On the bubble lattice generation in nonuniform pulse fields

Conclusion The dynamic equilibrium bubble domain lattices created by nonuniform magnetic field pulses were investigated in the region of pulse amplitudes where the domain structure behaved in the most regular way. The lattice formation was found possible in a rather narrow range of the bias field values (withinH _e<H<H _col) and in the area of the sample betweens ₂- ands ₁-boundaries. Detailed parameters on these boundaries (position, field and field gradient) were measured and calculated as functions of the bias field and the pulse field amplitude. The boundarys ₂ was understood as being the position in the sample where the total stray + applied field during the field pulses equalsH _e. A next magnetostatic calculation of the domain structure in the nonuniform field should give an appropriate theoretical formula fors ₁. High-speed photography has been used to record the immediate patterns of the domain structure both during a single field pulse and between the pulses. The obtained pictures made it possible to calculate the magnetic stray fields and stray field gradients when a simplified dipole model of the observed pattern was used. The attraction towards the wire has been compared with the repulsion due to the stray fields and their difference in the case of the equilibrium bubble lattice has been compared with the material coercivity term. The coercivity of the material was found to have very small influence on the translation of the domains during the process of the formation and spreading of the bubble lattice, which was attributed to the repetitive pulse component of the applied field.Dedicated to Jan Kaczér DrSc on the occasion of his 65-th birthday.The authors wish to thank L. Murtinová, P. Novotný and G. Vertésy for their helpful discussions, G. Vertésy for the ion implanted samples and L. Bodis and G. Battistig for taking part at the preliminary experiments.     

T1 and T2 effects during radio-frequency pulses in spoiled gradient echo sequences

Finite pulse durations in diverse pulse schemes lead to the reduction of the magnitude of the magnetization vector due to T₁ and T₂ effects during the radio-frequency pulses. This paper presents an analysis of the steady state signal in the presence of relaxation effects during radio-frequency pulses in MRI spoiled gradient echo sequences. It is shown that minor attenuations of the magnetization vector can have dramatic consequences on the measured signal, and may thus entail a loss in SNR benefits at high static magnetic fields if a careful analysis is not performed. It is emphasized that it is the time-integrated magnetization vector trajectory that matters for these effects and not only the pulse duration. Some experimental results obtained on a phantom at 3 T verify this analysis.     

FT-EPR with a Nonresonant Probe: Use of a Truncated Coaxial Line

A truncated transmission line probe (TLP) has been utilized to excite and detect time domain responses after pulsed excitation in electron paramagnetic resonance (EPR) spectroscopic experiments in the frequency range 200–400 MHz. The TLP device is a modified short-circuited coaxial line, which allows the irradiation of the sample by the traveling waveB₁fields in the frequency range of kilohertz to 30 GHz. In EPR studies at 300 MHz carrier frequency, with 10 W incident power, a 45° pulse is 45 ns in duration. This corresponds to a 0.9-GB₁field. Using the TLP, time-domain responses from the solidN-methyl pyridiniumtetra-cyanoquinodimethane (TCNQ) were collected at 200, 250, 300, and 350 MHz, with the range limited by the amplifiers. In addition two tubes containing TCNQ placed side-by-side vertically along the axis of the probe were used to collect time domain responses in the presence of magnetic field gradients to test the feasibility of two-dimensional imaging using a TLP. The magnetic field gradient was steered in thexzplane and 36 projections were collected at 5° intervals. Using filtered back-projection image reconstruction, the two-dimensional spatial image in thexzplane was obtained at good resolution.     

Single and Multiple-Selective Excitation Combined with Pulsed Field Gradients

Applications of selective, multiselective, and semiselective pulses with pulsed field gradients are described. The use of multiple-selective excitation and PFGs for coherence selection in the selective one-dimensional experiments results in spectra devoid of artifacts and with remarkable solvent suppression. Multiple-selective excitation is also employed in an experiment called Multigate, a variant of the well-known WATERGATE experiment, in order to achieve multiple solvent signal suppression. Finally, new pulse sequences are shown for recording pure absorption ω₁semiselective PFG NOESY, ROESY, and TOCSY experiments. The merits and limitations of these experiments are discussed.     

