose (1 Mrad) and high doses (30 and 40 Mrad), respectively. The FTIR spectra of irradiated P–E copolymer indicate an increase in the concentration of peroxide groups. The absorption bands of –C=O and –OH groups were increased and the decline in the intensity of –CH₃ group absorption band is reported.     

Why ALC μSR is superior for gas‐phase radical spectroscopy

ALC μSR spectra of the muonated ethyl and cyclohexadienyl radicals in the gas phase are reported. They have surprisingly narrow lines for a magnetic resonance type technique under conditions near ambient temperature and near 1 atmosphere pressure. The main reason for this behaviour is the dramatic reduction of electron spin relaxation in high magnetic fields. This revised version was published online in August 2006 with corrections to the Cover Date.     

Spin densities in some odd alternant radicals including linear polyene radicals

An approximate unrestricted Hartree-Fock method is used to calculate spin densities in the odd alternant radicals cyclohexadienyl, benzyl, perinaphthenyl and triphenylmethyl and the linear polyene radicals from allyl to C₁₉H₂₁. The results agree quite well with the exact calculations and with empirical estimates as well as with experimental data on hyperfine splitting constants and E.S.R. second moments.     

Molecular orbitals calculations of C70H radicals

Bombardment of C₇₀ with positive muons leads to the formation of muonated radicals C₇₀Mu. However, despite there being five distinct possible addition sites, only three radicals are easily observed with a fourth, very weak, signal also present. In order to provide guidance in assigning the radicals to specific sites, semi-empirical molecular-orbitals calculations of the analogous C₇₀H radicals were undertaken. The results indicate that addition at the equatorial carbons is energetically disfavoured, and allow a reasonable assignment of the other sites to be made.     

Addition reactions and longitudinal field spin relaxation of small radicals in gases: Mu+CO and Mu+N2O

Muon spin relaxation has been measured in longitudinal magnetic fields for Mu+CO and Mu+N₂O reactions. The interpretation of the results for these small molecules, which are quite different than those obtained in larger molecule systems (e.g., Mu‐ethyl and Mu‐t‐butyl radicals), are made with the phenomenological model for Mu‐radical spin relaxation previously proposed. Proper fitting procedures are important in these cases and are discussed in the present paper. This revised version was published online in August 2006 with corrections to the Cover Date.     

Photoinduced C70 radical anions in polymer:fullerene blends

Photoinduced polarons in solid films of polymer:fullerene blends were studied by photoluminescence (PL), photoinduced absorption (PIA) and electron spin resonance (ESR). The donor materials used were P3HT and MEH‐PPV. As acceptors we employed PC₆₀BM as reference and various soluble C₇₀‐derivates: PC₇₀BM, two different diphenylmethano‐[70]fullerene oligoether (C₇₀‐DPM‐OE) and two dimers, C₇₀–C₇₀ and C₆₀–C₇₀. Blend films containing C₇₀ revealed characteristic spectroscopic signatures not seen with C₆₀. Light‐induced ESR showed signals at g ≥ 2.005, assigned to an electron localized on the C₇₀ cage. The formation of C₇₀ radical anions also leads to a subgap PIA band at 0.92 eV, hidden in the spectra of C₇₀‐based P3HT and MEH‐PPV blends, which allows for more exact studies of charge separated states in conjugated polymer:C₇₀ blends. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Radio-frequency muon spin resonance (RFμSR) experiments on condensed matter

In this paper we present an overview of the radio-frequency muon spin resonance (RFμSR) technique, an analogue to continuous-wave NMR, and an introduction to time-integral (TI) and time-differential (TD) RFμSR on muons in diamagnetic or in paramagnetic environments. The general form of the resonance line for TI-RFμSR as well as the expression for the time-dependence of the longitudinal muon spin polarization at resonance are given. Since RFμSR does not require phase coherence of the muon spin ensemble, this technique allows us to investigate muon species that are generated by transitions from, or in the course of reactions of, a precursor muon species even if in transverse-field (TF) μSR measurements the signal is lost due to dephasing. This ability of RFμSR is clearly demonstrated by measurements on doped Si. In this example, at low temperatures, a very pronounced signal from a muon species in diamagnetic environment has been found in RFμSR measurements, whereas in TFμSR experiments only a very small signal from muons in diamagnetic environment could be detected and a large fraction of the implanted muons escaped detection. These findings could be interpreted in terms of the delayed formation of a diamagnetic muonium-dopant complex, and, due to the large diamagnetic RFμSR signal, the RFμSR technique is a unique tool to study how the variation of parameters and experimental conditions such as illumination affects formation and behavior of these complexes. First results obtained on illuminated boron doped Si are reported. However, as illustrated by the example of experiments on the muonated radical in solid C₆₀, results from conventional TI-RFμSR cannot always be interpreted unambiguously since different parameters, namely the fraction of muons forming the investigated muon species, the longitudinal and the transverse relaxation rates, have similar effects on height and shape of the RFμSR resonance line. These ambiguities, however, may be resolved by collecting time-differential data. With this extension RFμSR becomes a very powerful complementary method to TFμSR in the studies of dynamic effects.     

