High-Resolution H NMR Spectroscopy in the Solid State: Very Fast Sample Rotation and Multiple-Quantum Coherences

In the past few years, solid-state ¹H NMR spectroscopy under fast magic-angle spinning (MAS) has developed into a versatile tool for elucidating structure and dynamics. Dipolar multiple-quantum (MQ), in particular double-quantum (DQ), MAS spectroscopy has been applied to a variety of materials and provided unique insight, e.g., into the structure of hydrogen-bonded systems. This review intends to present solid-state ¹H DQ and MQ MAS spectroscopy in a systematic fashion with a particular emphasis on methodological aspects, followed by an overview of applications.     

High-resolution 1H NMR spectroscopy in the solid state: very fast sample rotation and multiple-quantum coherences

In the past few years, solid-state 1H NMR spectroscopy under fast magic-angle spinning (MAS) has developed into a versatile tool for elucidating structure and dynamics. Dipolar multiple-quantum (MQ), in particular double-quantum (DQ), MAS spectroscopy has been applied to a variety of materials and provided unique insight, e.g., into the structure of hydrogen-bonded systems. This review intends to present solid-state 1H DQ and MQ MAS spectroscopy in a systematic fashion with a particular emphasis on methodological aspects, followed by an overview of applications.     

New insights into plant cell walls by vibrational microspectroscopy

Vibrational spectroscopy provides non-destructively the molecular fingerprint of plant cells in the native state. In combination with microscopy, the chemical composition can be followed in context with the microstructure, and due to the non-destructive application, in-situ studies of changes during, e.g., degradation or mechanical load are possible. The two complementary vibrational microspectroscopic approaches, Fourier-Transform Infrared (FT-IR) Microspectroscopy and Confocal Raman spectroscopy, are based on different physical principles and the resulting different drawbacks and advantages in plant applications are reviewed. Examples for FT-IR and Raman microscopy applications on plant cell walls, including imaging as well as in-situ studies, are shown to have high potential to get a deeper understanding of structure–function relationships as well as biological processes and technical treatments. Both probe numerous different molecular vibrations of all components at once and thus result in spectra with many overlapping bands, a challenge for assignment and interpretation. With the help of multivariate unmixing methods (e.g., vertex components analysis), the most pure components can be revealed and their distribution mapped, even tiny layers and structures (250 nm). Instrumental as well as data analysis progresses make both microspectroscopic methods more and more promising tools in plant cell wall research.     

Rapid sonochemical synthesis of MCM-41 type benzene-bridged periodic mesoporous organosilicas

Benzene-bridged periodic mesoporous organosilicas (PMOs) with the MCM-41 were synthesized by a rapid sonochemical process via co-condensation of tetraethoxysilane (TEOS) and 1,4-bis(triethoxysilyl) benzene (BTEB) under basic conditions within a few minutes using cetyltrimethylammoniumbromide (CTMABr) as a structure-directing agent. The molar ratio of the silicon precursors and the synthesis time were varied in order to investigate their influence on the structural ordering of the materials. The characteristics of the materials were evaluated by X-ray diffraction (XRD), N₂-sorption, transmission electron microscopy (TEM) and solid-state NMR spectroscopy. The resultant materials exhibited well-ordered hexagonal mesostructures with surface areas in the range of 602–1237 m²/g, pore volumes of 0.37–0.68 cm³/g, and pore diameters in the range of 2.5–3.5 nm. Two dimensional ²⁹Si{¹H} heteronuclear correlation (HETCOR) NMR spectra confirmed the formation of a single mesophase with various Q (from TEOS) and T (from BTEB) silicon species located randomly within the pore walls due to the co-condensation of BTEB and TEOS, which excluded the possibility of formation of island or two separate phases within such a short synthesis time. The prime advantage of the present synthesis route is that it can effectively reduce the total synthesis time from days to a few minutes, much shorter than the conventional benzene-bridged PMOs synthesis methods.     

