Linewidth Data Research Articles

Anisotropic superexchange and spin-resonance linewidth in diluted magnetic semiconductors.

The Dzyaloshinski-Moriya (DM) anisotropic superexchange constant and the resulting electron-paramagnetic-resonance (EPR) linewidth in Mn-based II-VI-compound diluted magnetic semiconductors (DMS) such as ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te are calculated quantitatively. An Anderson Hamiltonian, developed in a previous study of isotropic superexchange, describing correlated Mn 3d states hybridized with semiconducting s- and p-derived levels, is generalized to include the anion spin-orbit coupling responsible for anisotropic superexchange. DM exchange is shown to be the dominant anisotropic interaction, with magnitude \ensuremath{\sim}5% of isotropic superexchange. The EPR line shape is calculated with use of a moment expansion of the magnetic response function to first order in inverse temperature together with a maximum-entropy ansatz. The calculated infinite-temperature linewidths are in good agreement with extrapolated experimental values. A novel fit of the theoretical temperature dependence to existing experimental linewidth data provides the first empirical value for the anisotropic exchange constant, in excellent agreement with the theoretical value. Calculated chemical trends for the exchange constants yield the experimentally expected linewidth trends.

Effect of oxygen stoichiometry on softening of Raman active lattice modes in YBa2Cu3Ox.

Temperature-dependent frequency-shift and linewidth data of the Raman active 337-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{CuO}}_{2}$ in-plane bond-bending mode have been measured for five different oxygen stoichiometries between x=7.00 and x=6.14 in polycrystalline samples. Softening and line-broadening effects observed for the x=7.00 sample are sharply reduced for x=6.87 even though the superconducting transition temperature is similar. With a further reduction of oxygen content below x=6.68 both the soft-mode behavior and the line-broadening effects completely disappear. These unexpected results could suggest a large variation in the interaction between the electronic system and the lattice as the oxygen content is varied.

Predissociation of O2 in the B state

Dispersed LIF spectra of O2 in the Schumann–Runge band were measured with a modified tunable ArF laser in a flame. Spin-state selective predissociation of the B state was directly observed in fluorescence excitation spectra, revealing the relative coupling matrix elements of the triplet components to the manifold of repulsive states. Such data determines the symmetries of the important predissociating curves for each observed B-state vibrational level and shows that past interpretation of absorption linewidth data is in error. Due to the fast predissociation, quench-free emission spectra arising from laser prepared single rovibronic levels in the B state were observed even in an atmospheric flame. Fluorescence to X-state vibrational levels as high as v″=35 was observed and relative emission probabilities were derived.

Dynamics of energy flow from CH overtone excitations: Theoretical and experimental studies of CH3C≡CH

A study has been made of the vibrational energy flow mechanisms and time scales pertaining to the overtone stretch excitations of methyl and acetylenic CH stretches in propyne. Classical trajectories are used to interpret the experimental data for the overtone linewidths, as well as to analyze the role that individual modes play in determining energy flow. The full anharmonic potential surface for these calculations, including all modes, has been developed from spectroscopic and structural information, including the linewidth data. The principal results are: (1) The trajectory calculations show a localization transition, corresponding to a switch over from normal-mode behavior for CH3 excitations up to v≅3 to a local-mode CH excitation within the CH3 moiety for excitations of v≳6, with transition behavior for v=4,5. (2) The acetylenic CH shows local-mode behavior from v=1. Extremely long lifetimes are found for the excitations of this mode, and the trajectories indicate that the experimental width is predominantly rotational. (3) The rocking and deformation modes are dominant receiving modes in the relaxation of the methyl stretch. (4) A shorter lifetime is calculated for the v=6 vs the v=5 or v=7 overtones of the methyl C–H stretch. Experimental results are qualitatively consistent with this prediction. The origin of this shorter lifetime is a band of resonances between the stretch excitation and combinations of rocking, deformation, and pseudorotation modes. (5) CH3 internal rotation figures importantly in the relaxation of some levels (v=5, 8 of CH3) where it ‘‘closes the energy gap’’ for achieving resonant energy transfer. (6) For v=8 of the methyl CH, some direct energy transfer to both C–C≡C stretching modes is seen. The switching on of the stretches as receiving modes is a consequence of sufficiently strong interactions between the excited H and the C–C≡C chain, which take place at these high vibrational energies. (7) Evidence is found for long distance ‘‘through-space’’ energy transfer due to long-range dipole–dipole forces. This transfer occurs from the acetylenic to the methyl CH stretches. This result is illustrated for the v=2 excitation of the acetylenichydrogen, and constitutes a direct demonstration of intramolecular long-distance, through-space v–v energy transfer. These results demonstrate the potential importance of large amplitude modes such as rocking and deformation as initial receiving modes for vibrational energy from excited CH overtones. On the time scale probed here (∼1 ps), despite the availability of many degrees of freedom, the transfer process is dominated by specific energy transfer channels and by the specific behavior of individual modes, rather than by statistical considerations, which will certainly prevail on longer time scales.