Radiofrequency pulse sequences which compensate their own imperfections. 1980

Radiofrequency pulse sequences are described which have the same overall effect as a single 90° or 180° pulse but which compensate the undesirable effects of resonance offset and spatial inhomogeneity of the radiofrequency field H₁. These “composite” pulses are built up from a small number of conventional pulses which rotate the nuclear magnetization vectors about different axes in the rotating frame, while in the intervals between pulses a limited amount of free precession may be allowed to occur. Insight into the way in which pulse imperfections are compensated is obtained by computer simulation of trajectories of families of nuclear spin “isochromats” representing a distribution of H₁ intensity or resonance offset. Composite 90° pulses are suggested as a method of reducing systematic errors in spin-lattice relaxation times derived from progressive saturation or saturation-recovery experiments, and as the preparation pulse of a spin-locking experiment. A test of the effectiveness of the composite 180° pulse sequence has been made by using it for population inversion in a spin-lattice relaxation measurement, where T₁ is derived from the null point in the recovery curve, a technique known to be very sensitive to pulse imperfections.     

Far-infrared pump-probe measurement of the lifetime of the 2p?1 shallow donor level in n-GaAs

The low temperature lifetime of electrons excited in the 2p^–1 donor level of n-GaAs has been studied in a far-infrared pump-probe experiment. The measurement has been carried out using a pulsed far-infrared molecular gas laser working at a wavelength of 292µm, with the sample in a magnetic field of 5.1 T, resonant with the 1s_o–2p^–1 transition. Two FIR pulses are sliced from one FIR-laser pulse by means of optical switching techniques using two Q-switched Nd:YAG lasers. The first pulse is used to saturate the transition, while the second pulse probes the return of the population in the excited state towards thermal equilibrium as a function of the time delay after the excitation pulse. The value of 350±50 ns found for the lifetime falls in line with CW saturation results on materials with other doping concentrations.     

Localized cross-polarization in solids

Single quantum heteronuclear cross-polarization in solids is strongly sensitive to resonance offsets. In the presence of main field- or radio-frequency field gradients, the cross-polarization efficiency, therefore, shows a strong spatial dependence, which represents a new principle for localized NMR in solids. Since slices-selective excitation is achieved simultaneously to cross-polarization, the suggested pulse sequences avoid the use of shaped pulses, the application of which is problematic with solid. The dependence of the localization efficiency on experimental and sample parameters is analyzed theoretically for a spin-1/2 system in the presence of a static or a radio-frequency magnetic field gradient. The resulting slice profiles and the calculated dependence of the slice thickness on the parameters of the basic cross-polarization procedures are discussed and confirmed experimentally on the example of¹H-³C spin systems.     

Simulations of NMR experiments employing static and/or radio-frequency field gradients: application to slice selection by the inhomogeneous radio-frequency field of a standard coil

A general computer program has been developed in order to simulate any nuclear magnetic resonance (NMR) experiment. It includes, in addition to the action of radio-frequency (rf) pulses and of gradients of both the static and the rf magnetic fields, the effects of inhomogeneity of the latter. It has been used here for devising a slice selection procedure on the basis of the inhomogeneity of the rf field delivered by a standard coil (e.g., a saddle-shaped coil). Two sequences have been investigated, a DANTE-like pulse train and a very simple one (named S²P for slice selection with 2π pulses), (2π - τ) _n , where 2π corresponds to the flip angle of the region to be selected, whereas τ has to be chosen according to the relaxation time values;n, the number of cycles, must be sufficiently large, its actual value being uncritical. Simulations show that performances (in terms of selectivity) of both sequences are comparable, while experimental verifications favor S²P for its robustness and for the absence of any signal loss.     

Magnetization-Grid Rotating-Frame Imaging Technique for Diffusion and Flow Measurements

