Apparent Linewidth Research Articles

Dissecting the Molecular Origin of g-Tensor Heterogeneity and Strain in Nitroxide Radicals in Water: Electron Paramagnetic Resonance Experiment versus Theory.

Nitroxides are common EPR sensors of microenvironmental properties such as polarity, numbers of H-bonds, pH, and so forth. Their solvation in an aqueous environment is facilitated by their high propensity to form H-bonds with the surrounding water molecules. Their g- and A-tensor elements are key parameters to extracting the properties of their microenvironment. In particular, the gxx value of nitroxides is rich in information. It is known to be characterized by discrete values representing nitroxide populations previously assigned to have different H-bonds with the surrounding waters. Additionally, there is a large g-strain, that is, a broadening of g-values associated with it, which is generally correlated with environmental and structural micro-heterogeneities. The g-strain is responsible for the frequency dependence of the apparent line width of the EPR spectra, which becomes evident at high field/frequency. Here, we address the molecular origin of the gxx heterogeneity and of the g-strain of a nitroxide moiety (HMI: 2,2,3,4,5,5-hexamethylimidazolidin-1-oxyl, C9H19N2O) in water. To treat the solvation effect on the g-strain, we combined a multi-frequency experimental approach with ab initio molecular dynamics simulations for structural sampling and quantum chemical EPR property calculations at the highest realistically affordable level, including an explicitly micro-solvated HMI ensemble and the embedded cluster reference interaction site model. We could clearly identify the distinct populations of the H-bonded nitroxides responsible for the gxx heterogeneity experimentally observed, and we dissected the role of the solvation shell, H-bond formation, and structural deformation of the nitroxide in the creation of the g-strain associated with each nitroxide subensemble. Two contributions to the g-strain were identified in this study. The first contribution depends on the number of hydrogen bonds formed between the nitroxide and the solvent because this has a large and well-understood effect on the gxx-shift. This contribution can only be resolved at high resonance frequencies, where it leads to distinct peaks in the gxx region. The second contribution arises from configurational fluctuations of the nitroxide that necessarily lead to g-shift heterogeneity. These contributions cannot be resolved experimentally as distinct resonances but add to the line broadening. They can be quantitatively analyzed by studying the apparent line width as a function of microwave frequency. Interestingly, both theory and experiment confirm that this contribution is independent of the number of H-bonds. Perhaps even more surprisingly, the theoretical analysis suggests that the configurational fluctuation broadening is not induced by the solvent but is inherently present even in the gas phase. Moreover, the calculations predict that this broadening decreases upon solvation of the nitroxide.

Influence of exterior acoustic noise on narrow linewidth laser measurements using self-homodyne optical fiber interferometer

Spectral linewidth measurements of a HeNe laser were obtained by using a self-homodyne optical fiber Mach-Zehnder interferometer located in a vacuum controlled chamber. The data showed an apparent broadening of the highly-coherent laser linewidth from few hertz up to kilohertz with the change in the air chamber pressure, evincing an influence of external acousto-optic effects on the interferometer optical fiber. To support the results, a theoretical harmonic acoustic analysis was developed and then related to the correspondent interferometer phase noise spectrum measurements. Lorentzian approximations of the experimental laser apparent linewidth suggest that the interferometer was working in a quasi-coherent regime for all tested chamber air pressure.

Modeling galaxy evolution at high-redshift in highly overdense and normal regions

AbstractWe present results from high-resolution, zoom-in cosmological simulations to study the effect of feedback from galactic outflows on the physical and Lyα properties of high-redshift galaxies in highly overdense and normal environments at z >∼6. The Lyα properties have been obtained by post-processing the simulations with a Monte-Carlo radiative transfer (RT) code. Our results demonstrate that galactic outflows play an important role in regulating the growth of massive galaxies in overdense regions as well as the temperature and metallicity of the intergalactic medium. In particular, we find that galactic outflows are necessary to reproduce the observed Lyα luminosity functions as well as the apparent Lyα luminosity, line width and equivalent width distributions of luminous Lyα emitters at z ∼ 6.