Inelastic light scattering in halide and oxide glasses: Intrinsic Brillouin linewidths and stimulated Brillouin gain

Brillouin scattering measurements on various multicomponent halide and oxide glass compositions were performed as a function of composition, lattice temperature and configurational temperature. The intrinsic Brillouin linewidth measurements, the Brillouin intensities and the Brillouin frequency shifts allowed the calculation of phonon attenuations, the Pockels' elastoopic coefficients and the stimulated Brillouin scattering gain coefficients. From the parameters obtained in the above measurements we are able to calculate the threshold energy for the onset of stimulated Brillouin scattering in halide and oxide glasses. The results show that the threshold power for stimulated Brillouin scattering is larger in some of the halide glass compositions than some of the silica based glasses. This finding has important ramifications as a selection criteria for halide-based glasses versus oxide-based glasses in their use as possible single mode waveguide materials. The phonon attenuation values of some of the halide glasses, as measured from the linewidth data have magnitudes that correspond more to a liquid than to a solid. These anomalous phonon attenuation findings will be discussed in the light of existing theories.

Spin-wave excitations in amorphous Fe78B13Si9

Inelastic neutron scattering measurements have been used to study the long wavelength spin dynamics of the high Tc amorphous ferromagnetic alloy Fe78B13Si9 (Metglas 2605S2). Spin waves were observed over the accessible wave-vector range of 0.06 Å−1≤q≤0.12 Å−1, for temperatures between 473 K (0.67 Tc) and 705 K (0.99 Tc). The magnon dispersion curves exhibit the conventional quadratic relationship E=D(T)q2+Δ, typical of an isotropic ferromagnet, where the small energy gap Δ≊0.05 meV is attributed primarily to the dipole-dipole interaction. An estimate of the T=0 value of D≊156 meV Å2 was obtained from a plot of D(T) vs T extrapolated to low temperatures, while the Curie temperature of 710 K was obtained from the extrapolation of D(T) to zero at high temperatures. Spin-wave linewidth data for T/Tc≊0.95 showed the q4 wave-vector dependence expected for magnon-magnon interactions.

Analysis of static and dynamic light-scattering data in a critical binary-liquid mixture along isoconcentration paths.

We report measurements of the intensity and linewidth of the light quasielastically scattered from a binary critical mixture of n-hexane and nitrobenzene along various constant-concentration paths. The agreement between the intensity data and the scaling theory for the static quantities, and of the linewidth data with the dynamic-mode coupling theory for the transport coefficients and for the decay rate of the order parameter, including background effects, is very good.

EPR of layered magnetic metal-amino acid salts. II. Cu(L-Met) 2

Single crystals of bis(L-methioninato)copper(II), Cu(L-Met) 2, were studied by EPR at 9.7 and 33.6 GHz, at 300 K. The position and the peak-to-peak linewidth of the single observed EPR line were measured in three perpendicular planes of the samples. This single resonance is due to the collapse of the resonances of the two magnetically inequivalent copper ions in the lattice caused by the exchange interaction. The components of the molecular g tensor for isolated copper ions were obtained using a model which assumes axial symmetry. The results indicate that the unpaired electron occupies the d( x 2 − y 2) orbital, and the orientation of the molecule obtained from the EPR data agrees with the crystallographic result. The linewidth data support a model which assumes exchange narrowing of the magnetic dipolar interaction in a two-dimensional magnetic lattice, an incomplete collapse of the hyperfine structure, and a frequency-dependent contribution in the planes where the g factors for the two sites of copper are different. An analysis of this latter contribution, allows to evaluate an exchange coupling constant | J′| = 0.10 K between inequivalent copper neighbors. Besides, the analysis of the hyperfine contribution to the linewidth gives | J| = 0.18 K for the average value of the exchange interaction of one copper ion with its six nearest neighbors within the layer. Concerning the possible superexchange paths between inequivalent copper ions, we suggest they can be of two types: the CuNH…OCu one, involving hydrogen bonds between equatorial nitrogens and equatorial oxygens, and the other consisting of CuOCOCu carboxylate bridges involving apical and equatorial oxygens. They are discussed in view of the experimental results.