A method for NMR imaging of magnetization patterns generated by a preparation radiofrequency pulse is reported. The technique is suitable for the simultaneous spatially and spectroscopically resolved acquisition of diffusion, flow, and spin-lattice relaxation data. The procedure is based on gradients of the RF amplitude B₁. A first preparation RF pulse produces a z-magnetization grid. After a certain evolution interval, the grid is imaged by a rotating-frame imaging technique using the same RF coil. Neither rotary nor Hahn echoes are intrinsic to the method. Transverse relaxation in the free-evolution intervals is irrelevant. High-power transmitters in combination with suitable probeheads normally produce RF pulses which are short relative to transverse relaxation in the presence of RF, so that spin-lattice relaxation is the only time-limiting factor. Gradients of the main magnetic field induced by variations of the magnetic susceptibility are uncritical. The proposed "real-space detection" method is compared with stimulated or rotary-echo "wave number encoding" procedures for diffusion experiments. It is shown that the imaging procedure presented not only makes visible the spatial (apart from the spectral) distribution of transport properties which otherwise are concealed in the wave-number encoded signal, but also renders the measuring procedure insensitive to inhomogeneities of the B₁ gradient, which needs neither to be constant nor to be uniformly oriented. Extremely inhomogeneous B₁ gradient distributions should even make single-scan diffusion experiments feasible. The magnetization-grid rotating-frame imaging procedure can be employed for the two-dimensional measurement and representation of the probability P(z₁, 0|z₂, t) that a particle is at a position z₁ at a time 0 and at a position z₂ at a time t.     

Solvent Suppression in High-Resolution 1H NMR Spectroscopy Using Conventional and Phase Alternated Continuous Wave Free Precession

Solvent suppression is frequently mandatory in ¹H high-resolution nuclear magnetic resonance (NMR), especially for those experiments designed for non-deuterated solvent, normally used in protein and in vivo analysis, and also in liquid chromatography-NMR. Here, simple pulse sequences, which are based on continuous wave free precession (CWFP), consisting of a train of pulses separated by a time interval $ T_{\text{p}} \ll T_{2}^{*} $ , is applied to suppress one or more solvent signals in ¹H high-resolution NMR experiments, because of its dependency on the offset frequency. The conventional CWFP pulse sequence, that uses pulses with the same phase and duration, introduces some phase anomaly in the Fourier-transformed spectrum. This problem is minimized when the pulses are applied with phase alternation by π/2 in relation to the preceding pulse. Some problems with signal intensity can also be minimized using a shorter pulse width. Both CWFP and phase alternated CWFP can be easily used to suppress two solvent signals simultaneously, just using the correct T _p value, that must be equal to the inverse of frequency difference (?ν) between both signals to be suppressed. After modifications, we could introduce the CWFP train into 2D routine pulse sequences.     

Control of susceptibility-related image contrast by spin-lock techniques

Macroscopic magnetic field inhomogeneities might lead to image distortions, while microscopic field inhomogeneities, due to susceptibility changes in tissues, cause spin dephasing and decreasing T₂ relaxation time. The latter effects are especially observed in the trabecular bone and in regions adjacent to air-containing cavities when gradient-echo sequences are applied. In conventional MRI, these susceptibility-related signal voids can be avoided by applying spin-echo (SE) techniques. In this study, an alternative method for the examination and control of susceptibility-related effects by spin-lock (SL) radiofrequency pulses is presented: SL pulses were applied in two different susceptibility-sensitive sequence types: (a) between the jump and return 90° pulses in a 90°_x−τ−90°_−x magnetization-prepared Fast Low Angle Shot (FLASH) sequence and (b) between the 90° pulse and the 180° pulse in an asymmetric SE sequence. The range of Larmor frequencies used for spin locking can be determined for different B₁ amplitudes of the SL pulses, allowing control of image contrast by the amplitude of the SL pulses.     

Weak-pulse transparency enhancement in an optically dense three-level medium induced by a 2π pulse in a neighboring transition (V-scheme)

The coherent V-configuration interaction between an optically dense resonantly-absorbing three-level medium (neon) and two ultrashort superradiance pulses with converging wave fronts is investigated experimentally and theoretically. Both separate and combined propagation of pulses with wavelengths λ ₁=614.3 nm (strong field, θ ₁⩾π) and λ ₃=594.5 nm (weak field, θ ₃≈/20) are studied. For propagation of a separate strong-field pulse, supertransparency of the absorbing medium was observed, which is associated with the generation of a soliton-like pulse at the difference frequency (Δν≈1700 MHz) and the dispersion-diffraction stabilization effect. Under these conditions a weak-field pulse is completely absorbed. Combined propagation of the pulses leads to novel effects. A below-threshold pulse (weak field) was observed to pass through the absorber while interacting coherently with a strong-field pulse at a neighboring transition. It is shown theoretically that absorption of the weak pulse is reduced for two reasons: first, as a result of incoherent transparency of the resonance transition caused by emptying of the lower level by the field of the strong pulse, and second, as a result of coherent transfer of polarization between the upper levels via the two-photon processes. When the conditions for combined propagation are met, the latter mechanism ensures inversionless amplification of a weak pulse over a wide band of frequencies. In this case, the gain can even exceed the linear absorption coefficient in absolute value. A difference in propagation velocities of the weak and strong pulses was recorded experimentally, along with a shift in the carrier frequency of the weak field towards the red (≈600 MHz). A mechanism for transfer of phase modulation from a strong pulse to a weak pulse via the common lower level is discussed theoretically. Zh. éksp. Teor. Fiz. 113, 71–88 (January 1998)     

