Single-line Spectrum Research Articles

Amplified resonant Raman scattering in conducting polymer thin films

Using picosecond pulsed laser excitation, we investigate the optical emission characteristics of poly(2,5-dioctyloxy-p-phenylenevinylene), (DOO-PPV), thin films at high excitation intensities (∼1–90 MW/cm2). We observe the presence of amplified resonance Raman scattering in the emission spectra of conducting polymer films. The effect results in sharp Raman lines (widths smaller than 3 Å) superimposed on a significantly broader, well known, spectrally narrowed emission band (width ∼10 nm) caused by the amplified spontaneous emission in the waveguided polymer film. At the highest used excitation intensities, Raman scattering dominates the DOO-PPV emission spectrum resulting in a highly monochromatic, single-line emission spectrum.

Peroxy Radical Formation from Plasma-Induced Surface Radicals of Polyethylene As Studied by Electron Spin Resonance

The nature of peroxy radical formation from plasma-induced surface radicals of polyethylene (PE), both low-density polyethylene (LDPE) and high-density polyethylene (HDPE), was studied by electron spin resonance with the aid of systematic computer simulations. It was found that peroxy radical formation varies with the structure of component radicals of plasma-irradiated PE, both LDPE and HDPE: Among three plasma-induced radicals of PE, dangling bond sites (DBS) undergo an instant conversion into the corresponding peroxy radicals in contact with oxygen, while the midchain alkyl radical is of very low reactivity with oxygen in both LDPE and HDPE. Computer simulations disclosed that ESR spectra of peroxy radicals are similar to each other in LDPE and HDPE, both being composed of two types of spectra, a partial g-averaging anisotropic spectrum and a nearly isotropic single line spectrum due to different molecular motional freedom at the trapping sites of peroxy radicals.

Plasma-induced free radicals of polycrystalline dicarbohydrates studied by electron spin resonance

We report here special features of plasma-induced free radicals of dicarbohydrates, such as maltose and cellobiose, on its comparison with those of amylose and cellulose. The simulated spectra disclosed that the observed spectra of maltose consist of four kinds of spectral components, two isotropic spectra [doublets (I) and triplets (II)], both being assigned to hydroxylalkyl radicals and an anisotropic spectrum [doublet of doublets (IV)], assigned to an acylalkyl radical, and a smeared-out single line spectrum (V), assigned to an immobilized dangling-bond site (DBS) at the surface cross-linked region. On the other hand, those of cellobiose consist of five kinds of spectral components, I, II, IV, V and triplet spectrum (III) different in hyperfine splitting constant (HSC) from II, but assignable to the other hydroxylalkyl radicals.

EPR Studies on the Effects of Dopant Cadmium (II) Ions on the Low Magnetic Dimensionality of Copper Calcium Acetate Hexahydrate

EPR investigations on Cd(II) doped copper calcium acetate hexahydrate of the composition Cu1-x Cdx Ca(Ac)4·6H2O (x=0.004–0.858) have been reported in the present communication. In the single crystal having as low as 0.4% Cd(II) concentration, single line spectrum having enhanced linewidth (7% enhancement) is obtained along the chain axis ( c-axis). For crystals having ≥8.2% Cd(II) concentration hyperfine structure appears. A large number of weak lines due to dimers, trimers and chains of finite length appear superimposed on the main line. It is inferred that Cd(II) impurity ions act as good reflectors and have profound influence on the spin diffusion process in the present 1D system. Along each of d- and e-axes in the tetragonal plane, doublet structures in the concentrated as well as in the dilute crystals are obtained. The doublet splitting (δHd) initially increases with increasing dilution; then the rate of increment gradually slows down and practically stops at Cd(II) concentration ∼55%. A model has been proposed to interpret the observed behaviour of δHd. Half-field lines due to forbidden ΔM s=±2 transition within the dimer triplet have been observed in the dilute crystals.

The Structure of Hexamethyltungsten, W(CH3)6: Distorted Trigonal Prismatic with C3 Symmetry