Heteronuclear NMR studies of cobalamins. 31P NMR observations of cobalamins bound to a haptocorrin from chicken serum.

A vitamin B12-binding protein (haptocorrin) from chicken serum has been purified to homogeneity by photodissociative affinity chromatography and characterized by gel electrophoresis and UV-visible spectrophotometry of its aquocobalamin, hydroxocobalamin, and cyanocobalamin complexes. The haptocorrin is a glycoprotein with a molecular mass of about 70 kDa and a protein moiety of about 40 kDa. 31P NMR resonances of the haptocorrin-cobalamin complexes are relatively broad singlets (with or without proton decoupling) shifted downfield by 0.7-1.0 ppm from the position of the free cobalamin resonances. From the line width data, the relaxation of the phosphorus nucleus is found to be dominated by chemical shift anisotropy with a very minor (13%) component from dipolar interaction with the two nearest neighbor protons. The rotational correlation time of the haptocorrin at 25 degrees C is estimated to be 85 ns and the activation energy for rotational correlation 3.9 +/- 0.3 kcal mol-1. The downfield shift of the 31P resonances of cobalamins upon binding to the haptocorrin cannot be due to hydrogen bonding phosphodiester moiety or displacement of the axial base by a group on the protein. Calculations also show that the downfield shift is very unlikely to be due to dipolar deshielding of the phosphorus nucleus by the ring current of an aromatic residue of the protein. It is concluded that the downfield shift of the 31P resonance must be due to sterically induced changes in phosphodiester conformation which may, or may not, involve steric compression of the axial Co-N bond.

Similarity between the Cu(2)-O-planes in tetragonal and orthorhombic YBa2Cu3O7−δ as probed by 89Y NMR

89 Y-NMR linewidth and relaxation rate data are reported for YBa 2 Cu 3 O 7−δ in the orthorhombic metallic and the tetragonal nonmetallic modifications. The temperature dependence of the observed relaxation rates is Korringa like with almost equal Korringa constants for both compounds. These results are correlated to the Cu(2) NMR data of Warren et al. and might be seen as a support of any theory which decouples charge transport from spin fluctuations.

Mobility of individual 5-fluorouridine residues in 5-fluorouracil-substituted Escherichia coli valine transfer RNA : A 19F nuclear magnetic resonance relaxation study

19F nuclear magnetic resonance (n.m.r.) relaxation parameters of 5-fluorouracil-substituted Escherichia coli tRNA 1 Val were measured and used to characterize the internal mobility of individual 5-fluorouridine (FUrd) residues in terms of several models of molecular motion. Measured relaxation parameters include the spin-lattice ( T 1) relaxation time at 282 MHz, the 19F 1H NOE at 282 MHz, and the spin-spin ( T 2) relaxation time, estimated from linewidth data at 338 MHz, 282 MHz and 84 MHz. Dipolar and chemical shift anisotropy contributions to the 19F relaxation parameters were determined from the field-dependence of T 2. The results demonstrate a large chemical shift anisotropy contribution to the 19F linewidths at 282 and 338 MHz. Analysis of chemical shift anisotropy relaxation data shows that, relative to overall tumbling of the macromolecule, negligible torsional motion occurs about the glycosidic bond of FUrd residues in 19F-labeled tRNA 1 Val, consistent with the maintenance of base-base hydrogen-bond and/or stacking interactions at all fluorouracil residues in the molecule. The dipolar relaxation data are analyzed by using the “two-state jump” and “diffusion in a cone” formalisms. Motional amplitudes (θ) are interpreted as being due to pseudorotational fluctuations within the ribose ring of the fluorinated nucleoside. These amplitudes range from approximately 30 ° to 60 °, assuming a correlation time ( τ i,2 ) of 1.6 ns. By using available 19F n.m.r. assignment data for the 14 FUrd residues in 5-fluorouracil-substituted tRNA 1 Val, these motional amplitudes can be correlated directly with the environmental domain of the residue. Residues located in tertiary and helical structural domains show markedly less motion (θ ∼- 30 to 35 °) than residues located in loops (θ ∼- 45 to 60 °). A correlation between residue mobility and solvent exposure is also demonstrated. The amplitudes of internal motion for specific residues agree quite well with those derived from X-ray diffraction and molecular dynamics data for yeast tRNA Phe.