Pervasiveness of cluster excitations as seen in the Mössbauer spectra of magnetic materials

Magnetic cluster excitations in various physical systems (e.g., soliton bearing one-dimensional solids, metallic alloys, amorphous materials, small particle aggregates, magnetically ordered substances near T_C, transition metal di-chloride graphite intercalation compounds, etc.) are described. Use of Fe-57 Mössbauer effect spectroscopy as a probe of the spin dynamics for inverse autocorrelation times between 10⁷Hz to 10¹⁰ Hz is emphasized. Particular attention is given to systems which exhibit local or long range magnetic order and whose Mössbauer spectra must therefore be described by more than one autocorrelation (or dwell) time for fluctuations between different allowed hyperfine field directions on a given site.     

Multivariate Image Analysis of Magnetic Resonance Images with the Direct Exponential Curve Resolution Algorithm (DECRA): Part 2: Application to Human Brain Images

Owing to the heterogeneity of living tissues, it is challenging to quantify tissue properties using magnetic resonance imaging. Within a single voxel, contributions to the signal may result from several types of¹H nuclei with varied chemical (e.g., −CH₂−, −OH) and physical environments (e.g., tissue density, compartmentalization). Therefore, mixtures of¹H environments are prevalent. Furthermore, each unique type of¹H environment may possess a unique and characteristic spin–lattice relaxation time (T₁) and spin–spin relaxation time (T₂). A method for resolving these unique exponentials is introduced in a separate paper (Part 1. Algorithm and Model System) and uses the direct exponential curve resolution algorithm (DECRA). We present results from an analysis of images of the human head comprising brain tissues.     

Spectral deconvolution of NMR cross polarization data sets

The COmponent-REsolved (CORE) strategy has been employed, for the first time to solid state NMR spectroscopy. CORE was used to extract two time-dependent spectral components in 24 ²⁹Si{¹H} NMR spectra, recorded on a meso-structured silica material under conditions of cross polarization evolution. No prior assumptions were made about the component bandshapes, which were both found to be skewed to higher chemical shifts. For the silica fragments close to protons this skewness could be rationalized by a distribution of the degree of condensation in the silica network; however, for the other component the non-Gaussian shape was unexpected. We expect that the same strategy could be applied to a range of experiments in solid-state NMR spectroscopy, where spectral distributions or kinetic parameters need to be accurately extracted.     

Solid-State Si, Cd, Sn, and P NMR Studies of II-IV-P2 Semiconductors

Solid-state ²⁹Si, ¹¹³Cd, ¹¹⁹Sn, and ³¹P MAS NMR spectra are reported on a series of II-IV-P₂ compounds. In favorable cases (e.g., high degree of crystallinity, low concentration of unpaired electrons), well-defined spectra, with sharp lines for each specific nearest-neighbor configuration, are observed; in such cases, expected J coupling patterns are also seen. High-resolution solid-state NMR studies of this type provide useful information on structure (disorder), doping, and electron-mediated coupling in semiconductor systems.     

Noble metals induced magnetic properties of graphene

The adsorption energies, stable configurations, electronic structures, and magnetic properties of the graphene with noble metal (NM=Pt, Ag, and Au) atom adsorption were investigated using first-principles density-functional theory. It is found that the bridge site is the most stable adsorption site for the Pt adatom; the Ag adatom can be stabilized almost equally at the bridge or the top site, while the Au adatom prefers to be adsorbed at top site. The Pt-graphene interaction is stronger than the interaction of Ag-graphene and Au-graphene, since the Pt atom has an unsaturated electronic d-shell (d⁹s¹). While there is no net magnetic moment for the Pt adatom, the Ag and Au adatoms still exhibit magnetic character on the graphene. The magnetic moments of the NM-graphene systems may be quenched (e.g., Pt-graphene), reduced (e.g., Ag-graphene) or not changed (e.g., Au-graphene) as compared with the values before adsorption. Therefore, the magnetic character of the adatom-graphene system can be turned by adsorbing different NM atoms on the graphene.     

