Two-dimensional nutation exchange NQR spectroscopy: direct determination of rotational angles

A new method for the direct determination of rotational angles based on 2-dimensional nutation exchange NQR spectroscopy is proposed. The method involves the detection of exchange processes through NQR nutation spectra recorded after the mixing interval. The response is analysed of a system of spins I = 3/2 in zero applied field, experiencing electric quadrupole couplings, to the three-pulse sequence with incrementing pulse widths. The systems investigated here were hexachloroethane and tetrachloroethylene, which show threefold and twofold reorientational jumps about the carbon-carbon axis, respectively. It is shown that the 2D nutation exchange NQR spectrum exhibits characteristic ridges, which reveal the motional mechanism in a model-independent fashion. The angles through which the molecule rotates can be read directly from elliptical ridges in the 2D spectrum, which are also sensitive to the value of asymmetry parameter of the electric field gradient tensor.     

Relaxation effects in two-dimensional zero-field nutation NQR spectroscopy

The relaxation and off-resonance effects on the NQR (nuclear quadrupole resonance) nutation frequency and on the damping constant of the nutation spectrum for spinsI = 3/2 are considered. Using the solution of dynamic equations for nuclear magnetisation in the rotating frame, we obtained a general expression for determination of the asymmetry parameter η, which includes the relaxation time and frequency offset. Specific requirements for the RF (radio frequency) field allowing the presence of nutation and the sufficient resolution of nutation spectra are determined. It is shown that at low RF field it is necessary to take into account theT ₂ relaxation effect on the location of frequency singularities in the nutation spectrum. The values ofH ₁ andT ₂ must be exactly known in nutation NQR spectroscopy. The 2D NQR spin-echo separation technique was used for determination of transverse relaxation of individual spin packets in case of inhomogeneously broadened NQR lines, so that the asymmetry parameter could then be determined by the nutation experiment with consideration of relaxation effects and frequency offset. The theoretical results are confirmed experimentally for the narrow and inhomogeneously broadened 2D-nutation NQR spectra in several molecular compounds containing the³⁵Cl and⁷⁵As nuclei.     

Two-dimensional NQR spectroscopy of forbidden transitions

Two-dimensional NQR spectroscopy has been realized experimentally for a direct method of detecting and recording forbidden transitions. The cross peaks of the two-dimensional spectra were used to assign the lines in the complex spectra.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 73–75, August, 1991.     

A Al MAS, MQMAS and off-resonance nutation NMR study of aluminium containing silica-based sol-gel materials

Aluminium containing hybrid materials were prepared via the sol-gel method using aluminium sec-butoxide complexed with ethylacetoacetate (Al(OBu^s)₂EAA or Al(OBu^s)₃/EAA mixtures). As silanes, phenyltrimethoxysilane (PhTMS) or phenyltriethoxysilane (PhTES), 3-glycidoxypropyl trimethoxysilane (Glymo) and tetraethylorthosilicate (TEOS) were used. After room temperature drying of the samples the ²⁷Al single pulse excitation (SPE) magic angle spinning (MAS) NMR shows that octahedral (5 ppm) and tetrahedral (55 ppm) coordinated aluminium species are present in the materials. The relative amount of these two species depends on the preparation method. However, the Al(IV)/Al(VI) ratio is lower than 3 (typically 2.3) in all materials, indicating the presence of a small amount of an aluminate phase. Annealing of the samples at 100, 150 and 200 °C results in the formation of an extra signal at 30 ppm (peak maximum measured at 11.7 T). Based on the resonance frequency this signal is generally assigned to a pentahedrally coordinated aluminium species. Hydration/dehydration processes of annealed samples were studied with ²⁷Al SPE MAS NMR, multiple-quantum MAS NMR (MQMAS) and off-resonance nutation NMR. Upon hydration of the annealed sample the signal intensity around 30 ppm decreases in intensity and at the same time the intensity of the signal around 55 ppm increases by the same amount (tetrahedrally coordinated aluminium). The MQMAS spectra reveal that the signal around 30 ppm is not caused by a fivefold-coordinated aluminium species but mainly by tetrahedrally coordinated aluminium species in a distorted environment, experiencing large quadrupole induced shifts and small chemical shifts due to conformational changes in the polymeric network. From the MQMAS NMR spectra it can be concluded that the linebroadening observed in the ²⁷A1 MAS NMR spectra is due to both a distribution in isotropic chemical shifts and a distribution in quadrupole coupling constants (C_qcc = e²qQ/h). Hydration of the sample results in a decrease of the average C_qcc for the tetrahedrally coordinated aluminium from 6 to 4 MHz, whereas the average C_qcc of the octahedrally coordinated aluminium is hardly influenced (4 MHz). These MQMAS results are confirmed by off-resonance nutation experiments.     