Ab initio and density functional calculations show that the equilibrium structure of hexamethyltungsten is a distorted trigonal prism of C3 symmetry (with local C3v symmetry for the WC6 skeleton). A regular prismatic D3 structure (with D3h skeleton) is found to be ca. 20 kJ mol-1 higher in energy at correlated levels of theory. It is a transition state connecting two C3 minima. These results extend a recent gas-phase electron diffraction study which favored a regular prismatic structure but could not rule out a distortion to C3v. The failure of a previous theoretical study to locate the distorted minimum is due to the neglect of electron correlation and to some other restrictions during the structure optimizations. Correlation is important, e.g. for the description of hyperconjugative “agostic” C−H → W interactions which are found to be pronounced in W(CH3)6. Structures optimized with gradient-corrected or hybrid density functionals, or at the MP2 level, describe these interactions well. The observed single-line 13C and 1H NMR spectra are explained by dynamic motions due to the low D3 inversion and methyl rotation barriers. 13C chemical shifts calculated using density functional theory differ by ca. 18 ppm between the two nonequivalent sets of methyl groups in the distorted trigonal prismatic structure. Low-temperature NMR experiments could be useful to confirm this value and thus the distortion. Harmonic vibrational frequency analyses are consistent with experimental results and have been used to characterize stationary points on the potential energy surface. Differences between the structural preferences of W(CH3)6 and WH6 are investigated via detailed bonding analyses.

Synthesis and magnetic behaviour of polyradical: poly(1,3-phenyleneethynylene) with π-toporegulated pendant stable aminoxyl and imine N-oxide–aminoxyl radicals

The palladium-catalysed polycondensation of N-tert-butyl-N-(3,5-diethynylphenyl)aminoxyl with 2-(3,5-diiodophenyl)-4,4,5,5-tetramethyl-3-oxido-2-imidazolin-3-ium-1-yloxyl in pyridine–triethylamine at room temperature afforded poly(1,3-phenyleneethynylene) with pendant aminoxyl and imine N-oxide–aminoxyl radicals as a light blue powder in 78–83% yields. The number-average molecular weights (n) of the polyradical determined by GPC were 2670–3030 and the spin concentrations determined by EPR were 1.91–2.02 × 1021 spins g−1. Both solution (CH2Cl2) and powder EPR measurements of the polyradicals gave a hyperfine-smeared single line spectrum, suggesting that spin-exchange narrowing took place due to the high spin concentrations of the polyradicals. The magnetic susceptibility (χ) measurements of the polyradicals were carried out on a superconducting quantum interference device (SQUID) magnetometer in the temperature range of 1.8–300 K. The χ Tvs. T plots showed that the polyradicals were a paramegnetic species yielding a weak antiferromagnetic coupling (θ = −1.5 K) below 30 K. The absence of the through-bond ferromagnetic interaction is accounted for by masking of the interaction by the stronger through-space antiferromagnetic interactions.

Electron paramagnetic resonance and cyclic voltammetry studies of the cations and anions of α-aminoanthraquinones obtained by electrochemical oxidation/reduction

The 1,4-diaminoanthraquinone dyes Disperse Violet 1 and Disperse Blue 14 have been studied by cyclic voltammetry and EPR spectroscopy. The cation radicals, obtained by in situ electrochemical one-electron oxidation, can be characterised as aromatic diamino radicals (semiquinonediimines), with a spectroscopic pattern similar to those of Wurster's Blue-type radicals. The anion radicals, obtained by one-electron reduction, are of the semiquinone type. For the cation radicals all amino protons are unequivocally identified by deuterium exchange.The new data show that single-line spectra observed from the antitumour antibiotics mitoxantrone and ametantrone must have arisen by a process more complex than simple one-electron oxidation of the parent compounds.

Mössbauer studies of pressure-induced amorphization in the molecular crystalSnBr4

A single line spectrum is characteristic of the Moessbauer data obtained in the 0--5 GPa range, indicating the preservation of fourfold symmetry of the SnBr{sub 4} molecule in the crystal. Above 5 GPa, near {ital P}{sub {ital c}}, a {ital quadruple} {ital interaction} takes place concurrent with a dramatic increase in the {ital recoil{minus}free} {ital fraction} $({ital f})---. The value of the quadrupole splitting reaches a maximum of 0.9 mm/s at {ital P}{similar_to}15 GPa and remains constant thereafter. By {similar_to}9 GPa the absorption area, which is proportional to {ital f}, increases by 30--40 % over the lowest pressure value and then remains constant at higher pressure. These results are consistent with the formation of a molecular species, e.g., a (SnBr{sub 4}){sub 2} dimer, lacking the {ital T}{sub {ital d}} symmetry at the original Sn{sup 4+} site and having optical phonons {ital hard} enough not to be excited by the nuclear recoil process. Molecular association into (SnBr{sub 4}){sub 2} dimers, the building block of the high-pressure disordered state, also explains many of the experimental features of the Raman data. Upon decompression, Moessbauer (and Raman) data suggest that these dimers dissociate into monomers at {similar_to}5 GPa; however, a disordered structure of SnBr{submore » 4} persists as pressure is decreased further. Crystallization is fully recovered below 1 GPa. The nature of the pressure-induced amorphization of the insulator SnBr{sub 4} is discussed in terms of the structural and valence properties of the analogous metallic SnI{sub 4}.« less

