Improving N nuclear quadrupole resonance detection of trinitrotoluene using off-resonance effects

The off-resonance dependence of the amplitudes of the six dominant ¹⁴N nuclear quadrupole resonance (NQR) lines in commercial polymorphic trinitrotoluene (TNT) sample were experimentally determined for a wide range of experimental parameters when irradiated with the spin-lock spin-echo (SLSE) pulse sequence. We find that the amplitudes off-resonance dependence follows a sinc-like function with an additional modulation due to the spacing between the RF pulses. This dependence can be very well modeled with expressions we have derived for a single site ¹⁴N NQR in paranitrotoluene (PNT). The results can be immediately used for the reduction of the number of free parameters used in the robust signal processing models for the TNT NQR detectors.     

The transient processes in multi-pulse nitrogen-14 NQR

It has been demonstrated that transient processes, observed in a single crystal of NaNO₂ acted upon by pulse sequences MW-2 and MW-4 and their modifications with 180° flip angle of the pulses (Solid State Nucl. Magn. Resonance 10 (1997) 63; Sov. Phys.—JETP 88(5) (1999) 1580), which manifest themselves in the oscillating form of the NQR signals envelope, can be explained in the frames of a two-particle model. It has been proved that the nature of echo signals in the effective field of multi-pulse sequences received by the inversion of the phase of the sequence pulses or by introducing an additional 180° pulse is connected with re-focusing of accumulated digressions of the flip angle from the ideal 180° pulse. Experimental results of observing single and multiple echoes in a number of powdered nitrogenated substances in the effective field of various sequences at room temperature have been presented.     

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.     

Nuclear quadrupole resonance echoes from hexamethylenetetramine

We investigated the echo phenomenon of nuclear quadrupole resonance (NQR) from hexamethylenetetramine (HMT). We detected the pure NQR echo signal of HMT with a short pulse interval. The intensity of the echo signal increased as the pulse interval time was decreased. We observed that a clean echo signal was generated even when the pulse interval was shorter than the decay time constant T₂^*. Since the short interval time gives a strong echo, our result insists that shorter interval time is preferred for the NQR detection.     

Polarization enhancement technique for nuclear quadrupole resonance detection

We demonstrate a dramatic increase in the signal-to-noise ratio (SNR) of a nuclear quadrupole resonance (NQR) signal by using a polarization enhancement technique. By first applying a static magnetic field to pre-polarize one spin subsystem of a material, and then allowing that net polarization to be transferred to the quadrupole subsystem, we increased the SNR of a sample of ammonium nitrate by one-order of magnitude.     

NQR frequencies in polycrystalline samples and their correlation with σ and k i values

NQR frequencies are studied in 6 compounds at 77°K and room temperature. The frequencies of the substituted chlorobenzene derivatives are compared with values calculated using the σ andk values of Hammett and Biedenkapp and Weiss respectively. The poor agreement between observed and calculated values is ascribed to the presence of two or more substituent groups other than chlorine and the consequent interaction between them. The poor agreement with Nagarajan’s equation at room temperature is ascribed to the increase in temperature. The σ andk values for the orthobenzamide group are estimated from the observed frequencies as 0.275 and 0.369 respectively.     

Spin diffusion and nuclear spin-lattice relaxation in irradiated solids: a multiple-pulse NQR study

We present a detailed theoretical and experimental NQR multiple-pulse spin-locking study of spin-lattice relaxation and spin diffusion processes in the presence of paramagnetic impurities in solids. The relaxation function of the nuclear spin system at the beginning of the relaxation process is given by exp , where T_1ρ is spin-lattice relaxation time in rotating frame and =d/6, d is the sample dimensionality. Then the relaxation proceeds asymptotically to an exponential function of time, which was attributed to the spin-diffusion regime. Using the experimental data obtained from the analysis of those two relaxation regimes in γ-irradiated powdered NaClO₃, spin diffusion coefficient has been determined and the radius of the diffusion barrier has been estimated.     