An improved method for the detection of malignancy by proton NMR spectroscopy of plasma

The ratio of intensity (RI) of the plasma lipid methyl proton NMR resonance from two Carr-Purcell-Meiboom-Gill spectra acquired with a short (2.4 msec) and long (120 msec) pulse delay time is proposed to provide a potentially more sensitive and less variable marker of human malignancy than the linewidth parameter previously suggested ( E.T. Fossel, J.M. Carr, and J. McDonagh (1986) N. Engl. J. Med. 315 , 1369–1376). Linewidth and transverse (T 2) relaxation data are presented which demonstrate that the observed correlation between methyl and methylene lipid linewidth and the methyl RI parameter arises from a combination of T 2 differences and the relative amounts that fast- and slow-relaxing components contribute to the composite methyl resonance.

EPR of layered magnetic metal—amino acid salts. I. Cu(L-PHE)2

EPR measurements in single crystals of Cu(L-PHE)2, the copper derivative of the amino acid L-phenylalanine, were performed at 300 K and 9.7 GHz. The gyromagnetic factor, g, and the linewidth of the single EPR line were measured in three perpendicular planes of the sample. The gyromagnetic tensor was obtained, and its principal values are g1 = 2.211, g2 = 2.134, and g3 = 2.075. The single resonance is explained by the collapse of the resonances of the two magnetically non-equivalent copper sites due to the exchange interaction. The molecular g-factor of isolated copper ions is obtained from a model assuming axial symmetry. The results are g| = 2.266 and g⊥ = 2.075, indicating a d(x2 - y2) ground orbital. The orientations of the molecules in the crystal obtained by this model are in agreement with the crystallographic values. The linewidth data support a model which assumes exchange narrowing of the magnetic dipolar interaction in a two-dimensional magnetic structure, an incomplete collapse of the hyperfine structure, and contributions arising from non-isotropic exchange. A mean value ¦J¦=0.19 K is calculated for the isotropic exchange interactions between one copper and its six copper neighbors in Cu(L-PHE)2. Also, a lower limit ¦J′¦ > 0.02 K for the exchange coupling J′ between non-equivalent copper neighbors is obtained.

The ionization behavior of bile acids in different aqueous environments.

The ionization behavior of cholic acid, deoxycholic acid, and chenodeoxycholic acid in a variety of physiologically important molecular environments was studied using 13C NMR spectroscopy. The apparent pKa of the carboxyl group was determined from titration curves obtained from the dependence of the carboxyl carbon chemical shift on pH. Using 90% 13C isotopic substitution of the carboxyl carbon, a complete titration curve was obtained for cholate at a concentration below its critical micelle concentration and solubility limit in water. Incorporation of 12 mole % bile acid into mixed micelles with its taurine conjugate prevented precipitation of the unconjugated bile acid, and titration curves for cholic, deoxycholic, and chenodeoxycholic acids in the mixed micelles were obtained. The apparent pKa was also determined for 13C-enriched bile acids complexed with bovine serum albumin and in egg phosphatidylcholine vesicles. For monomers, micelles, and BSA complexes of all three bile acids and for deoxycholic and chenodeoxycholic acid in vesicles, one magnetic environment was observed. In contrast, two environments, both titratable, were detected for cholic acid in phosphatidylcholine vesicles. The apparent pKa's of the bile acids in the different environments ranged from 4.2 to 7.3. At pH 7.4, as monomers or bound to albumin, the bile acids were fully ionized, but when associated with phosphatidylcholine vesicles they were only partially ionized. In addition, aspects of the molecular motion and relative hydrophobicity of the bile acid carboxyl group in the environments studied were discerned from chemical shift, line-width, and lineshape data.

Heats of combustion and electron spin resonance of ultrafine graphite

Heats of combustion, ( ΔH c) and ESR measurements on ultrafine graphite, an amorphous “degraphitized” form, depended on prior exposure of the material to air. Air-free samples: ΔH c increased from 18 to 35% over the literature value for polycrystalline graphite; the ESR signal was a symmetrical Lorentzian line centered at g = 2.0034. Air-exposed samples; ΔH c was about 4% higher than the literature value; the ESR signal was reversibly and asymmetrically broadened to 4 times the air free value and was nearly Gaussian. The different reactivities of edge and planar carbon atoms in the finely divided material are invoked to explain the combustion data. The asymmetric broadening of the ESR signal on exposure to air is accounted for by postulating anisotropic dipolar coupling between physisorbed molecular oxygen and the unpaired spins. The linewidth data support a model of thin graphite platelets roughly 10 Å thick and 30 Å in diameter.