Electron Spin Resonance Study on Plasma-Induced Surface Radicals of Ethylene–Tetrafluoroethylene Copolymer

We have undertaken Ar plasma irradiation on ethylene–tetrafluoroethylene (1:1) alternating copolymer (ETFE) powder, and the radicals formed were studied by electron spin resonance (ESR). The room temperature ESR spectrum of plasma-irradiated ETFE shows a six-line spectrum (sextet) similar to that of polyethylene (PE), due to a small difference in hyperfine splitting constant between β-H and β-F atom. Thus, the major radical formed was assigned to the midchain alkyl radical generated by hydrogen elimination. The ESR spectral parameters for a representative sextet deduced from the simulated spectra are as follows: g=2.0029, Nα=2.19 mT (1H), Hβ=2.88 mT (2H), and Fβ=2.80 mT (2F). It was also found that the sextet spectrum gradually turned toward a broad single-line spectrum as the plasma duration increased indicating the successive formation of dangling bond sites (DBS) due to the occurrence of cross-linked surface reaction.

Spectrochemistry of Plasma-Induced Free Radicals in Cellulose Derivatives.

We report here the specific features of plasma-induced free radicals of cellulose derivatives such as ethylcellulose (EC) and hydroxyethylcellulose (HEC) on its comparison with those of cellulose. The electron spin resonance (ESR) spectra of Ar plasma-irradiated EC and HEC consist of three kinds of discrete spectral components, two isotropic spectra [doublets (I) and triplets (II), both being assigned to hydroxylalkyl radicals] and one anisotropic spectrum [doublet of doublets (IV) assigned to an acylalkyl radical], and a single broad line spectrum (III). The special feature here is the fact that the spectrum (III) is a major component, contrary to cellulose, which was assigned to an immobilized dangling-bond site (DBS) at the cross-linked region. The results suggest that plasma-induced cross-linking reactions are very predominant in EC and HEC relative to that of cellulose, due to the presence of alkyl substituents in EC and HEC.

Phonon softening in (C2H5)4NFeX4 (X=Cl, Br)

Mossbauer spectra of the Fe(III) compounds (C2H5)4NFeX4 (X=Cl, Br) exhibit the following anomalies above ∼ 260 K: (i) thef-factor drops with respect to normal thermal behavior, (ii) the second-order Doppler shift drops similarly, (iii) the single-line spectrum broadens, becoming asymmetric at room temperature. These phenomena are discussed in relation to possible phase transitions in these systems.

Compton scattering in jets: A mechanism for approximately 0.4 and less than or approximately 0.2 MeV line production

We show that gamma-ray line emission at approximately 0.4 and less than or approximately 0.2 MeV can be produced by Compton scattering of beamed radiation in the jets of Galactic black hole candidates. This mechanism has the novel feature of not invoking the presence of e(exp +)-e(exp -) pairs. To produce the two lines, we employ a symmetric double-sided jet with bulk flow velocity of about 0.5c and incident beam radiation with a hard energy spectrum. We show that the two lines can be seen at viewing-angle cosines relative to the jet ranging from 0.2 to 0.6. This comprises 40% of the total solid angle. In addition, the line radiation is approximately 10% polarized. Depending on the bulk flow and viewing angle, the model can produce lines at other energies as well. In particular, a broad feature near 1 MeV can be seen by viewing the jet close to its axis. Our model can also accommodate single-line spectra if the beamed gamma-ray emission or the jets themselves are asymmetric.

Effect of Deuteration on the Phase Transitions and on the Critical Dynamics in Ammonium Hexachlorometallates