A comparison between MEM and FFT for the spectral analysis of geophysical signals

Summary The results obtained by the maximum entropy method (MEM) for the spectral analysis of real signals (the magnetic-field observations performed by ISEE-2 during periods of solar-wind turbulence upstream of the Earth's bow shock) are compared with those obtained by the traditional fast Fourier transform (FFT) technique. The major relative importance of the dominant wave form suddenly emerges in the MEM analysis and the total frequency excursion of peaks is often smaller than the frequency resolution allowed by the FFT. The expected occurrence of spurious peaks might be less dramatic then in other cases; however, the definite choice of the best power spectrum remains often ambiguous. This research has been supported by the Ministero per la Pubblica Istruzione (40%) and by the Gruppo Italiano di Fisica Cosmica.     

Study of As-Sb-Se chalcogenide glasses by NQR and EPR spectroscopy methods

In this paper experimental results obtained by both ⁷⁵As NQR and EPR spectroscopy are presented for the three-component system As-Sb-Se. The ⁷⁵As NQR spectra of glasses of structures (As₂Se₃)_0.78 (Sb₂Se₃)_0.22, (As₂Se₃)_0.75 (Sb₂Se₃)_0.25, (As₂Se₃)_0.5 (Sb₂Se₃)_0.5 have broad lines with two Sb-NQR lines (corresponding to the Sb₂Se₃ units) and two ⁷⁵As-NQR lines (corresponding to the As₂Se₃ units). Differences in the EPR spectra of the different glasses arise because of the different amounts of arsenic and antimony in their structure.     

Caesium and hydrated lead chlorates studied by35Cl NQR spectroscopy

Molecular dynamics of caesium and lead chlorates (hydrated form) has been studied by³⁵Cl nuclear quadrupole resonance (NQR) spectroscopy. The occurrence of only one resonance line for the two compounds in the whole temperature range studied testifies to the crystallographically equivalent positions of their molecules in elementary cells. The relaxation processes in the two chlorates have been found to be analogous to those in the other chlorates not undergoing phase transitions. Analysis of the temperature dependence of the spin-lattice relaxation time has indicated that at low temperatures the dominant relaxation mechanism consists of torsion vibrations (ClO₃⁻ anion libration), whereas at 260 K the hindered rotation of the chlorate ion about the threefold axis sets on and its activation energy is 50.0 kJ/mol. The temperature dependence of the resonance frequency for the two chlorates studied is best described by the Brown model extended over the low-temperature range, which indicates large anharmonicity of the vibrations.     

Phase transition in MEM(TCNQ)2 single crystals by infrared reflection spectroscopy

Between 280 and 320 K the MEM⁺ ions in the salt MEM(TCNQ)₂ show an order-disorder phase transition. Moreover MEM(TCNQ)₂ undergoes the phase transition at 335 K. The influence of these two transitions on the i.r. reflectivity spectrum is studied.     

DTA and NQR studies of polymorphism in p-chlorofluorobenzene

Differential thermal analysis, and the ³⁵Cl Nuclear Quadrupole Resonance frequency (ν_Q) and spin-lattice relaxation time (T₁) were measured as a function of temperature in p-chlorofluorobenzene. Three different phases were found according to the thermal history of the sample and only one was stable up to the melting point. In one of the phases, the presence of molecular reorientations can be inferred from T₁ data with activation energy of 3.7 kcal/mol. A value of 13.9 kJ/mol for the fusion heat of this compound has been obtained.     