Transfer of1H and13C dynamic nuclear polarization from immobilized nitroxide radicals to flowing liquids

In this study,¹H and¹³C dynamic nuclear polarization (DNP) was generated at a magnetic field strength of 0.33 T utilizing silica phase immobilized nitroxide (SPIN) samples. The polarization was subsequently transferred to flowing liquids and monitored at a magnetic field strength of 4.7 T. These solid/liquid intermolecular transfer (SLIT) experiments provide efficient polarization transfer without the necessity of the free radical system present in the monitoring fluid. Specifically, ultimate¹H SLIT DNP Overhauser enhancements of ?56 and ?110 have been observed for benzene and chloroform in the presence of SPIN system 2, respectively. The¹³C SLIT DNP enhancement for benzene is dominated by three-spin effects and poor leakage factors (f _c). However, a particularly favorable case is the chloroform/SPIN 2 system which exhibits a scalar dominated enhancement. For this case, positive enhancements 40–60 times the¹³C thermal Boltzmann magnetization at 4.7 T have been observed. The large scalar dominated¹³C DNP enhancement for this system represents one of the largest experimental enhancements reported to date. The¹³C DNP spectra for other samples which exhibit favorable scalar¹³C dominated enhancements (e.g., Freon 113) are also presented. Three different SPIN systems were also prepared and characterized in the present study.     

Analysis of EPR-time profiles of transient radicals with unresolved spectra

A method for analysing EPR-time profiles of transient radicals in solution with unresolved hyperfine structure is proposed. It is based on considering the magnetic field integral of the magnetization, i.e., the total EPR signal intensity, instead of single components of overlapping EPR transitions. For a radical system involving chemical kinetics, chemically induced electron polarization (CIDEP), and spin relaxation, an analytical solution is found for the evolution of the integral magnetization in the Laplace domain. The solution in the time domain is given for the case of negligible saturation, i.e., omega2(1)T1T2 < 1. The formulae presented are suitable to avoid equivocal multi-parameter fits when analysing the results of time-resolved continuous-wave EPR experiments for the observables, which characterize the chemical kinetics, CIDEP, and electron spin relaxation of radical systems.     

Active terahertz metamaterials

In this paper we present an overview of research in our group in terahertz (THz) metamaterials and their applications. We have developed a series of planar metamaterials operating at THz frequencies, all of which exhibit a strong resonant response. By incorporating natural materials, e.g., semiconductors, as the substrates or as critical regions of metamaterial elements, we are able to effectively control the metamaterial resonance by the application of external stimuli, e.g., photoexcitation and electrical bias. Such actively controllable metamaterials provide novel functionalities for solid-state device applications with unprecedented performance, such as THz spectroscopy, imaging, and many others.     

Multicomponent loschmidt echo and mixing in the quantum dynamics of systems with dense discrete spectra

A dynamic problem for a system coupled to a reservoir possessing a dense discrete spectrum of states has been analytically solved under two simplifying assumptions proposed by Zwanzig [15], according to which the reservoir spectrum is equidistant and all the system-reservoir coupling matrix elements are identical (i.e., independent of reservoir states). It is demonstrated that a multicomponent Loschmidt echo arises in each recurrence cycles, the number of components being equal to the cycle number. At a certain critical cycle number, the components of neighboring cycles exhibit mixing. As a result, the dynamics of the system transforms from a regular to stochastic-like dynamics, in which an arbitrarily small coarse graining (inherent in any real system) of the results of measurements or uncertainty in the initial conditions leads to (i) the loss of one-to-one correspondence between the discrete spectrum of eigenvalues and the state of the system and (ii) the loss of invariance with respect to the time reversal. Interrelation between the mixing of cycles with the entanglement of trajectories, on the one hand, and the overlap of resonances in classical systems with mixing, on the other hand, is discussed. The properties of the proposed model are consistent with a variety of the kinetic regimes of vibrational relaxation (from exponential decay to irregular, weakly damping oscillations) observed in various objects. Common features are average distances between neighboring levels on the order of 1–10 cm^?1 and recurrence cycles on a time scale of 10^–13–10^?11 s, which is studied using femtosecond spectroscopy techniques.     