Transition of local moments coupled to itinerant electrons in the quasi one-dimensional conductor, copper phthalocyanine iodide.

Copper phthalocyanine iodide is a molecular metal whose conducting stacks incorporate a one-dimensional array of local moments strongly coupled to conduction electrons. Below 20 K the EPR $g$ value of the coupled system increases anomalously and at 8 K the EPR signal broadens abruptly and becomes unobservable. Anomalies in the proton NMR spin-lattice relaxation are observed at the transition temperature. Magnetic susceptibility measurements and NMR linewidth data both indicate that there is little if any static magnetic order below 8 K.

Proton magnetic resonance study of molecular dynamics in (NH 4) 2CdI 4

Proton magnetic resonance and spin-lattice relaxation studies have been carried out on (NH 4) 2CdI 4 as a function of temperature (77–400 K) and Larmor frequency (10, 20 and 30 MHz). The T 1 data indicate isotropic tumbling of ammonium ions at equivalent sites till 160 K. There is an indication of a phase transition at 265 K, the activation energy for molecular reorientation increases from 2.8 kcal/mole to 4.6 kcal/mole. The relaxation results and the linewidth data support the presence of two inequivalent sites at low temperatures, one having an environment corresponding to near-rigid-lattice limit and the other undergoing fast reorientations. The behaviour of the free induction decay with temperature below 120 K suggests a coherent motion for the faster species.

Observation of a correlation length finite-size effect in Rayleigh scattering from thin critical fluid films.

Rayleigh linewidth data gathered at small scattering angles on critical-mixture films of thickness 2L are found to collapse about a single universal curve, ${\mathrm{\ensuremath{\Gamma}}}^{\mathrm{*}}$(K${\ensuremath{\xi}}^{\mathrm{eff}}$), where ${\mathrm{\ensuremath{\Gamma}}}^{\mathrm{*}}$(x) is the 3D reduced Rayleigh linewidth function, and ${\ensuremath{\xi}}^{\mathrm{eff}}$ is an effective correlation length obeying a general finite-size scaling relation, ${\ensuremath{\xi}}^{\mathrm{eff}/\mathrm{L}=\mathrm{F}(\ensuremath{\xi}/\mathrm{L})}$. The latter reveals that ${\ensuremath{\xi}}^{\mathrm{eff}}$ grows with \ensuremath{\xi} until \ensuremath{\xi}\ensuremath{\approxeq}L, when its growth terminates, implying the existence of surface layers of thickness \ensuremath{\xi} formed by preferential wall-fluid interactions.

FMR study of crystallization in the metglas 2826 amorphous alloy

The ferromagnetic resonance technique was used to study the annealing behavior of the metallic glass Fe 40Ni 40P 14B 6 (Metglas 2826). The peak-to-peak FMR linewidth, Δ H pp, was measured at room temperature and 9.5 GHz for several times of isothermal annealing in the temperature range 350–375°C. After an initial decrease, attributed to structural relaxation, the linewidth increases with annealing time for all temperatures investigated. The activation energy for crystallization, E c, is estimated from the times to Δ H pp = 20 to 80 mT as (412±23) kJ/mol. The results suggest that, for sufficiently long annealing times, there is a well-defined relation between Δ H and the volume fraction of the transformed phase. If this relation is assumed to be linear, the transformed fraction, as derived from linewidth data, satisfies the Johnson-Mehl-Avrami equation with the exponent n equal to 1.67±0.13.

Intramolecular vibrational energy redistribution and the gas phase overtone spectra of selectively deuterated neopentanes

The gas phase overtone spectra of neopentane-d6 [(CH3)2C(CD3)2] and neopentane-d9 [(CH3)C(CD3)3] are measured in the regions of ΔvCH =2–5. The peak positions are analyzed in terms of the local mode model with the assumption of methyl group independence. The overtone linewidths are shown to be consistent with intramolecular vibrational decay of the overtone states. The coupling scheme for the vibrational decay is similar to that applied to the linewidth data of benzene by Sibert, Reinhardt, and Hynes.