Broadband 13C-13C adiabatic mixing in solution optimized for high fields

Experiments which require mixing among spins with large frequency differences are generally performed with sequences based on composite pulses or computer-optimized cycles. Adiabatic pulses generally offer several advantages over other approaches, including greater single spin inversion bandwidths and tolerance to RF inhomogeneity. Here, a novel theoretical framework is presented in order to understand how spin-spin interactions are influenced by adiabatic inversion pulses, and insights from this approach are used to design more efficient adiabatic coherence exchange experiments. For very large frequency differences, this new approach generally offers improved results over previously applied mixing sequences, as applied to 13C-13C experiments which are the basis of modern sidechain assignment techniques in proteins. It is also anticipated that the approach presented here will be applicable to the analysis of various alternative approaches to adiabatic mixing.     

Implementation of all-optical Toffoli gate in Λ-systems

An optical Toffoli gate is the essential logical element, which permits the implementation of a reversible optical processor. We propose a simple realization of such a gate in films of crystals doped with rare-earth ions. The proposed scheme is based on adiabatic population transfer in a ??-system by means of counterintuitive and intuitive sequences of short laser pulses. We also discuss possibilities for experimental realization of the proposed gate.     

REDOR with adiabatic dephasing pulses

The response of a spin (1/2) ensemble, at thermal equilibrium and experiencing chemical shift anisotropy (CSA), to the application of adiabatic inversion pulses has been studied under magic-angle spinning (MAS). Numerical simulations and experimental studies on such systems, carried out under slow spinning conditions, show that the response to adiabatic inversion pulses has much more favorable characteristics than the response to conventional rectangular pulses. We have also explored the possibilities of employing adiabatic 180 degrees pulses as dephasing pulses in rotational-echo double-resonance (REDOR) experiments. Our results show that it is indeed possible to employ such adiabatic inversion pulses conveniently in REDOR experiments to eliminate resonance offset and H(1) inhomogeneity effects which may arise from the usage of conventional rectangular 180 degrees pulses. Copyright 2000 Academic Press.     

Fast-MAS total through-bond correlation spectroscopy using adiabatic pulses

Pulse sequences consisting of adiabatic pulses for total through-bond correlation spectroscopy (TOBSY) under magic-angle spinning (MAS) are introduced. Above a certain threshold, the polarization transfer achieved with these sequences is largely insensitive to the amplitude and homogeneity of the radiofrequency field employed. An experimental transfer efficiency of up to 76% was achieved in a two-spin system using the sequence WiW9(24)(1) at a MAS frequency of 26.67 kHz. Applications to resonance assignments in the dipeptide L-Val-L-Phe and in the cyclic decapeptide antamanide are demonstrated.     

Transverse relaxation in the rotating frame induced by chemical exchange

In the presence of radiofrequency irradiation, relaxation of magnetization aligned with the effective magnetic field is characterized by the time constant T1rho. On the other hand, the time constant T2rho characterizes the relaxation of magnetization that is perpendicular to the effective field. Here, it is shown that T2rho can be measured directly with Carr-Purcell sequences composed of a train of adiabatic full-passage (AFP) pulses. During adiabatic rotation, T2rho characterizes the relaxation of the magnetization, which under adiabatic conditions remains approximately perpendicular to the time-dependent effective field. Theory is derived to describe the influence of chemical exchange on T2rho relaxation in the fast-exchange regime, with time constant defined as T2rho,ex. The derived theory predicts the rate constant R2rho,ex (= 1/T2rho,ex) to be dependent on the choice of amplitude- and frequency-modulation functions used in the AFP pulses. Measurements of R2rho,ex of the water/ethanol exchanging system confirm the predicted dependence on modulation functions. The described theoretical framework and adiabatic methods represent new tools to probe exchanging systems.     

Compensated adiabatic inversion pulses: broadband INEPT and HSQC

When adiabatic fast passage is used to flip nuclear spins, sites with different chemical shifts are inverted at different times, causing refocusing errors. By mapping the phase evolution diagrams, we show that these effects can be accurately compensated with matched pairs of adiabatic pulses, either opposed or in the same sense, depending on the application. Applied to well-known heteronuclear polarization transfer experiments such as INEPT and HSQC, the requisite evolution of J-vectors is achieved irrespective of chemical shift or the duration of the adiabatic sweeps. By replacing conventional 180 degrees pulses, these new adiabatic sequences offer an order of magnitude improvement in effective bandwidth for the X-spins. Alternatively the experiments can be carried out with significantly reduced radiofrequency power. One- and two-dimensional spectra of (13)C in 13-cis-retinal at 600MHz have been used to demonstrate these advantages.     