Nonlinear cavity optomechanics with nanomechanical thermal fluctuations

Although the interaction between light and motion in cavity optomechanical systems is inherently nonlinear, experimental demonstrations to date have allowed a linearized description in all except highly driven cases. Here, we demonstrate a nanoscale optomechanical system in which the interaction between light and motion is so large (single-photon cooperativity C0≈103) that thermal motion induces optical frequency fluctuations larger than the intrinsic optical linewidth. The system thereby operates in a fully nonlinear regime, which pronouncedly impacts the optical response, displacement measurement and radiation pressure backaction. Specifically, we measure an apparent optical linewidth that is dominated by thermo-mechanically induced frequency fluctuations over a wide temperature range, and show that in this regime thermal displacement measurements cannot be described by conventional analytical models. We perform a proof-of-concept demonstration of exploiting the nonlinearity to conduct sensitive quadratic readout of nanomechanical displacement. Finally, we explore how backaction in this regime affects the mechanical fluctuation spectra.

Unconventional electroabsorption in monolayer MoS2

Signal modulation in optoelectronics is obtained by modulation of either the refractive index or the absorbance by an electric field. However, electromodulators have not kept up with the miniaturization of other electronic and optical components. Here we show a strong transverse electroabsorption signal in a monolayer of the 2D semiconductor MoS2. The electroabsorption spectrum is dominated by an apparent linewidth broadening of around 15% at a modulated voltage of only Vpp = 0.5 V. Contrary to known variants of the Stark effect, the broadening increases linearly with the applied field strength and arises from a linear variation of the distance between the strongly overlapping exciton and trion resonances. The achievable modulation depths exceeding 0.1 dB nm−1 bear the scope for extremely compact, ultrafast, energy-efficient electroabsorption modulators for integrated photonics, including on-chip optical communication.

Fluorescence Excitation and Emission Spectroscopy on Single MEH-PPV Chains at Low Temperature

Fluorescence emission and excitation spectra of single MEH-PPV polymer molecules dispersed in thin PMMA films have been recorded at 1.2 and 20 K. We observe single as well as multichromophore emission in single chain emission spectra, whereby the relative fractions depend on the two different molecular weights (50 and 350 kDa) studied. The molecular weight also affects the distribution of peak emission maxima, which is monomodal (bimodal) for the low (high) molecular weight. The appearance of an additional "red" subpopulation for the high molecular weight sample is attributed to interactions of multiple chromophores from a sufficiently flexible single chain. The comparison of emission spectra appearing in the "blue" as well as "red" subpopulations suggests that these intrachain interactions rather lead to ground-state aggregates than excimers. Independent of the molecular weight, large variations in spectral shape and apparent line width in the emission spectra have been observed. Occasionally, we find very narrow purely electronic zero-phonon lines both in emission and in excitation spectra, with line widths down to the instrumental resolution. In accordance with earlier literature data it is argued that linear electron-phonon coupling should be quite strong for MEH-PPV in PMMA, leading to only a small fraction of chromophores exhibiting zero-phonon lines. In addition, spectral diffusion, which manifests itself by several time-dependent line shifting and broadening phenomena, contributes to the substantial variations of spectral shapes. Excitation experiments with particularly stable chromophores provide an upper limit for the optical line width (approximately 0.1 cm(-1)), which at 1.2 K can actually approach the lifetime-limited homogeneous width. Raising the temperature to 20 K leads to line broadening and typically, to disappearance of zero-phonon lines. The failure to observe zero-phonon lines of chromophores supposedly serving as donors in intramolecular energy transfer is tentatively attributed to fast transfer rates, resulting in strongly broadened lines which escape detection even at 1.2 K.