Thermoactivated molecular motions in solids and determination of the energy of their activation by NQR spectroscopy methods

The activation energy of thermoactivated molecular motions in solids is determined by examining the influence of these motions on the temperature dependence of the nuclear quadrupole spin-lattice relaxation rate and quadrupole resonance (NQR) signal intensity for chlorine-35. In the latter case, the heating of the crystals is accompanied by the fading of resonance signals, which is analyzed using a linear correlation between the activation energy of this motion and the fade-out temperature. The correlation parameters are demonstrated to be dependant on the type of molecular motion. The relaxation method is shown to be more effective in studying molecular motion and evaluating its activation energy as compared to the NQR signal fading technique.     

Carbon-Based Materials: Growth,Properties, MEMS/NEMS Technologies,and MEM/NEM Switches

Micro- or nanoelectromechanical system (MEMS/NEMS) is a multidisciplinary field, which has witnessed explosive growth during the past decades. The current materials for MEMS are dominated by Si thanks to the fully based technology for CMOS. However, Si has its own intrinsic limitations such as poor mechanical or tribological properties and poor thermal stability. Carbon-based materials such as diamond, carbon nanotube, and graphene possess excellent properties such as low mass, high Young's modulus, high thermal conductivity, hydrophobic surface, and tailorable electronic configuration, which make these materials promising for MEMS/NEMS applications with diverse and much better performance than Si. In this review, we describe the recent progress of carbon-based materials for MEMS/NEMS with focus on diamond, carbon nanotube, and graphene. The growth of these carbon materials is briefly described and only selected properties are discussed with respect to MEMS/NEMS applications. The fabrication process for suspended structures of these materials toward MEMS/NEMS is comprehensively reviewed from the recent literatures. Special attention is devoted to the state-of-the-art of MEM/NEM switches based on these carbon materials, one of the most important fields in MEMS/NEMS. Finally, the application of carbon-based MEM/NEM switches to microwave field is critically analyzed.     

Two-dimensional chemical-shift tensor correlation spectroscopy. Analysis of sensitivity and optimal measurement directions

Chemical-shift tensors can be determined from a single crystal placed in six or more orientations in a magnetic field. The sensitivity of this determination as a function of the selected crystal orientations is considered with a statistical figure of merit. A variety of configurations is examined, and it is found that the set of six orientations based on the vertices of an icosahedron optimizes the figure of merit and results in the most sensitive determination of the tensor. The relationship of these optimal orientations to those used previously in two-dimensional chemical-shift tensor correlation spectroscopy is discussed. It is shown that the high symmetry of the icosahedron simplifies the design on which the construction of a new sample orienting mechanism may be based.     

NMR and NQR studies on superconducting Sr2RuO4

We review our nuclear-magnetic resonance (NMR) and nuclear-quadrupole-resonance (NQR) studies in superconducting Sr₂RuO₄, which have been performed in order to investigate the gap structure and the pairing symmetry in the superconducting state and magnetic fluctuations in the normal state. The spin-lattice relaxation rate (1/T₁) of a high-quality sample with shows a sharp decrease without a coherence peak just below T_c, followed by a T³ behavior down to 0.15 K. This result indicates that the superconducting gap in pure Sr₂RuO₄ is a highly anisotropic character with a line-node gap. The Knight shift, which is related to the spin susceptibility, is unchanged in the superconducting state irrespective of the direction of the applied fields and various magnitude of the field. This result strongly suggests that the superconducting pairs are in the spin-triplet state, and the spin direction of the triplet pairs is considered to be changed by small fields of several hundred Oe.     

Two-dimensional hyperfine sublevel correlation spectroscopy: powder features for S=1/2, I=1

The lineshapes of two-dimensional magnetic resonance spectra of disordered or partially ordered solids are dominated by ridges of singularities in the frequency plane. The positions of these ridges are described by a branch of mathematics known as catastrophe theory concerning the mapping of one 2D surface onto another. We systematically consider the characteristics of HYSCORE spectra for paramagnetic centers having electron spin S=1/2 and nuclear spin I=1 in terms of singularities using an exact solution of the nuclear spin Hamiltonian. The lineshape characteristics are considered for several general cases: zero nuclear quadrupole coupling; isotropic hyperfine but arbitrary nuclear quadrupole couplings; coincident principal axes for the nuclear hyperfine and quadrupole tensors; and the general case of arbitrary nuclear quadrupole and hyperfine tensors. The patterns of singularities in the HYSCORE spectra are described for each case.     