Nitrogen-14 nuclear quadrupole resonance (NQR): dramatic sensitivity enhancement by large and fast temperature lowering

We have observed that, when going rapidly from ambient temperature down to liquid nitrogen temperature, the nitrogen-14 NQR signal (for transitions involving the m=0 spin state, nitrogen-14 being a quadrupolar nucleus of spin I=1) is increased by a factor of ca. 10(2). While Boltzmann statistics cannot explain this enhancement, the strong temperature dependence of the quadrupolar interaction is very likely to be at the origin of this phenomenon. Indeed, the quadrupolar Hamiltonian becomes time dependent and is prone to induce transitions toward the spin state associated with m=0. Its binding and slow relaxing properties result in a durable increased population and consequently in an increased intensity of NQR lines originating from the state m=0.     

14N Pulsed nuclear quadrupole resonance. 4. Two-pulse sequences for the determination of T1 and T2 relaxation times

A general theory, based on density matrix calculations, has been developed for the special case of a two-pulse sequence applied to spin 1 (¹⁴N) nuclear quadrupole resonance (NQR) of a powder sample. It is shown that the homolog of the NMR inversion-recovery experiment leads easily to the spin-lattice relaxation time T ₁ (associated with the diagonal elements of the density matrix) provided that an appropriate phase cycling is used. Conversely, in spite of two-step phase cycling schemes adapted to spin-spin relaxation measurements, the homolog of the NMR Hahn spin-echo sequence may pose some problems if the results are displayed in the magnitude mode. First, at short decay times, the echo may be corrupted by unwanted signals. Secondly, in that case, the amplitude of the resulting signal can evolve unexpectedly and differently as a function of the phase of the second pulse. Thirdly, at long decay times, the echo maximum occurs earlier than expected. All these problems in principle disappear with a complete four-step phase cycling scheme and the echo decay curve yields reliably the spin-spin relaxation time T ₂ (associated with off-diagonal elements). This theory allowed the exploitation of many test experiments performed at different frequencies on hexamethylenetetramine (HMT) and sodium nitrite.     

Secondary signals in two-frequency nuclear quadrupole resonance on (14)N nuclei with I=1

Our experimental and theoretical studies show that using two-frequency excitation of (14)N nuclei it is possible to observe secondary NQR signals at one of the three possible transitions due to irradiation of another adjacent transition. As a result of the pulse sequence applied to the adjacent transition the spin-echo signals on the detected transition are observed after essential time interval from the initial single pulse on this frequency. Experiments have been performed on the (14)N nuclei in the sodium nitrite (NaNO(2)) and the military explosive hexahydro-1,3,5-trinitro-s-triazine C(3)H(6)N(6)O(6) (RDX).     

Low-limit photo-acoustic detection of solid RDX and TNT explosives with carbon dioxide laser

We report the absorption spectra of RDX and TNT explosive samples in solid form at room temperature using a carbon dioxide laser-based photo-acoustic technique. A continuous wave ¹²C¹⁶O₂ laser, tunable between the 9.25–10.74 μm wavelength region, was used as an energy source. Interference-free limits of detection of solid RDX and TNT in simulated samples are estimated to be 10.0 and 16.5 ppbw, respectively. Published in Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 1, pp. 113–118, January–February, 2006.     