Observation of Very Narrow Linewidths in the Fluorescence Excitation Spectra of Single Conjugated Polymer Chains at 1.2 K

Fluorescence emission and excitation spectra of single poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] polymer molecules embedded in poly(methylmethacrylate) have been recorded at 1.2 K. The ubiquitous spectral diffusion causes large variations in the spectral shape and apparent linewidth in the emission spectra. Nevertheless, we find very narrow (approximately 0.1 cm(-1)) purely electronic zero-phonon lines in the excitation spectra, which are 2 orders of magnitude smaller than previous estimates of the homogeneous linewidth. These results complement the molecular description of the low energy transitions in conjugated polymers.

EPR and Solid-State NMR Studies of Poly(dicarbon monofluoride) (C2F)n

Poly(dicarbon monofluoride) (C2F)n was studied by electron paramagnetic resonance (EPR) and solid-state nuclear magnetic resonance (NMR). The effects of physisorbed oxygen on the EPR and NMR relaxation were underlined and extrapolated to poly(carbon monofluoride) (CF)n and semi-covalent graphite fluoride prepared at room temperature. Physisorbed oxygen molecules are shown to be an important mechanism of both electronic and nuclear relaxations, resulting in apparent spin-lattice relaxation time and line width during NMR and EPR measurements, respectively. The effect of paramagnetic centers on the 19F spin-lattice relaxation was underlined in accordance with the high electron spin density determined by EPR. 19F magic angle spinning (MAS) NMR, 13C MAS NMR, and 13C MAS NMR with 19F to 13C cross polarization (CP) underline the presence of two types of carbon atoms, both sp3 hybridized: some covalently bonded to fluorine and the others linked exclusively to carbon atoms. Finally, a C-F bond length of 0.138 +/- 0.002 nm has been determined thanks to the re-introduction of dipolar coupling using cross polarization.

Solid‐State Magic‐Angle Spinning NMR of Outer‐Membrane Protein G from Escherichia coli

Uniformly 13C-,15N-labelled outer-membrane protein G (OmpG) from Escherichia coli was expressed for structural studies by solid-state magic-angle spinning (MAS) NMR. Inclusion bodies of the recombinant, labelled protein were purified under denaturing conditions and refolded in detergent. OmpG was reconstituted into lipid bilayers and several milligrams of two-dimensional crystals were obtained. Solid-state MAS NMR spectra showed signals with an apparent line width of 80-120 Hz (including homonuclear scalar couplings). Signal patterns for several amino acids, including threonines, prolines and serines were resolved and identified in 2D proton-driven spin-diffusion (PDSD) spectra.

Lineshapes inmagnetic resonance spectra

In magnetic resonance, and in particular, in superparamagneticresonance studies at variable temperatures, a correlation betweenthe apparent resonance magnetic field and the apparent linewidth isoften observed. In order to account for this correlation, weconsider the resonance lineshapes resulting from differentphenomenological equations of damped motion of the magnetic momentsin the cases of a linear paramagnet and of a perfect softferromagnet. The Bloch-Bloembergen, modified Bloch, Gilbert,Landau-Lifshitz and Callen equations are analysed. In most caseswe obtain analytical expressions for the apparent resonance-fieldshift. Finally, we report an experimental variable-temperaturestudy of the superparamagnetic resonance of ultrafineFe2O3 particles in sol-gel glass. Computer simulationsusing the Landau-Lifshitz lineshape provide good fits of theresonance spectra at different temperatures for the same magneticand morphological parameters of the particles.

Effects of ischemia on intracellular rubidium in pig and rat hearts: 87Rb NMR imaging and spectroscopic study