Abstract Recent and novel data obtained from chlorine NQR measurements on natural and deuterated (NH4)2MCl6 compounds are discussed with special regard to the influence of the ammonium-ion dynamics on the structural stability of these crystals. The temperature dependence (4.2 K to 350 K) of the chlorine NOR frequency vQ and relaxation rates T1 -1 , T2 -1 obtained from the natural ammonium salts of Sn, Pd, Os, Pb, Te, Se and from the deuterated salts of Sn, Te and Se are analysed. Slight deviations from the normal temperature behaviour of vQ and T1 -1 are found in Sn, Pd and Os compounds which stay cubic in the whole temperature range. The ammonium compounds of Pb and Te undergo a structural transformation between 80 K and 90 K from the cubic to a trigonal phase which is distinguished by the preservation of the single line spectrum of the chlorine NQR below rel. The observed divergence of T1 -1 at the transition point can be described in terms of a spin-phonon process in the presence of an overdamped soft mode. Deuteration of (NH4)2TeCl6 only slightly affects the transition of Tc2 but leads to new structural changes at lower temperatures. Whereas the natural compound stays trigonal down to 4.2 K the deuterated crystal undergoes two additional structural transformations at Tc2 = 48 K and Tc3 = 28 K which are correlated with a slowing down of the deuteron motion. Approaching Tc2 from above, the spin-lattice relaxation rate and the spin-spin relaxation rate of the chlorine NQR exhibit distinct anomalies which are attributed to limited jumps of the octahedron in a shallow potential. The barrier height of this potential deduced from the chlorine NQR spin-lattice relaxation rate is 400 K. The transition at Tc2 is explained by the condensation in one minimum of this potential. At Tc3 a long range correlation is formed which is accompanied by a rotation of the octahedron about its fourfold axis. A similar mechanism is adopted for the transitions observed in (NH4)2SeCl6 at Tc= 24 K and in (ND4)2 SeCl6 at Tc = 48 K.

A model describing the single and multiple line spectra of tunable microcrystal lasers

A theoretical model is derived by which the occurrence of the single and multiple emission line spectra of tunable microcrystal lasers is described. These spectra in general exhibit several emission lines, arising from the Stark-split atomic levels of the lasing ions in the crystal field. The model is based on the shift of the resonator frequencies due to the thermally induced shift of the optical resonator length and demonstrates that the coincidence of the resonator frequencies with the laser gain lines leads to the emission of single or multiple line spectra of microcrystal lasers. These spectra can be described by the model. The model is given as a set of criteria. In this way not only can predictions of the single emission-line tuning range be made but also resonator lengths can be optimized in order to obtain a maximum tuning range. Furthermore, Q-switched operation can be achieved for specific parameters by periodically shifting the resonator frequencies. The linewidth of the gain used in this model depends on the laser threshold and is folded with the thermal shift of the atomic transitions. Therefore the centre wavelength of the gain is assumed to be constant. The advantage is that this experimentally relevant linewidth can be measured easily with microcrystal lasers themselves, whereas spectroscopic data do not take laser threshold behaviour into account. It is shown that the results of the model are in good agreement with experimental data measured for two different Nd: YAG crystals. Simply by inserting other material and laser parameters, the model can easily be applied to other laser crystals and other wavelengths.

ESR Dosimetry with Magnesium Sulphate

Abstract The aim of the investigation is to enhance the ESR dosemeter sensitivity in the therapy dose range below 10 Gy, where the ESR spectrum of the a-alanine radical becomes increasingly difficult to evaluate for dosimetry. Anhydrous magnesium sulphate, MgSO4, appears to be an excellent candidate from the spectrometric point of view. On irradiation with 60Co ? rays the stable radical SO3 is produced whose ESR signal amplitude increases linearly with dose up to about 105 Gy. Ignoring the abundant isotopes 17O (0.04%) and 33S (0.74%), the spectrum of SO3 is a single line at g = 2.0036 of width 0.5 mT which is stable at temperatures up to at least 120 oC. Using the conventional peak-to-peak method of dosemeter readout, the MgSO4 dosemeter is somewhat more sensitive than the traditional alanine dosemeter of the same mass. Its main advantage over the alanine dosemeter is, however, the ease of computer enhancement and the possibility of integration which the single-line spectrum offers.

Electron paramagnetic resonance characterization and thermal decay of radicals trapped in a photopolymerized triacrylate monomer

Radicals trapped during the photopolymerization and photo-cross-linking of 2-ethyl-2-hydroxymethylpropane-1,3-diol triacrylic ester have been studied by EPR spectroscopy at different temperatures (25–180 °C) and their thermal decay has been followed at 120 °C and 150 °C. Performing a fitting procedure on the EPR signals showed that they are composed of two kinds of patterns, which superimpose with the same g value. One of them is a single-line spectrum, while the other is hyperfine-structured. An exchange between two radical conformations is revealed by the latter, with the rate depending both on temperature and on the molar fraction of radicals giving the single-line spectrum. This suggests that both species are involved in the exchange mechanism. Termination occurs preferentially between radicals giving the same EPR pattern. Second-order termination rate constants depend on the photoinitiator concentration in the irradiated photoreactive mixture and give information about the microstructure of the polymer.