Efficient coherent population transfer among three states in NO molecules by Stark-Chirped rapid adiabatic passage

We present the experimental demonstration of a novel, efficient, and selective technique to prepare population inversion. The technique is an extension of Stark-chirped rapid adiabatic passage (SCRAP), i.e., SCRAP among three states. In this process a Lambda-type quantum system is driven by two laser pulses, the pump and Stokes pulses, which are appropriately detuned from transition frequencies. A third laser pulse induces a dynamic Stark shift in the upper energy level, and the timing of all three pulses is controlled in order to prepare population inversion between the two lower states in the Lambda-type level scheme. Our data on population transfer in nitric oxide (NO) molecules clearly show that SCRAP among three states provides an advantageous alternative to such techniques as stimulated Raman adiabatic passage.     

Multiband Adiabatic Inversion Pulses

The conditions necessary to obtain multiple adiabatic population inversion at different frequencies in a two-level system are defined. It is shown how any pulse that produces adiabatic inversion in a single frequency band can be modified to become a multiband adiabatic inversion pulse. Using Floquet formalism, the interaction between the different inversion bands is described and shown to create effective nonlinear irradiation fields. By controlling the reference phase of the single-inversion-frequency-band pulses, these effective irradiation fields can be minimized or canceled. These pulses can be used for multiple selective excitation or selective population inversion in coherent spectroscopy. NMR experiments confirming the theoretical predications are shown. The experimental results agree very well with the theoretical predictions.     

Tagging of the magnetization with the transition zones of 360 degrees rotations generated by a tandem of two adiabatic DANTE inversion sequences

A new technique is presented for generating myocardial tagging using the signal intensity minima of the transition zones between the bands of 0 degrees and 360 degrees rotations, induced by a tandem of two adiabatic delays alternating with nutations for tailored excitation (DANTE) inversion sequences. With this approach, the underlying matrix corresponds to magnetization that has experienced 0 degrees or 360 degrees rotations. The DANTE sequences were implemented from adiabatic parent pulses for insensitivity of the underlying matrix to B(1) inhomogeneity. The performance of the proposed tagging technique is demonstrated theoretically with computer simulations and experimentally on phantom and on the canine heart, using a surface coil for both RF transmission and signal reception. The simulations and the experimental data demonstrated uniform grid contrast and sharp tagging profiles over a twofold variation of the B(1) field magnitude.     

Cross-correlations between low-γ nuclei in solids via a common dipolar bath

Correlation of chemical shifts of low-γ nuclei (such as 15N) is an important method for assignment of resonances in uniformly-labeled biological solids. Under static experimental conditions, an efficient mixing of low-γ nuclear spin magnetization can be achieved by a thermal contact to the common reservoir of dipole-dipole interactions in order to create 15N-15N, 13C-13C, or 15N-13C cross-peaks in a 2D correlation spectrum. A thermodynamic approach can be used to understand the mechanism of magnetization mixing in various 2D correlation pulse sequences. This mechanism is suppressed under magic-angle spinning, when mixing via direct cross-polarization with protons becomes more efficient. Experimental results are presented for single-crystalline and powder samples of 15N-labeled N-acetyl-L-15N-valyl-L-15N-leucine (NAVL). In addition to the thermodynamic analysis of mixing pulse sequences, two different new mixing sequences utilizing adiabatic pulses are also experimentally demonstrated.     

A Vector Model of Adiabatic Decoupling

A vector model of adiabatic decoupling is enunciated for an IS-coupled system of two spin- heteronuclei in the high-power limit of ideal adiabatic pulses. The observed S-spin magnetization evolves according to a time-dependent coupling that scales as thezcomponent of an I-spin vector which evolves due to the applied decoupling irradiation. Simple analytical expressions are derived both on and off resonance for the reduced coupling during an ideal sech/tanh inversion pulse and for the resulting signal when either in-phase or antiphase magnetization is present at the start of decoupling. The resulting model allows one to readily envision decoupling experiments, make accurate estimates of sideband intensity, and assess the relative performance of different decoupling schemes. The utility of the model is further demonstrated by applying it to several recently proposed methods for reducing sidebands. In the limit of ideal adiabatic pulses, the predictions of the vector model are almost identical to those of quantum mechanics. At the lower RF power levels used in practical adiabatic decoupling applications, where the pulses are no longer perfectly adiabatic, phase cycles are employed to achieve performance that approximates the ideal limits derived here, so the vector model is more generally applicable, as well. These limits establish standards for future determination of the most efficient parameters for practical applications of broadband adiabatic decoupling in a single transient.     