87Rb MR imaging and spectroscopy were used to study the effects of ischemia on the properties of K+ in cardiac tissue. Isolated pig and rat hearts perfused by the Langendorff method with Krebs-Henseleit buffer were preloaded with Rb+. Ischemia (Isc) was induced by 120-min occlusion of the left anterior descending artery in the pig hearts or by stopping perfusion for 33 min in the rat hearts. Serial 87Rb MR images or spectra from the anterior (An) LV wall of pig hearts were acquired continuously. The intensities of the Rb images of the An and posterior (Pos) walls were similar and stable during the first 45 min of ischemia. The intensity of signal from the An wall (Isc) then gradually increased by 60 ± 8% relative to the preischemic value (vs. 31 ± 5% increase in Pos wall) and necrosis (19 ± 5% of the LV wall mass) developed upon reperfusion. The Rb+ content was lower in the ischemic (An) than in the normal (Pos) area (22.3 ± 3 vs. 28.4 ± 1.3 mmol/g wet wt). A similar pattern was observed in the peak heights of 87Rb spectra from the An wall, which increased by 40 ± 16% (vs. 21 ± 11% in control) due to a 12% decrease in the apparent Rb linewidth (LW) and a 24 ± 14% increase in the peak area. The Rb peak comprised narrow (297 ± 21 Hz) and broad (1098 ± 40 Hz, 59 ± 3% of total area) Lorentzian components. The LW of the broad component decreased by 14%, while the narrow component did not change markedly. In the rat hearts ischemia caused a 33 ± 4% increase in the 87Rb peak height as a result of peak narrowing (13 ± 1%), and an increase in peak area (17 ± 5%). The decreases in LW and increases in Rb+ visibility can be explained by an increase in Rb+ mobility caused by displacement of Rb+ from anionic binding sites by H+ (ischemic acidosis) and changes in intracellular compartmentalization of Rb+. Magn Reson Med 44:193–200, 2000. © 2000 Wiley-Liss, Inc.

Effects of ischemia on intracellular rubidium in pig and rat hearts: (87)Rb NMR imaging and spectroscopic study.

87Rb MR imaging and spectroscopy were used to study the effects of ischemia on the properties of K(+) in cardiac tissue. Isolated pig and rat hearts perfused by the Langendorff method with Krebs-Henseleit buffer were preloaded with Rb(+). Ischemia (Isc) was induced by 120-min occlusion of the left anterior descending artery in the pig hearts or by stopping perfusion for 33 min in the rat hearts. Serial (87)Rb MR images or spectra from the anterior (An) LV wall of pig hearts were acquired continuously. The intensities of the Rb images of the An and posterior (Pos) walls were similar and stable during the first 45 min of ischemia. The intensity of signal from the An wall (Isc) then gradually increased by 60 +/- 8% relative to the preischemic value (vs. 31 +/- 5% increase in Pos wall) and necrosis (19 +/- 5% of the LV wall mass) developed upon reperfusion. The Rb(+) content was lower in the ischemic (An) than in the normal (Pos) area (22.3 +/- 3 vs. 28.4 +/- 1.3 mmol/g wet wt). A similar pattern was observed in the peak heights of (87)Rb spectra from the An wall, which increased by 40 +/- 16% (vs. 21 +/- 11% in control) due to a 12% decrease in the apparent Rb linewidth (LW) and a 24 +/- 14% increase in the peak area. The Rb peak comprised narrow (297 +/- 21 Hz) and broad (1098 +/- 40 Hz, 59 +/- 3% of total area) Lorentzian components. The LW of the broad component decreased by 14%, while the narrow component did not change markedly. In the rat hearts ischemia caused a 33 +/- 4% increase in the (87)Rb peak height as a result of peak narrowing (13 +/- 1%), and an increase in peak area (17 +/- 5%). The decreases in LW and increases in Rb(+) visibility can be explained by an increase in Rb(+) mobility caused by displacement of Rb(+) from anionic binding sites by H(+) (ischemic acidosis) and changes in intracellular compartmentalization of Rb(+). Magn Reson Med 44:193-200, 2000.

