Formation and decay of radicals during vacuum-UV irradiation of poly(dimethylsiloxane)

Formation of radicals in a poly(dimethylsiloxane) (PDMS) elastomer upon 172 nm vacuum-UV (VUV) irradiation was investigated by electron paramagnetic resonance spectroscopy (EPR). Kinetics of radical buildup and decay was investigated under varying conditions and found to depend strongly on the degree of pre-modification of the polymer. Furthermore, radicals were observed only when oxygen was absent during in situ VUV irradiation. The EPR spectrum is dominated by a broad signal, assignable to unspecified carbon-centered radicals. Stationary radical concentrations during VUV irradiation in nitrogen atmosphere can reach 0.2 mol/L in an about 260 nm thick surface layer, consistent with the remaining carbon concentration in the modified PDMS as measured by XPS. Gas diffusion is not limiting kinetics of the modification process. Our findings are consistent with a mechanism where oxygen molecules from the atmosphere diffuse in PDMS and quench radical sites in the polymer

Cryogenic Q-band (35GHz) probehead featuring large excitation microwave fields for pulse and continuous wave electron paramagnetic resonance spectroscopy: performance and applications.

The construction and performance of a Q-band (35GHz) cryogenic probehead for pulse electron paramagnetic resonance and continuous wave electron paramagnetic resonance measurements with down-scaled loop gap resonators (LGRs) is presented. The advantage of the LGR in comparison to TE(012) resonators lies in the large B(1) microwave (mw) fields that can be generated with moderate input mw power. We demonstrated with several examples that this allows optimal performance for double-quantum electron coherence, HYSCORE, and hyperfine decoupling experiments employing matched and high turning angle mw pulses with high B(1)-fields. It is also demonstrated that with very low excitation power (i.e. 10-40 mW), B(1)-fields in LGRs are still sufficient to allow short mw pulses and thus experiments such as HYSCORE with high-spin systems to be performed with good sensitivity. A sensitivity factor Lambda(rs) of LGRs with different diameters and lengths is introduced in order to compare the sensitivity of different resonant structures. The electromagnetic field distribution, the B(1)-field homogeneity, the E(1)-field strength, and the microwave coupling between wave guide and LGRs are investigated by electromagnetic field calculations. The advantage and application range using LGRs for small sample diameters is discussed

Distance determination from dysprosium induced relaxation enhancement: a case study on membrane-inserted WALP23 polypeptides

Membrane incorporated synthetic a-helical polypeptides labelled with Dy(III) chelate complexes and nitroxide radicals were studied by the inversion recovery (IR) technique and Dy(III)-nitroxide distances were obtained. A comparison of obtained distances with the previously reported Gd(III)-nitroxide double electron–electron resonance (DEER) calibration data was performed and revealed reliability of the IR-based technique for the distance determination in membrane-incorporated biomacromolecules. The presented distance determination technique is ‘spectroscopically orthogonal’ to DEER-based distance measurements and can be potentially combined with DEER to study multiply spin-labelled biomacromolecules. The key steps of the data processing, the types of obtained distance information and the areas of possible application of the technique are discussed

EPR study of ceria nanoparticles containing different concentration of Ce³⁺ ions

© 2018 Elsevier B.V. The electron paramagnetic resonance of Ce3+ ions in ceria is a rather controversial issue. In this report we analyze the EPR line with g ∼1.97 in the absorption spectra of ceria powders, which has been assigned in many studies to Ce3+ ions. Such an assignment is not in agreement with the experimental data nor with the theory of spin-lattice relaxation of rare earth ions in solids. Our statement is supported by our experimental results based on EPR spectroscopy of nanocrystalline ceria, synthesized in various ways, as well as by magnetization measurements of as-prepared and vacuum-annealed ceria nanoparticles. The possible model of the paramagnetic center, which is assigned to the EPR line with g ∼1.97 in ceria, is an electron trapped near the surface Ce3+/Ce4+ redox pair

EPR study of ceria nanoparticles containing different concentration of Ce³⁺ ions

© 2018 Elsevier B.V. The electron paramagnetic resonance of Ce3+ ions in ceria is a rather controversial issue. In this report we analyze the EPR line with g ∼1.97 in the absorption spectra of ceria powders, which has been assigned in many studies to Ce3+ ions. Such an assignment is not in agreement with the experimental data nor with the theory of spin-lattice relaxation of rare earth ions in solids. Our statement is supported by our experimental results based on EPR spectroscopy of nanocrystalline ceria, synthesized in various ways, as well as by magnetization measurements of as-prepared and vacuum-annealed ceria nanoparticles. The possible model of the paramagnetic center, which is assigned to the EPR line with g ∼1.97 in ceria, is an electron trapped near the surface Ce3+/Ce4+ redox pair

Cryogenic 35GHz pulse ENDOR probehead accommodating large sample sizes: Performance and applications.

The construction and performance of a cryogenic 35GHz pulse electron nuclear double resonance (ENDOR) probehead for large samples is presented. The resonator is based on a rectangular TE(102) cavity in which the radio frequency (rf) B(2)-field is generated by a two turn saddle ENDOR coil crossing the resonator along the sample axis with minimal distance to the sample tube. An rf power efficiency factor is used to define the B(2)-field strength per square-root of the transmitted rf power over the frequency range 2-180MHz. The distributions of the microwave B(1)- and E(1)-field, and the rf B(2)-field are investigated by electromagnetic field calculations. All dielectrics, the sample tube, and coupling elements are included in the calculations. The application range of the probehead and the advantages of using large sample sizes are demonstrated and discussed on a number of paramagnetic samples containing transition metal ions