Calorimetric and single crystal X-ray study of the phase transition of (PyH)2PdCl4

Polymorphic transition of pyridinium tetrachloropalladate(II) was investigated by heat capacity measurements and by single crystal X-ray structural analysis. A large λ-type anomaly was detected at 240 K in the temperature dependence of the heat capacity. The low-temperature phase (LTP) belongs to the triclinic space group with a=6.856(1), b=7.293(1), c=7.721(1) Å, α=75.180(2)°, β=71.081(2)°, γ=81.109(3)° at 100 K, and the high-temperature phase (HTP) to the same space group with a=7.217(2), b=7.470(2), c=7.880(2) Å, α=73.438(3)°, β=65.195(3)°, γ=82.727(4)° at 293 K. The pyridinium cations are ordered antiferroelectrically in LTP. In HTP, however, an orientational disorder of the cation was observed. The energy difference between potential wells for the reorientation of pyridinium ion in HTP is discussed referring to the results of the present single crystal X-ray and heat capacity as well as the previous ¹H NMR measurements. A five-site disorder model is shown to be consistent with both of the observations of ¹H NMR and X-ray study.     

Double resonance experiments in low magnetic field: Dynamic polarization of protons by N and measurement of low NQR frequencies

The possibilities of dynamically polarizing proton spin system via the quadrupole ¹⁴N spin system in low magnetic field are analyzed. The increase of the proton magnetization is calculated. The polarization rate of the proton spin system is related to the transition probabilities per unit time between the ¹⁴N quadrupole energy levels and proton energy levels. The experiments performed in 1,3,5-triazine confirm the results of the theoretical analysis. A new double resonance technique is proposed for the measurement of nuclear quadrupole resonance frequencies ν_Q of the order of 100 kHz and lower. The technique is based on magnetic field cycling between a high and a low static magnetic field and observation of the proton NMR signal in the high magnetic field. In the low magnetic field the quadrupole nuclei and protons resonantly interact at the proton Larmor frequency ν_H = ν_Q/2. The quadrupole nuclei are simultaneously excited by a resonant rf magnetic field oriented along the direction of the low static magnetic field. The experimental procedure is described and the sensitivity of the new technique is estimated. Some examples of the measurement of low ¹⁴N and ²H nuclear quadrupole resonance frequencies are presented.     

An N Soliton Resonance Solution for the KP Equation: Interaction with Change of Form and Velocity

A family of solutions of the KP hierarchy obtained by Sato is analyzed for the KP equation. It is found that these solutions describe processes of interaction of an arbitrary number of resonant solitons. The interacting structures correspond to the one observed by Miles within the context of diffraction of solitons. It is seen that in solutions consisting in interaction processes, the Miles structures share certain branches in such a way that they can be interpreted as a multiple diffraction. It is also observed that, under interaction, these resonant solitons change both their forms and their velocities.     

Comparison of gadolinium chelates with manganese-DPDP for liver lesion detection and characterization: Preliminary results

Nonspecific extracellular gadolinium chelate (NEGd) was prospectively compared with manganese (Mn)-DPDP (Mn) for the detection and characterization of focal liver lesions of various histology. Seventeen patients with known or suspected focal liver lesions underwent NEGd and Mn-enhanced studies at 1.5 T. Study findings were correlated with histology (five patients), computed tomography (CT) examinations (17 patients), and 4- to 13-month imaging follow-up by CT and/or MR (five patients). NEGd studies were performed as serial postcontrast spoiled gradient echo (SGE) sequences, and Mn studies were performed as SGE sequences 15 and 30 min postcontrast and T₁-weighted, fat-suppressed spin echo at 16 min. NEGd and Mn images were prospectively interpreted in a separate blinded fashion. Lesion detection and characterization were determined. NEGd and Mn-enhanced images demonstrated 61 and 49 lesions, respectively (p = .1, NS). A total of 60 and 33 lesions were characterized on NEGd and Mn images, respectively, which was significantly different (p = .008). No differences were observed for the detection and characterization of liver metastases; whereas there was a trend for superior detection and characterization for hepatocellular carcinoma with NEGA.     