Surface-enhanced raman scattering and nonlinear optics applied to electrochemistry

The most recently developed diagnostic technique in metal-electrolyte and metal-gas interfaces adapts spontaneous Raman scattering and nonlinear optical generation, techniques normally applied to bulk media, to surface science investigation. For certain metallic surfaces, an enormous increase exists in the Raman (as much as 10⁶ to 10⁸ times) and nonlinear optical signals resulting from submonolayer coverage of molecular adsorbates at the interface. Spontaneous Raman scattering and nonlinear optical scattering are well developed in both theory and practice for the analysis of molecular structure and concentration in bulk media. Instrumentation to generate and detect these inelastically scattered signals is readily available and is adequate for adaption to surface science. However, the mechanism (or mechanisms) giving rise to such a large enhancement at the interfaces is still being actively researched and remains controversial. Theoretical and experimental investigations related to the underlying physics of this enhancement and the application of such surface enhancement as a vibrational probe for adsorbates on the metal surface have been labeled “surface-enhanced Raman scattering” (SERS) and “surface-enhanced nonlinear optics”. Soon after the recognition that molecules adsorbed onto metal electrodes under certain conditions exhibit an anomalously large Raman scattering efficiency,^1–3 it became evident that such a phenomenon makes possible an in situ diagnostic probe for detailed and unique vibrational signatures of adsorbates in the ambient phase (electrolyte and atmospheric gas surroundings). Optical spectroscopy in the visible range has a much higher energy resolution (e.g., 0. I cm-I) than is presently available in electron energy loss spectroscopy (EELS), as well as the capability to measure much lower frequency modes (e.g., as low as 5 cm^?1) than is possible in infrared spectroscopy. Perhaps the most significant attribute of SERS and surface-enhanced nonlinear optical scattering is that the surrounding media in front of the interface (e.g., several meters of gas and several centimeters of liquid) do not introduce optical loss or overwhelmingly large signals. The recognition that SERS is capable of performing vibrational spectroscopy with this resolution, frequency range, and in such dense surroundings has therefore brought an explosion of activity to the field since 1977.     

Spin dynamics in strongly coupled spin-correlated radical pairs: Stochastic modulation of the exchange interaction and ST−1 mixing in different magnetic fields

We investigate the effect of stochastic modulation of the exchange interaction J_ex on singlet (S)-triplet (T) transitions in radical pairs. These transitions limit the lifetime of the photo-generated radical pairs in covalently linked porphyrin-quinone systems that have been developed for biomimetic modeling of photosynthetic electron transfer processes. In order to explain transient electron paramagnetic resonance (EPR) results in different magnetic fields, i.e., with X-band ((0.34 T)/ (9.5 GHz)) and W-band ((3.4 T)/(95 GHz)) time-resolved EPR, we have to assume that J_ex is modulated over a range of 20000 G, which is wide enough that S is temporarily almost degenerate with T₀ as well as with T_?1. This large modulation of J_ex is caused by restricted rotational diffusion of the quinone subunit with respect to the porphyrin subunit. However, because of the small interradical distance of about 1.0–1.4 nm, the radical pair is continuously kept in the strong coupling limit and, therefore, we observe only EPR transition between the triplet sublevels. We find an approximation to solve the stochastic Liouville equation valid for rotational diffusion on an intermediate time scale, i.e., the diffusion rate D_R is smaller than the singlet electron recombination rate K_S ～ 10⁹ s^?1, but larger than the ST transition rates κ₀, κ_?1 < 10⁶ s^?1.     