C60 solid state rotational dynamics and production and EPR spectroscopy of fullerenes containing metal atoms

The narrow single-line NMR spectrum of solid C60 at ambient temperature demonstrates that the molecules rapidly reorient within the crystal lattice. The rate of this motion can be quantitatively probed using solid-state NMR by measuring the relaxation rate, 1/T1. The dominant contribution to this rate at ambient temperature arises from the combination of rotation and chemical shift anisotropy (CSA). The contribution, 1/T1CSA, is isolated by measuring the magnetic field dependence of T1, allowing the molecular reorientational correlation time, tau , to be determined from T1CSA and the CSA tensor components. At 283 K tau =9.1 ps, only three times longer than the calculated tau for free rotation, and shorter than the value measured for C60 in solution (15.5 ps). This tau corresponds to a rotational diffusion constant D=1.8*1010 s-1. Below 260 K a second phase with a much longer reorientation time is observed, consistent with reports of an orientational phase transition in solid C60. In both phases tau shows Arrhenius behaviour with apparent activation energies of 1.4 and 4.2 kcal mol-1 for the high-temperature (rotator) and low-temperature (ratchet) phases, respectively. The authors succeeded in producing quantities of fullerene cages containing metal atoms sufficient to allow the first characterization of such a species by electron spin resonance spectroscopy. The EPR spectrum of La@C82 shows an octet of lines centered in the region characteristic of fullerene anion radicals, with a small hyperfine splitting which indicates that the spin is predominantly associated with the carbon shell, leaving the lanthanum atom in its preferred +3 oxidation state.

Electron paramagnetic resonance spectra of the Fe2(CO)–8 radical trapped in single crystals of PPN+FeCo(CO)–8

Anisotropic, single-line EPR spectra observed in the temperature range 4–80 K in single crystals of PPN+FeCo(CO)–8 doped with small amounts of the radical Fe2(CO)–8 have been assigned to two electronically and geometrically distinct forms of the radical which exist in thermal equilibrium. The least energetic isomer has the orientation and the low symmetry of the host-crystal molecule with a single carbonyl bridge, one five-coordinate Fe and one six-coordinate Fe nucleus. The unpaired spin in this isomer is essentially confined to the five-coordinate Fe nucleus in a dxy orbital. The high-temperature species is believed to have a second carbonyl bridge and C2v symmetry with a SOMO of the representation B2 and unpaired spin density shared equally between the metal nuclei. At temperatures in excess of 80 K the spectra of these two isomers are replaced by that of a third electronic doublet with g-tensor components which are approximately the average of those for the other two. The temperature dependence of the spectra is characteristic of exchange between two sites, and the third species is believed to be a fluxional variant of the low-temperature isomers.

Effect of thickness on the linewidth of a scattering Mössbauer spectrum

The Mössbauer line broadening is of interest for diffusion and relaxation studies. In scatterer samples consisting of light elements and a Mössbauer probe in very low abundance having an internal conversion coefficient of high value, beside resonant scattering other phenomena also occur. Bara and Bogacz (Nucl. Instr. and Meth. 186 (1981) 561) have given expressions for line shapes taking into account all such significant phenomena. Starting from this expression for the line shape of a scattering spectrum, linewidth of a single line spectrum has been analytically found and expressed as a Taylor series expansion in terms of the thickness of the scatterer.

A deuterium and phosphorus-31 nuclear magnetic resonance study of the interaction of melittin with dimyristoylphosphatidylcholine bilayers and the effects of contaminating phospholipase A2.

The interaction of bee venom melittin with dimyristolphosphatidylcholine (DMPC) selectively deuteriated in the choline head group has been studied by deuterium and phosphorus-31 nuclear magnetic resonance (NMR) spectroscopy. The action of residual phospholipase A2 in melittin samples resulted in mixtures of DMPC and its hydrolytic products that underwent reversible transitions at temperatures between 30 and 35 degrees C from extended bilayers to micellar particles which gave narrow single-line deuterium and phosphorus-31 NMR spectra. Similar transitions were observed in DMPC-myristoyllysophosphatidylcholine (lysoPC)-myristic acid mixtures containing melittin but not in melittin-free mixtures, indicating that melittin is able to stabilize extended bilayers containing DMPC and its hydrolytic products in the liquid-crystalline phase. Melittin, free of phospholipase A2 activity, and at 3-5 mol% relative to DMPC, induced reversible transitions between extended bilayers and micellar particles on passing through the liquid-crystalline to gel phase transition temperature of the lipid, effects similar to those observed in melittin-acyl chain deuterated dipalmitoylphosphatidylcholine (DPPC) mixtures [Dufourc, E. J., Smith, I. C. P., & Dufourcq, J. (1986) Biochemistry 25, 6448-6455]. LysoPC at concentrations of 20 mol% or greater relative to DMPC induced transitions between extended bilayers and micellar particles with characteristics similar to those induced by melittin. It is proposed that these melittin- and lysoPC-induced transitions share similar mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)