2-D electron spin transient nutation spectroscopy of Lanthanoid ion Eu2+(8S7/2) in a CaF2 single crystal on the basis of FT-pulsed electron spin resonance spectroscopy: Transition moment spectroscopy

This work demonstrates the usefulness of pulsed electron spin resonance (ESR)-based two-dimensional electron spin transient nutation (2-D ESN) spectroscopy for complete assignments of complicated fine-structure hyperfine ESR spectra including hyperfine forbidden transitions from electronic and nuclear high-spin systems. The 2-D ESN spectroscopy is termed transition moment spectroscopy as spectra are acquired as a function of transition moment instead of transition energy used in conventional spectroscopy. We have applied the novel spectroscopic technique to Eu²⁺ ion (S=7/2,I=5/2), which has two isotopes (¹⁵¹Eu [47.9%] and¹⁵³Eu [52.1%]), in a CaF₂ single crystal as a model system. We have completely identified the complicated fine-structure hyperfine ESR spectra by invoking the spectral simulation of the 2-D ESN spectra on the basis of transition moment analyses. The analyses are based on exact numerical calculations of the transition moments as well as a perturbation-based analytical approach combined with reduced rotation matrices for the nuclear part of the transition moment. This is the first example of the spectral simulation for 2-D ESN spectra including the hyperfine allowed and forbidden transitions in high-spin systems. In addition, we have made simulation of the fine-structure forbidden transitions, which reproduces the angular variations of the observed spectra at liquid helium temperatures.     

Solid-state NMR/NQR and first-principles study of two niobium halide cluster compounds

Two hexanuclear niobium halide cluster compounds with a [Nb₆X₁₂]²⁺ (X=Cl, Br) diamagnetic cluster core, have been studied by a combination of experimental solid-state NMR/NQR techniques and PAW/GIPAW calculations. For niobium sites the NMR parameters were determined by using variable B_o field static broadband NMR measurements and additional NQR measurements. It was found that they possess large positive chemical shifts, contrary to majority of niobium compounds studied so far by solid-state NMR, but in accordance with chemical shifts of ⁹⁵Mo nuclei in structurally related compounds containing [Mo₆Br₈]⁴⁺ cluster cores. Experimentally determined δ_iso(⁹³Nb) values are in the range from 2400 to 3000 ppm. A detailed analysis of geometrical relations between computed electric field gradient (EFG) and chemical shift (CS) tensors with respect to structural features of cluster units was carried out. These tensors on niobium sites are almost axially symmetric with parallel orientation of the largest EFG and the smallest CS principal axes (V_zz and δ₃₃) coinciding with the molecular four-fold axis of the [Nb₆X₁₂]²⁺ unit. Bridging halogen sites are characterized by large asymmetry of EFG and CS tensors, the largest EFG principal axis (V_zz) is perpendicular to the X-Nb bonds, while intermediate EFG principal axis (V_yy) and the largest CS principal axis (δ₁₁) are oriented in the radial direction with respect to the center of the cluster unit. For more symmetrical bromide compound the PAW predictions for EFG parameters are in better correspondence with the NMR/NQR measurements than in the less symmetrical chlorine compound. Theoretically predicted NMR parameters of bridging halogen sites were checked by ^79/81Br NQR and ³⁵Cl solid-state NMR measurements.     