HECADE: HMQC- and HSQC-Based 2D NMR Experiments for Accurate and Sensitive Determination of Heteronuclear Coupling Constants from E.COSY-Type Cross Peaks

Two-dimensional pulse sequences for the determination of heteronuclear long-range coupling constants are presented. The sequences are based on the HMQC/HMBC or HSQC technique with subsequent optional homonuclear I-spin transfer. However, they yield tilted cross-peak patterns displaying antiphase heteronuclear coupling constants in the projections of both dimensions, which allow accurate determination of the couplings even in cases where the linewidth is of comparable magnitude. Two characteristic pulse-sequence elements were implemented to shape theF₁domain: the first element allows an arbitrary scaling of the heteronuclear coupling splittings relative to S-spin chemical-shift differences, whereas the second element achieves homonuclear broadband decoupling among the I spins in the HMQC/HMBC experiments and thus allows purely absorptive representations of such spectra. In comparison with established (ω₁) X-half-filtered TOCSY spectra, the signal dispersion inF₁is significantly improved and largely under experimental control. Furthermore, heteronuclear couplings of (I₁, S) pairs where S is either quaternary or carries one or more I spins that do not belong to the same I-coupling network as I₁can also be measured. The implementation of pulsed field gradients results in good suppression of spectral artifacts.     

Quasistatic magnetic fields generated by a nonrelativistic intense laser pulse in uniform underdense plasma

Quasistatic magnetic fields generated by nonrelativistic intense linearly polarized (LP) and circularly polarized (CP) laser pulses in an initially uniform underdense plasma in the collision-dominated limit are investigated analytically. Using a selfconsistent analytical model, we perform a detailed derivation of quasistatic magnetic fields in the laser pulse envelope in the collision-dominated limit to obtain exact analytical expressions for magnetic fields and discuss the dependence of magnetic fields on laser and plasma parameters. Equations for quasistatic magnetic fields including both axial component B_z and the azimuthal one B_θ are derived simultaneously from such a selfconsistent model. The dependence of quasistatic magnetic field on incident laser intensity, transverse focused radius of laser pulse, electron density and electron temperature is discussed.     

Tunable,sub-nanosecond KrF-Raman laser in the ultraviolet

A 0.5 cm^–1 bandwidth injection-locked KrF laser pumps a rare-gas Brillouin cell to produce a reflected pulse with a leading edge risetime of 1 ns, tunable from 248.1 to 248.7 nm. Consistent with Lamb theory of laser amplifiers, subsequent excimer amplification of this pulse produces an intense 500 ps spike on the pulse leading edge. Stimulated Raman scattering then separates the spike from the parent pulse, yielding a tunable short pulse at the first Stokes (S ₁) wavelength. Varying the Raman cell length results in a variable Raman threshold and an adjustable short pulse duration: 250 ps pulses at energies of 3–4 mJ at 268 nm with a 50 cm methane cell and 350 ps, 5 mJ pulses from a 100 cm cell are measured with a streak camera. First pass Raman conversion of the spike toS ₁ followed by second pass backward Raman amplification, where the parent 248 nm pulse serves as the pump beam for the reflectedS ₁ pulse, yields simultaneousS ₁ pulses of 20–25 mJ in the 800 ps range andS ₂ pulses of 550 ps at 5–6 mJ near 290 nm. This laser will avoid collision effects during laser excitation and enable quantitative, single pulse imaging of OH radicals in turbulent combustion because of its high pulse energy.     