Magnetic hyperfine fields of vacancy-associated119Sn in ion-implanted nickel

Magnetic hyperfine fields of¹¹⁹Sn impurity defects in nickel have been investigated by Mössbauer emission spectroscopy. Radioactive¹¹⁹Xe isotopes were implanted, annealing was performed after¹¹⁹Xe had decayed to¹¹⁹Sb. At least five different components with well-defined magnetic hyperfine fields, isomer shifts and Debye temperatures are identified in the rather complex spectra. One of these (B=2T) is known to be due to substitutional Sn. The hyperfine fields of the other components are pronouncedly larger (B=9T, B=15T, and B=17T, respectively, for single crystals). These defects are proposed to be Sn-multivacancy defects.     

In-situ phase analysis by a versatile miniaturized Mössbauer spectrometer

The element iron plays a major role in modern industries and technologies as for instance car-industry, mineral processing and steel production and power plants. For quality control, process monitoring and device inspection (e.g., pipes in power plants) a fast, non-destructive and sensitive analytical method is desirable. ⁵⁷Fe Mössbauer spectroscopy is able to determine the different iron phases (e.g., oxides, sulfides, nitrates, carbonates and carbides) and therefore would be the ideal tool to perform this job. We have developed a miniaturized backscattering Mössbauer spectrometer for space applications which will be modified and used for industrial applications under certain circumstances. The instrument is designed in a modular way which would allow to adapt it to different applications. The instrument has approximately the size of a soft drink can, a weight of about 0.5 kg, and a power consumption of about 3 watts.     

Use of Infrared Spectroscopy for In-Field Measurement and Phenotyping of Plant Properties: Instrumentation,Data Analysis,and Examples

Abstract: Recent developments in agricultural technology have led to a demand for a new era of nondestructive methods of plant analysis in the field rather than in the laboratory. The combination of fundamental science (e.g., plant physiology, biochemistry, and other disciplines), multivariate data analysis, and spectroscopy will enable the development of technologies for reliable and rapid on-farm or in-field low-cost testing. This will enable both farmers and scientist to maximize sales in existing markets and to target new markets with differentiated products or select new varieties, better soil management, among other issues. Infrared (IR) spectroscopy has been successfully applied for the determination of composition analysis in several fields (e.g., agriculture, pharmaceutical, etc.) and in product quality assessment and production control. The IR spectrum can give a global signature of composition (fingerprint), which, with the application of chemometric techniques, can be used to elucidate particular compositional characteristics not easily detected by traditional targeted chemical analyses. This article highlights the principles of IR spectroscopy, commercially available instruments and software, and calibration issues including calibration development, networking, and transfer. In addition, recent and potential applications of IR spectroscopy in grains, fruits, and other plant tissues are presented and discussed.     

Human behavior in online social systems

We present and study data concerning human behavior in four online social systems: (i) an Internet community of friends of over 10⁷ people, (ii) a music community website with over 10⁶ users, (iii) a gamers’ community server with over 5 × 10⁶ users and (iv) a booklovers’ website with over 2.5 × 10⁵ users. The purpose of those systems is different; however, their properties are very similar. We have found that the distribution of human activity (e.g., the sum of books read or songs played) has the form of a power law. Moreover, the relationship between human activity and time has a power-law form, too. We present a simple interest-driven model of the evolution of such systems which explains the emergence of two scaling regimes.     

Evidence of anomalous multiplet line shapes in optically thick,laser-produced plasmas

We report observations of anomalous line shapes for the transitions 2p?3d (²P?²D) emitted by the Li-like ions N(V), O(VI), F(VII) in laser-produced plasmas. These transitions are normally doublets but show completely different characteristics (e.g., triplet structures or invension of two-component intensity ratios) in the plasmas. The observed line profiles are accounted for in terms of opacity and Doppler effect produced by plasma expansion. This interpretation is independent of the particular transition involved, i.e., multiplet structures can generate more complicated features with various unexpected new components, anomalous intensity ratios, etc.