Studies of molecular dynamics of thiazides by<Superscript>35</Superscript>Cl NQR spectroscopy and DFT calculation

Molecular dynamics of three derivatives of 1,2,4-benzothiadiazine-1,1-dioxide, hydrochlorothiazide (HCTZ), althiazide (ATZ) and chlorothiazide (CTZ), was studied by³⁵Cl nuclear quadrupole resonance (NQR) spectroscopy. The temperature dependence of the resonance frequency was analyzed within the 6 known standard models. The activation energies estimated from the temperature dependence of the³⁵Cl NQR frequency assuming the Bayer model were 1.07, 2.35 and 2.76 kJ/mol for HCTZ, ATZ and CTZ respectively, which confirms that HCTZ is less rigid than CTZ and ATZ is much more rigid than HCTZ, and suggests that the mechanism of relaxation is based on small amplitude librations. The characteristic temperatures estimated from the Bayer model, with that for CTZ (332.5 K) being much higher than for HCTZ (132.1 K), mean that the intermolecular interactions in CTZ are much stronger than in HCTZ, as suggested by the melting point of CTZ being higher than that for HCTZ. For ATZ the characteristic temperature (288 K) takes an intermediate value, which suggests that the intermolecular interactions in this compound are stronger than in HCTZ and weaker than in CTZ. A significant narrowing of the resonance³⁵Cl NQR line observed for all these compounds at room temperature, relative to that at the liquid nitrogen temperature, suggests an averaging of dipolar interactions as a result of fast rotation of nonquadrupole nuclei in the vicinity of the quadrupole nuclei, when 2πν_Qτ_c ≫ 1 (a rotation of the −NH₂ group in the direct neighborhood of the chlorine nuclei) or a change in the gradient orientation with its value preserved (which is equivalent to rotation of the quadrupole nucleus Cl). The influence of the rotations of the −NH₂ and −CH₂SCH₂CH=CH₂ groups (ATZ) or −CHCl₂ group (TCTZ) on the³⁵Cl NQR frequency was modelled by the B3LYP/6-31G^* method. The frequencies of the libration vibrations calculated from the temperature dependence of the NQR resonance frequency were compared with experimental ones and those implied by the density functional theory, infrared and Raman spectra. For HCTZ the anomalies in the temperature dependence of the³⁵Cl NQR frequency, the lack of hysteresis and small but notable changes in the slope and the jump in the frequency observed at 253 K which does not exceed 0.05 MHz suggest a second-order phase transition at 253 K.     

基于激光吸收光谱技术的燃烧场气体温度和浓度二维分布重建研究

基于可调谐半导体激光吸收光谱技术和代数迭代算法（ART）实现燃烧场温度和浓度二维分布重建．采用时分复用技术,在1kHz扫描频率下分别扫描H2O的两条吸收谱线,7205．25和7416．05cm^-1,对温度分布在300-1100K范围内的气体温度场进行了重建．研究了投影角度和投影光线数目对温度场和浓度场重建结果的影响,并将温度场重建结果与热电偶测量结果进行比较,结果表明,采用四个投影方向时,温度场重建结果与热电偶测量结果除中心低温区域外基本符合．当光线数目减少时,通过在两条光线间增加虚拟光线,代入到迭代算法中,增加光线数目,提高了温度场和浓度场的重建效果．但此方法受到燃烧场温度梯度大小的影响,即在两条光线之间气体温度梯度较大,增加虚拟光线提高温度场重建效果不明显．     

Experimental study of the structure of chalcogenide glassy semiconductors in three-component systems of Ge-As-Se and As-Sb-Se by means of NQR and EPR spectroscopy

The structure of chalcogenide glassy semiconductors in three-component systems of Ge-As-Se and As-Sb-Se has been studied by means of both NQR (nuclear quadrupole resonance) and EPR (electron paramagnetic resonance) spectroscopy. It is investigated that in the glasses of both systems the value of the electric field gradient at the resonating nuclei grows with increasing concentration of the clusters As₂Se₃ and Sb₂Se₃, thereby increasing the NQR resonance frequencies. It appears that for the Ge-As-Se system the structural transition from a two-dimensional to three-dimensional structure occurs at average coordination number $\bar r$ = 2.45. The EPR spectral parameters of glasses depend on the composition, the average coordination number and the temperature, and these are discussed. The effect of ”ageing” for CGS (chalcogenide glassy semiconductors) of As-Sb-Se system due to partial crystallization of the sample is observed from the EPR spectra.