The effect of quadrupole relaxation on nuclear magnetic resonance multiplets

A theory of the broadening of multiplet components in magnetic resonance spectra of coupled nuclei due to electric quadrupole spin-lattice relaxation is presented. It is shown that the broadening of the components of the 1:1:1 triplet for nuclei of spin 1/2 coupled to a nucleus of spin 1, is 3/2 as great for the outer lines as for the central one, provided that the rate of quadrupole relaxation is not too large. This is in agreement with experimental observations on ¹⁴NH₃. A theory of the line shape of the partly collapsed multiplet at higher rates of quadrupole relaxation is also given.     

Diffusion in nuclear quadrupole resonance

With the aid of the method of random trajectories, formulas have been obtained for the first time for diffusion damping of the amplitudes of nuclear quadrupole spin echo signals because of molecular vibration-rotation in a two-frequency four-pulse program for spin 5/2 and zero asymmetry parameter. The formulas are similar to the result of Hahn for diffusion in NMR.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 14–16, June, 1985.     

Oxygen isotope effect in high-T C superconductors: Cu nuclear quadrupole resonance relaxation rates

A scaling relationship has been observed between the⁶³Cu(2) spin-lattice relaxation rates 1/T ₁,(18) and 1/T ₁(16) observed in¹⁸O- and¹⁶O-exchanged YBa₂Cu₄O₈ and the normalized temperatureT/T _c(18 or 16). This relation implies thatT _C is decreased by the change in antiferromagnetic (AF) spin fluctuation rate. Thus oxygen isotope effect observed in high-T _C superconductors does not indicate any importance of electron-phonon interaction but it appears to originate from the change in AF fluctuations.     

Two-frequency effects in nuclear quadrupole resonance

A number of new effects which occur in the two-frequency excitation of a multilevel spin-system in nuclear quadrupole resonance are described. Three programs for observing the effect of trapping, used to detect some transitions in a multilevel system in terms of others, for an accurate determination of the asymmetry parameter of the gradient of the electric field, to identify lines of the multiplet spectrum of nuclear quadrupole resonance, and to investigate the phenomenon of cross-relaxation, are proposed. Quantitative estimates are given of the value of the trapping when two neighboring transitions are excited. The phenomenon of slow beats between signals of a two-frequency echo and the frequency of the transition ±3/2±5/2 for = 0 when an external magnetic field is applied is explained. The reason for the short spin-spin relaxation times in the secondary echoes is established. The experimental method and a block diagram of a two-frequency nuclear quadrupole resonance system are described.This paper was presented at an All-Union Symposium on Magnetic Relaxation in June 1971 in Leipzig.Translated from Izvestiya VUZ, Fizika, No. 3, pp. 82–88, March, 1973.In conclusion we wish to thank V. A. Shishkin, M. Z. Yusupov, and A. D. Gordeev, for their help and for useful discussions.     

N nuclear quadrupole resonance in the barbiturates

The ¹⁴N nuclear quadrupole resonance frequencies at 77 K are reported for several barbiturates. Differences between charge densities in the π and various σ orbitals at the nitrogen site are obtained. The charge distributions are correlated with various substituents, and are discussed with emphasis on the effects of phenyl substituents, hydrogen bonding, and formation of the sodium salt.     

Photon-echo detected nuclear quadrupole resonance spectroscopy

Rare-earth-ion-doped crystals (REICs) have played an important role in quantum information processing due to their excellent coherent properties. In order to obtain the information regarding the hyperfine structures of the rare-earth ions in REICs, optically detected nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) techniques based on RF resonance and various optical detection methods are widely employed in previous works. Here we demonstrate a new method of NQR spectroscopy based on the photon-echo detection. The hyperfine spectra of the ground state (⁷F₀) and the optically-excited state (⁵D₀) of ¹⁵¹Eu³⁺ in Y₂SiO₅ at zero field are obtained. This method can determine the hyperfine splittings within the ground state and the optically-excited state and is shown to be robust against electrical noise. Our results provide an alternative way for optical detection of NMR and NQR with high signal-to-noise ratio.     