Phospholipids chiral at phosphorus. Dramatic effects of phosphorous chirality on the deuterium NMR properties of the choline head group of phospholipids in the liquid crystalline phase

To probe the motional and conformational properties of the choline head group of 1,2-dipalmitoyl-sn-glycero-3-thiophosphocholine (DPPsC), the Rp, Sp, and Rp + Sp isomers of [alpha-D2]DPPsC, [beta-D2]DPPsC, and [delta-D9]DPPsC in the subgel, gel, and liquid crystalline phases were investigated with deuterium NMR, and the results were compared with those of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) labeled at the same positions. In the subgel phase (5 degrees C) all isomers of [alpha-D2]DPPsC and [beta-D2]DPPsC displayed amorphous line shapes characteristic of a restricted and disordered motional environment, whereas [delta-D9]DPPsC showed narrower and symmetric line shapes indicating substantial motions. For all three labeled positions the apparent line width of the Rp isomer is larger than those of Sp and Rp + Sp isomers, and the amorphous line shape of the Rp isomer also persists at 25 and 35 degrees C, which confirm the previous observation that the Rp isomer is unusually stable in the subgel phase and suggest that the Rp isomer is more rigid than the other isomers in the choline head group. In the gel phase (25 and 35 degrees C) narrower and symmetric line shapes were observed for Sp and Rp + Sp isomers, and the apparent line widths were comparable to those of DPPC. In the liquid crystalline phase there are dramatic differences between the spectra of DPPC and different isomers of DPPsC.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrical measurement of submicrometer contact holes

A technique to measure submicrometer-sized isolated features electrically is described. P.M. Hall's formula (1967-8) modified with an experimentally determined coefficient is used to obtain the hole diameter and the hole area from the electrically measured linewidth with a higher precision than the precision of the linewidth itself. By incorporating a large number of features in a single test structure, a gain in precision of better than 3 nm is shown analytically. The contact hole diameter and the area of the isolated features obtained in this way, were found to be accurate when compared with physical measurements using a scanning electron microscope. The test pattern used is a combination of two basic four-point probe linewidth-measuring structures and two modified ones. The modified structures contain a large number of contact holes in the 10- mu m lines running in the horizontal and vertical directions, which reduce the apparent electrical linewidth. Subtracting the reduced linewidth from that of a solid reference line removes the other contributions to linewidth variation. The size of the contact holes can then be derived from these results. The special test pattern, an analytic expression for interpretation of the results, and hole size data from a variety of exposure dosages are discussed. >

Mossbauer effect studies of Nd/sub 2/Fe/sub 14-x/Si/sub x/B and Y/sub 2/Fe/sub 14-x/Si/sub x/B

The Mossbauer effect spectra of the solid solutions of Nd/sub 2/Fe/sub 14-x/Si/sub x/B, with x equal to 0.00, 0.25, 0.50, 0.75, 1.0, 1.5, and 2.0, and Y/sub 2/Fe/sub 14-x/Si/sub x/Bi, with x equal to 0.0, 1.0, 1.5, and 2.0, have been measured at 295 K. The results are most consistent with a random distribution of the silicon over the smaller 16k/sub 1/, 16k/sub 2/, 8j/sub 1/, 4c, and 4e iron sites in Nd/sub 2/Fe/sub 14/B. Apparently, the silicon is, at least in part, excluded from the 8j/sub 2/ site, which is the site with the largest Wigner-Seitz volume. The results do not, however, rule out other possible silicon site occupancy models. The isomer shift increases slightly with increasing silicon content, whereas the quadrupole interaction remains virtually constant. The hyperfine field decreases with increasing silicon content, but the apparent linewidth increases. These changes have been modeled in terms of a distribution of hyperfine fields on each site which is determined by a binomial distribution of silicon on the Wigner-Seitz near neighbors of each iron site. >

Correction of Nonlinear Derivative Diode Laser Data in Standard Addition Analyses

Modulation techniques are widely used for derivative presentation of spectral lines for lineshape studies and quantitative analyses. Modulation amplitudes generating the most acceptable signal-to-noise ratios produce distortion in the recorded spectrum and alter the apparent linewidth and absorptivity of the band. Methods of determining modulation-broadened linewidths and absorptivities relative to their molecular values are presented for 2nd-derivative spectra of Doppler limited (Gaussian lineshape) infrared transitions of water vapor. The results are discussed in terms of using the effective absorptivities for correcting derivative data that no longer scale linearly with absorber concentration.