Correlation between the negative magnetoresistance effect and magnon excitations in single-crystalline CuCr1.6V0.4Se4

Structural, electrical and magnetic measurements, as well as electron spin resonance (ESR) spectra, were used to characterise the single-crystalline CuCr_1.6V_0.4Se₄ spinel and study the correlation between the negative magnetoresistance effect and magnon excitations. We established the ferromagnetic order below the Curie temperature T _C ≈ 193 K, a p-type semiconducting behaviour, the ESR change from paramagnetic to ferromagnetic resonance at T _C, a large ESR linewidth value and its temperature dependence in the paramagnetic region. Electrical studies revealed negative magnetoresistance, which can be enhanced with increasing magnetic field and decreasing temperature, while a detailed thermopower analysis showed magnon excitations at low temperatures. Spin–phonon coupling is explained within the framework of a complex model of paramagnetic relaxation processes as a several-stage relaxation process in which the V³⁺ ions, the exchange subsystem and conduction electron subsystem act as the intermediate reservoirs.     

Muon investigations of fullerenyl radicals

Four of the five possible isomeric C₇₀Mu radicals have been detected by transversefieldSR in a C₇₀ powder sample at 298 K. Their assignment is based on the results of semi-empirical MNDO calculations. There are significant changes in intensity and lineshape of the signals at low temperature. The first SR spectrum of a fullerenyl radical in solution has been obtained.     

The study of gamma irradiation effects on poly (glycolic acid)

We have investigated the effects of gamma irradiation on chemical structure, thermal and morphological properties of biodegradable semi-crystalline poly (glycolic acid) (PGA). PGA samples were subjected to irradiation treatment using a ⁶⁰Co gamma source with a delivered dose of 30, 60 and 90?kGy, respectively. Gamma irradiation induces cleavage of PGA main chains forming ～O?H₂ and ?H₂COO～ radicals in both amorphous and crystalline regions. The free radicals formed in the amorphous region abstract atmospheric oxygen and convert them to peroxy radicals. The peroxy radical causes chain scission at the crystal interface through hydrogen abstraction from methylene groups forming the ～?HCOO～ (I) radical. Consequently, the observed electron spin resonance (ESR) doublet of irradiated PGA is assigned to (I). The disappearance of the ESR signal above 190°C indicates that free radicals are formed in the amorphous region and decay below the melting temperature of PGA. Fourier transform infrared and optical absorption studies confirm that the groups are not influenced by gamma irradiation. Differential scanning calorimetry (DSC) studies showed that the melting temperature of PGA decreased from 212°C to 202°C upon irradiation. Degree of crystallinity increased initially and then decreased with an increase in radiation as per DSC and X-ray diffraction studies. Irradiation produced changes in the physical properties of PGA as well as affecting the morphology of the material.     

Addition and spin exchange rate constants by longitudinal field μSR: The Mu+NO reaction

The addition reaction Mu+NO+M→MuNO+M and the spin exchange reaction Mu(↑) +MO(↓)→Mu(↓)+NO(↑) have been measured by longitudinal field μSR at room temperature in the presence of up to 58 atm of N₂ as inert collider. The pressure dependence of the longitudinal relaxation rate due to the addition reaction (λ_c) demostrates that the system is still in the low pressure regime in this pressure range. The corresponding termolecular rate constant has been determined ask _0,Mu =(1.10±0.25)×10⁻³² cm⁶ molecules⁻² s⁻¹, almost 4 times smaller than the corresponding H atom reactionk _0,H=3.90×10⁻³² cm⁶ molecules⁻² s⁻¹ [I.M. Campbell et al., J. Chem. Soc. Faraday Trans. 1.71 (1975) 2097]. The average value of the spin exchange rate constants in the 2.5–58 atm pressure range,k _SE=(3.16±0.06)×10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, is in good agreement with previous values obtained by transverse field μSR [D.G. Fleming et al., J. Chem. Phys. 73 (1980) 2751].     