Nonstationary effects in nitrogen multipulsed nuclear quadrupole resonance

Multiple echoes in the envelope of the nuclear quadrupole resonance (NQR) signal were obtained in a field of multipulse sequences in powdered nitrogen-containing materials at room temperature. Echo signals were observed over a wide range of pulse rotation angles. It is shown that an analogue of the magic NMR echo can be obtained in the field of multipulse sequence.     

14N nuclear quadrupole resonance in carcinostatic phosphamides

Nitrogen-14 nuclear quadrupole resonance spectra of the anti-cancer drugs cyclophosphamide monohydrate, isonphosphamide and triphosphamide have been detected at 77 K. The electron distribution in the vicinity of the nitrogens possessing trigonal bonding configurations have been calculated in the framework of the Townes and Dailey theory.     

Composite pulse trains for nuclear quadrupole resonance

In the present work, a multipulse train comprising {±^0°|2^90°-τ-±^180°|2^90°} and {±^0°|2^90°-τ-±^0°|2^270°} composite pulses is suggested for the observation of nuclear quadrupole resonance (NQR) signals from remote objects. It is shown that these pulse trains provide efficient separation of interference signals in the multipulse train, which allows conditions of NQR signal storage for detecting explosives to be improved. Kaliningrad State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 54–57, September, 1999.     

Anisotropic relaxation and motion of half-integer quadrupole nuclei studied by central transition nuclear magnetic resonance spectroscopy

An efficient theoretical formalism and advanced experimental methods are presented for studying the effects of anisotropic molecular motion and relaxation on solid-state central transition NMR spectra of half-integer quadrupole nuclei. The theoretical formalism is based on density operator algebra and involves the stochastic Liouville–von Neumann equation. In this approach the nuclear spin interactions are represented by the Hamiltonian while the motion is described by a discrete stochastic operator. The nuclear spin interactions fluctuate randomly in the presence of molecular motion. These fluctuations may stimulate the relaxation of the system and are represented by a discrete relaxation operator. This is derived from second-order perturbation theory and involves the spectral densities of the system. Although the relaxation operator is valid only for small time intervals it may be used recursively to obtain the density operator at any time. The spectral densities are allowed to be explicitly time dependent making the approach valid for all motional regimes. The formalism has been applied to simulate partially relaxed central transition ¹⁷O NMR spectra of representative model systems. The results have revealed that partially relaxed central transition lineshapes are defined not only by the nuclear spin interactions but also by anisotropic motion and relaxation. This has formed the basis for the development of central transition spin-echo and inversion-recovery NMR experiments for investigating molecular motion in solids. As an example we have acquired central transition spin-echo and inversion-recovery ¹⁷O NMR spectra of polycrystalline cristobalite (SiO₂) at temperatures both below and above the α–β phase transition. It is found that the oxygen atoms exhibit slow motion in α-cristobalite. This motion has no significant effects on the fully relaxed lineshapes but may be monitored by studying the partially relaxed spectra. The α–β phase transition is characterized by structural and motional changes involving a slight increase in the Si–O–Si bond angle and a substantial increase in the mobility of the oxygen atoms. The increase in the Si–O–Si angle is supported by the results of ¹⁷O and ²⁹Si NMR spectroscopy. The oxygen motion is shown to be orders of magnitude faster in β-cristobalite resulting in much faster relaxation and characteristic lineshapes. The measured oscillation frequencies are consistent with the rigid unit mode model. This shows that solid-state NMR and lattice dynamics simulations agree and may be used in combination to provide more detailed models of solid materials.     