Zero temperature phase transitions in spin-ladders: Phase diagram and dynamical studies of Cu2(C5H12N2)2Cl4>

In a magnetic field, spin-ladders undergo two zero-temperature phase transitions at the critical fields H_c1 and H_c2. An experimental review of static and dynamical properties of spin-ladders close to these critical points is presented. The scaling functions, universal to all quantum critical points in one-dimension, are extracted from (a) the thermodynamic quantities (magnetization) and (b) the dynamical functions (NMR relaxation). A simple mapping of strongly coupled spin ladders in a magnetic field on the exactly solvable XXZ model enables to make detailed fits and gives an overall understanding of a broad class of quantum magnets in their gapless phase (between H_c1 and H_c2). In this phase, the low temperature divergence of the NMR relaxation demonstrates its Luttinger liquid nature as well as the novel quantum critical regime at higher temperature. The general behavior close these quantum critical points can be tied to known models of quantum magnetism. Received: 13 March 1998 / Received in final form and Accepted: 21 July 1998     

One-pot synthesis of carbon dots co-doped with N and S: high quantum yield governed by molecular state and fluorescence detection of Ag+

Fluorescent carbon-based nanoparticles, called chronically as carbon dots (CDs), were synthesised from citric acid (CA) and 2-Aminothiophenol (2AT) via an N and S co-doped hydrothermal method. After a series of micro-structural characterisation, N and S elements could be sufficiently doped by means of the heteroatom in the CDs solution. The as-prepared CDs solution showed blue colour fluorescence with the highest QY of 78.6%, and study on the UV–visible and PL spectra further revealed that the outstanding fluorescence of as-prepared CDs mainly originates from the generated molecular fluorophores instead of the surface state. Owing to the strong fluorescence, the as-prepared CDs can be used as a sensing probe for the detection of Ag⁺ with high sensitivity and selectivity. However, the changes of fluorescence intensity exhibited the complex nature of the quenching mechanism due to the –SH and –NH₂ groups on the fringes of carbonaceous cores or molecular fluorophores to aggregate into another fluorescent cores with the assistance of Ag⁺ ions, which promises a new approach for efficient detection of Ag⁺ for the application in industrial pollutants.

This figure shows citric acid (CA) and 2-Aminothiophenol (2AT) via an N and S co-doped hydrothermal method to prepare CDs with blue colour fluorescence and the highest QY of 78.6%. Owing to the excellent fluorescence, the as-prepared CDs can be used as a sensing probe for the detection of Ag⁺ with high sensitivity and selectivity, and the changes of fluorescence intensity exhibited the complex nature of the quenching mechanism due to the –SH and –NH₂ groups on the fringes of carbonaceous cores or molecular fluorophores to aggregate into another fluorescent cores with the assistance of Ag⁺ ions, which promises a new approach for efficient detection of Ag⁺ for the application in industrial pollutants.     

Electric field gradients in PrBa2 Cu 3 O 7 : LSDA+U results and comparison with experiment

Electric-field gradient (EFG) and asymmetry parameter (η) at all oxygen and copper sites of nonsuperconducting PrBa₂Cu₃O₇ (Pr123) compound were calculated using the full-potential (linearized)-augmented-plane-wave plus local orbitals method. Exchange and correlation effects were treated by LSDA+U for Cu(3d) and Pr(4f) electrons. The effects of changing screened Coulomb parameters U_Pr, U_Cu1, and U_Cu2 on the results were individually studied. The calculated EFG of O2 site is close to the EFG of O3 site at variance with the experimental result. It was shown that by increasing superconducting holes in O2 and O3 sites the EFG at these sites increase and vice versa. The most famous theories which have been proposed to explain the suppression of superconductivity in perfect (without any mis-substitution or other defects) Pr123 compound are not consistent with the experimental EFG at O2 and O3 sites. By replacing one Pr atom at the Ba site on unit cell of Pr123 (Pr_Ba), it was shown that Pr_Ba mis-substitution reduces the superconducting holes in both O2 and O3 sites and could be responsible for the suppuration of superconductivity in Pr123 samples. It is very probable that the unusual behaviors of experimental EFG at O2 and O3 sites of Pr123 are related to oxygen defects which are produced with Pr_Ba mis-substitution.