Mechanisms of inelastic scattering of low-energy protons by C6H6, C60, C6F12, and C60F48 molecules

The mechanisms of inelastic scattering of low-energy protons with a kinetic energy of 2–7 eV by C₆H₆, C₆F₁₂, C₆₀, and C₆₀F₄₈ molecules are studied using the methods of quantum chemistry and nonempirical molecular dynamics. It is shown that, for the C₆H₆ + proton and C₆₀ + proton systems, starting from a distance of 6 Å from the carbon skeleton, the electronic charge transfer from the aromatic molecule to H⁺ occurs with a probability close to unity and transforms the H⁺ ion into a hydrogen atom and the neutral C₆H₆ and C₆₀ molecules into cation radicals. The mechanism of interaction of low-energy protons with C₆F₁₂ and C₆₀F₄₈ molecules has a substantially different character and can be considered qualitatively as the interaction between a neutral molecule and a point charge. The Coulomb perturbation of the system arising from the interaction of the noncompensated proton charge with the Mulliken charges of fluorine atoms results in an inversion of the energies of the electronic states localized, on the one hand, on the positively charged hydrogen ion and, on the other hand, on the C₆F₁₂ and C₆₀F₄₈ molecules. As a result, the neutral molecule + proton state becomes the ground state. In turn, this inversion makes the electronic charge transfer energetically unfavorable. Quantum-chemical and molecular-dynamics calculations on different levels of theory showed that, for fluorine derivatives of some aromatic structures (C₆F₁₂, C₆₀F₄₈), the barriers to proton penetration through carbon hexagons are two to four times lower than for the corresponding parent systems (C₆H₆, C₆₀). This effect is explained by the absence of active π-electrons in the case of fluorinated molecules.     

Composition,temperature and radiation induced changes in gamma irradiated AAAMPS copolymer

Effect of composition, temperature and radiation dose in gamma irradiated acrylamide-2-acrylamido-2-methyl propane sulphonic acid (AA) copolymer has been investigated by electron spin resonance (ESR) and fourier transform infrared (FTIR) techniques. ESR spectra of gamma irradiated AA copolymer have been recorded under different conditions. The observed ESR spectra are analysed by computer simulation techniques, to separate the constituent component spectra. Magnetic parameters employed to simulate the component spectra enabled the identification of corresponding free radicals. The AA copolymer with low acrylamide content composed of macroradicals of the type ?CH₂?CH?CH₂? and methyl radicals (CH₃) whereas the copolymer with high acryl amide content possess methyl radicals and radicals of the type ?CH₂?C(CONH₂)?CH₂?/CH₃?C?CH₃. Reasons for the variation in the formation of free radicals have been explained. The observed changes in ESR spectra of irradiated AA copolymer at higher temperatures are thought to be due to the recombination of free radicals. Formation of free radicals found to be enhanced with the increase in dose of irradiation. FTIR spectra of pure and irradiated copolymers have also confirmed the previous results.     

Magnetic properties of alkali-intercalated C60 compounds probed by electron spin resonance

In this paper, we present a survey of our ESR results on alkali-intercalated C₆₀ compounds. As we are far from fully understanding the physical properties of fullerides, this article deals successively with the different phases in the phase diagram of A₁C₆₀ compounds. It focuses on both conducting and magnetic properties in metallic compounds, the spin ground state in insulating systems and phase transitions in A₁C₆₀ phases.     

ESR evidence for mirror symmetry conservation during radiation damage of X-irradiated single crystals of KClO4

Electron spin resonance (ESR) studies of ClO₃ and ClO₂ radicals in X-irradiated potassium perchlorate, KClO₄, single crystals are carried out to investigate the radiation decomposition pathways. The orientation of the maximum principal component of the³⁵Cl hyperfine tensor is determined by ESR and identified with that of the bond ruptured on irradiation. It is found that the weaker Cl-O(2)×2 bonds related by the mirror symmetry survive radiation damage, while the stronger Cl-O(3) and Cl-O(l) bonds get ruptured to form the ClO₃ and ClO₂ radicals, thus providing evidence for the important role played by the lattice symmetry during radiative decomposition.