Signal Enhancement Using 45° Water Flipback for 3D N-Edited ROESY and NOESY HMQC and HSQC

A novel implementation of the water flipback technique employing a 45° flip-angle water-selective pulse is presented. The use of this water flipback technique is shown to significantly enhance signal in 3D ¹⁵N-edited ROESY in a 20 kDa complex of the vnd/NK-2 homeodomain bound to DNA. The enhancement is seen relative to the same experiment using weak water presaturation during the recovery delay. This enhancement is observed for the signals from both labile and nonlabile protons. ROESY and NOESY pulse sequences with 45° water flipback are presented using both HMQC and HSQC for the ¹⁵N dimension. The 45° flipback pulse is followed by a gradient, a water selective 180° pulse, and another gradient to remove quadrature images and crosspeak phase distortion near the water frequency. Radiation damping of the water magnetization during the t₁ and t₂ evolution periods is suppressed using gradients. Water resonance planes from NOESY–HMQC and NOESY–HSQC spectra show that the HMQC version of the pulse sequences can provide stronger signal for very fast exchanging protons. The HSQC versions of the ROESY and NOESY pulse sequences are designed for the quantitative determination of protein–water crossrelaxation rates, with no water-selective pulses during the mixing time and with phase cycling and other measures for reducing axial artifacts in the water signal.     

Optical generation of spin coherence in single-charged (In,Ga)As/GaAs self-assembled quantum dots

The generation of electron spin coherence has been studied in n-modulation-doped (In,Ga)As/GaAs self-assembled quantum dots (QDs) which contain on average a single electron per dot. The doping has been confirmed by pump–probe Faraday rotation experiments in a magnetic field parallel to the heterostructure growth direction. For studying spin coherence, the magnetic field was rotated by 90° to the Voigt geometry, and the precession of the electron spin about the field was monitored. The coherence is generated by resonant excitation of the QDs with circularly polarized laser pulses, creating a coherent superposition of an electron, and a trion state. The efficiency of the generation can be controlled by the pulse intensity, being most efficient for (2n+1)π pulses.     

Coherent radiation in time separated fields

This paper reports on an experimental observation of coherent radiation after interaction with two standing-wave pulses having the time delayT in an absorbing rarefied gas. The radiation appears at the time 2T, 3T,... after the first pulse. The coherent radiation arises due to spacial transfer of polarization and to formation of a spacial polarization harmonic owing to a phase jump after two-photon interaction with the field of the second pulse. We observed intensity oscillations with frequency tuning of the exciting laser, varying the time delay between pulses. The paper was reported at theVth Vavilov Conference, Novosibirsk, USSR (June 1977)     

Generalized MAGSTE with bipolar diffusion-weighting gradient pulses

The generalized magic asymmetric gradient stimulated echo (generalized MAGSTE) sequence compensates background gradient cross-terms and can be adjusted to asymmetric timing boundary conditions which for instance are present in echo-planar MR imaging. However, its efficiency is not optimal because one of the two diffusion-weighting gradients applied in each interval usually must have a reduced amplitude to ensure the desired cross-term compensation. In this work, a modification of generalized MAGSTE is investigated where this gradient pulse is replaced by two gradient pulses with full amplitude but opposite polarities. It is shown that with these bipolar gradients (i) the sequence retains the cross-term compensation capability for an appropriate choice of the gradient pulse durations and (ii) the diffusion-weighting efficiency is improved, i.e. higher k and b values can be achieved without prolonging the echo time. These results are confirmed in MR imaging experiments on phantoms and in vivo in the human brain at 3 T using spin-echo and echo-planar MR imaging. In the examples shown, the b value could be increased between about 30% and 200% when using the bipolar gradient pulses. Thus, bipolar gradients may help to improve the applicability of the generalized MAGSTE sequence.     

14N pulsed nuclear quadrupole resonance. 3. Effect of a pulse train. Optimal conditions for data averaging

The calculations developed in this paper aim at determining the optimal conditions of a NQR experiment when a transition is monitored by means of a pulse train with pulses of identical duration and signal acquisition after each pulse; coherences are assumed to vanish by effective transverse relaxation prior to every new pulse. These calculations demonstrate that, as in NMR, a steady state is effectively reached for any value of the recycle time. However, by contrast with NMR, it is shown that, for optimal data averaging under steady state conditions, the recycle time T can be kept as low as possible (the only limitation is the acquisition time). Nutation curves (signal amplitude versus pulse length) calculated in the steady state case are shown to depend strongly on the ratio T/T ₁ (T ₁: longitudinal relaxation time). The signal growth as a function of T/T ₁under averaging of the first transients has been evaluated as well as the number of pulses necessary for reaching a steady state.