Coherent optical interference effects in external-cavity semiconductor lasers

A broadening of the apparent linewidth of the semiconductor laser modes with external optical feedback is observed. This is shown to be due to the coherent nature of the feedback and multiple reflections in the external cavity. A theory for the steady-state behavior of the external-cavity semiconductor laser taking into account such coherent optical effects is developed. The inclusion of these effects is also important in the interpretation of the threshold data of such lasers.

Optical feedback effects on the performance of Pb_1−xSnxSe semiconductor lasers

The effects of optical feedback on the performance of Pb(1-x)Sn(x)Se semiconductor lasers are reported. Experimental results are presented to demonstrate that uncontrolled optical feedback can occur from optics external to the laser cavity and that these external optical elements can result in the formation of a composite cavity. This composite cavity can sustain a number of narrow closely spaced emission modes with the spectral characteristics of these modes dependent upon the parameters of the cavity. With weak feedback, the spectral width of the amplitude envelope of the new lasing modes resembles that of the original laser mode with no optical feedback. With strongly coupled feedback the envelope of the external cavity modes substantially broadens the apparent linewidth of the laser. Results will be presented that show that these effects can be eliminated, and, moreover, they offer the potential for obtaining spectrally narrow singlemode emission from Pb-salt lasers through the use of carefully designed external cavities.

Dependence of the conformation of the polar head groups of phosphatidylcholine on its packing in bilayers. Nuclear magnetic resonance studies on the effect of the binding of lanthanide ions.

Proton magnetic resonance spectra of vesicles of various sizes composed of egg phosphatidylcholine (PC) with varying concentrations of cholesterol differed in the apparent line width of the signal of the methylene protons of PC (delta v1/2). They also varied in the extent of lanthanide-induced shifts of the 31P and 1H NMR signals of the corresponding nuclei of the polar head groups located on the outer surface of the vesicles (delta delta). The differences in the lanthanide-induced shifts of the 31P signals are fully accounted for by the ratio between the externally added lanthanide and the number of PC head groups available for interaction with the lanthanide ions. This was not the case ofr the changes in the 1H NMR spectra. Here delta delta decreased with increasing delta v1/2, suggesting that the packing of the PC paraffinic chaings in the bilayer affects the conformation of the polar head groups; tightening of the packing probably results in a more extended conformation of the head groups. This conclusion is also supported by the larger effect lanthanides have on the 1H chemical shift of the choline head groups on the outer surface of small unilamellar vesicles as compared to groups on the inner, tighter packed layer.

The structure of vesicular stomatitis virus membrane A phosphorus nuclear magnetic resonance approach

The proton decoupled 40.48 M Hz 31P NMR spectrum of intact and unperturbed membrane-enclosed vesicular stomatitis virus (serotype Indiana) exhibited two distinct maxima. These can be resolved into a narrow, symmetric line and a broad asymmetric line. The 31P NMR spectrum of a multilamellar (unsonicated) preparation of the extracted viral lipids exhibited a line shape similar to that of the intact virus. A sonicated vesicle preparation of the extracted viral lipids exhibited a narrow symmetric line. The narrow component in the intact virus spectrum may be attributed to small membrane fragments. Phospholipase C digestion of the intact virus resulted in substantial reduction in intensity of both components which suggests that much of the contribution to both peaks is due to phosphate in the phospholipid polar head groups. The phospholipid phosphates in both sonicated and unsonicated preparations of the extracted viral lipids exhibited substantially longer relaxation times than did those in the intact virus. The short relaxation time emanating from the intact virus preparation is caused by immobilization of the phospholipid head groups which could be due to lipid-protein interactions. Trypsin treatment of vesicular stomatitis virions, which results in complete removal of the exterior hydrophilic segment of the membrane glycoprotein, increased the 31P relaxation time to a value similar to that observed in the protein-free total lipid extracts; this finding provides supporting evidence for the role of virus glycoprotein in shortened relaxation times. A reversible temperature-dependent change in apparent line width and absence of an effect of cholesterol on the 31P phospholipid spectrum were also demonstrated.