Revision of spin echoes in pure nuclear quadrupole resonance

Goldman's spin-1/2 formalism has been used for describing the response of an I=3/2 spin system to a two-pulse sequence in a pure nuclear quadrupole resonance experiment. A detailed analysis of the polarization evolution and quadrupolar echo generation is carried out through the use of explicit expressions for secular homo- and heteronuclear dipolar interactions. In striking contrast with previous studies, it is predicted that Van Vleck's second moments governing a classical solid-echo or Hahn sequence differ from those obtained by equivalent means in magnetic resonance. In fact, it is shown that, although measured moments still complement each other, the combined use of standard sequences does not allow the separate determination of homo- and heteronuclear dipolar contributions to the linewidth, not even in an indirect manner. In this context, the importance and potential usefulness of a crossed coil probe are also briefly discussed.     

Emission of nuclear quadrupole resonance from polycrystalline hexamethylenetetramine

The angular dependence of the nuclear quadrupole resonance (NQR) signal intensity emitted from polycrystalline hexamethylenetetramine has been analytically investigated for all directions for non-contact detection of chemicals by nuclear quadrupole resonance. The field pattern of the NQR signal from a column sample was measured. The emitted patterns were the same as that from a united single magnetic dipole, which fitted well to the estimation based on quadrupole principle axis system. This result is helpful to design an antenna for NQR remote detection.     

Crossed-coil detection of two-photon excited nuclear quadrupole resonance

Applying a recently developed theoretical framework for determining two-photon excitation Hamiltonians using average Hamiltonian theory, we calculate the excitation produced by half-resonant irradiation of the pure quadrupole resonance of a spin-3/2 system. This formalism provides expressions for the single-quantum and double-quantum mutation frequencies as well as the Bloch-Siegert shift. The dependence of the excitation strength on RF field orientation and the appearance of the free-induction signal along an axis perpendicular to the excitation field provide an unmistakable signature of two-photon excitation. We demonstrate single- and double-quantum excitation in an axially symmetric system using 35Cl in a single crystal of potassium chlorate (omega(Q) = 28 MHz) with crossed-coil detection. A rotation plot verifies the orientation dependence of the two-photon excitation, and double-quantum coherences are observed directly with the application of a static external magnetic field.     

The two-frequency nuclear quadrupole resonance for explosives detection

The two-frequency nuclear quadrupole resonance (NQR) of¹⁴N nuclei is described for purposes of explosives detection. Two applications are known: two-frequency NQR for increasing the signal intensity, two-frequency NQR for improved reliability of explosives detection. The two-frequency experiments were carried out in hexahydro-1,3,5-trinitro-s-triazine C₃H₆N₆O₆ and sodium nitrite NaNO₂ as a substitute for octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocycine C₄H₈N₈O₈. The two-frequency sequences for NQR are proposed for increasing the amplitude of NQR signal and improvement of detection condition.     

Noise-resilient multi-frequency surface sensor for nuclear quadrupole resonance

A planar nuclear quadrupole resonance (NQR) sensor has been developed. The sensor is resilient to environmental noise and is capable of simultaneous independent multi-frequency operation. The device was constructed as an open multimodal birdcage structure, in which the higher modes, generally not used in magnetic resonance, are utilized for NQR detection. These modes have smooth distributions of the amplitudes of the corresponding radiofrequency magnetic fields everywhere along the sensor’s surface. The phases of the fields, on the other hand, are cyclically shifted across the sensor’s surface. Noise signals coming from distant sources, therefore, induce equal-magnitude cyclically phase-shifted currents in different parts of the sensor. When such cyclically phase-shifted currents arrive at the mode connection point, they destructively interfere with each other and are cancelled out. NQR signals of polycrystalline or disordered substances, however, are efficiently detected by these modes because they are insensitive to the phases of the excitation/detection. No blind spots exist along the sensor’s surface. The sensor can be used for simultaneous detection of one or more substances in locations with environmental noise.     

Remote detection of double nuclear quadrupole resonance spectra

Remote detection of the spectra of double nuclear quadrupole resonance of nitrogen-containing compounds is considered. A comparison is made of this technique with the cross-relaxation method.     

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.