Short laser pulse generation: Part one

The status of short laser pulse generation is reviewed. Achievements, limitations and potential of techniques producing pulses shorter than 50 ns, from the ultra-violet to the infra-red, are examined.In this half of the paper the physical principles underlying the generation of ultra-short pulses are introduced and the methods of Q-switching, gain switching, cavity dumping and mode-locking are considered.     

Compensation of refocusing inefficiency with synchronized inversion sweep (CRISIS) in multiplicity-edited HSQC

Use of adiabatic pulses in broadband inversion and decoupling is well known. Replacement of the rectangular pi pulses in the INEPT and rev-INEPT parts of the HSQC and gHSQC experiments with adiabatic pulses substantially improves the sensitivity of these experiments. However, modulation of cross peak intensity in multiplicity-edited HSQC or gHSQC experiments can be quite severe. These modulations arise during the multiplicity-editing periods due to the inefficient refocusing of the spin-echo caused by the mismatch of the echo delay with the one-bond coupling constant. These modulations (which we call echo modulations) are field strength (and hence spectral width) independent. Use of adiabatic pulses with the inversion sweep synchronized to the 1H-13C coupling constant range typically observed in a 13C spectrum will provide substantial improvement in sensitivity. The inversion profile problems associated with rectangular pi pulses can be moderately compensated by composite pulse schemes and these schemes could prove to be reasonable alternatives to adiabatic pulses. However, the adiabatic sweep provides a unique method to compensate the echo modulations for multiplicity-edited experiments. The origin and the compensation of refocusing inefficiency with synchronized inversion sweep (CRISIS) method to minimize these modulations is described.     

Low-power water suppression by hyperbolic secant pulses with controlled offsets and delays (WASHCODE)

A class of chemical-shift-selective (CHESS) water suppression (WS) schemes is presented in which the characteristic frequency-domain excitation profiles of "adiabatic" full-passage (AFP) RF pulses are utilized for frequency-selective excitation of the water resonance. In the proposed WS schemes, dubbed WASHCODE, hyperbolic secant (HS) pulses were used as the AFP pulses. Besides the high immunity of WS efficiency toward B(1) inhomogeneity, these sequences also exhibit extraordinary insensitivity to the dispersion of the water T(1) relaxation times. The actual performance of the proposed WS schemes was achieved in particular by optimizing the frequency offsets of WS HS pulses and the time intervals between them. To reduce the RF power requirements of these WS sequences for in vivo applications, HS pulses with the minimum possible frequency bandwidths were employed, which also substantially reduced the adverse effects on the observed proton MR spectra. The proposed WS schemes were evaluated by simulations based on the Bloch equations. Several WS sequences which looked particularly promising were verified experimentally on the human brain on a 3 T MR scanner using very short echo-time STEAM for volume selection and a standard single-loop surface coil for both signal transmission and reception. Routinely, water-suppression factors ranging from 2000 to 4000 were achieved in vivo without additional adjustment of parameters for individual subjects and without violating legal safety limits.     

Adiabatic and nonadiabatic preparation of a ground-state coherence in an optically thick lambda medium

Fractional stimulated Raman adiabatic passage (f-STIRAP) is used to develop a method for preparation of a two-photon coherence between long-lived ground states of an optically thick ensemble of lambda atoms. Despite strong depletion and reshaping of preparation pulses in the course of propagation, this method provides a stable predetermined value of the coherence uniformly established along the sample, from input to the output. Both adiabatic and nonadiabatic preparation regimes are considered.     

旋转磁流体系统的变分原理及稳定性条件

本文舍去多方近似,从完整的MHD方程出发,对一个具有粘性,可压缩性,及自引力的旋转MHD系统给出变分原理和稳定性条件。     

A comparison of NCO and NCA transfer methods for biological solid-state NMR spectroscopy

Three different techniques (adiabatic passage Hartman-Hahn cross-polarization, optimal control designed pulses, and EXPORT) are compared for transferring (15)N magnetization to (13)C in solid-state NMR experiments under magic-angle-spinning conditions. We demonstrate that, in comparison to adiabatic passage Hartman-Hahn cross-polarization, optimal control transfer pulses achieve similar or better transfer efficiencies for uniformly-(13)C,(15)N labeled samples and are generally superior for samples with non-uniform labeling schemes (such as 1,3- and 2-(13)C glycerol labeling). In addition, the optimal control pulses typically use substantially lower average RF field strengths and are more robust with respect to experimental variation and RF inhomogeneity. Consequently, they are better suited for demanding samples.     

Coherent momentum transfer due to interaction between three-level atoms and counterpropagating laser pulses

A theoretical analysis is presented of the change in the momentum of a three-level atom due to its interaction with counterpropagating laser pulses that overlap in time. The two lower energy states of the atom are metastable; i.e., a lambda-type configuration of atomic levels is considered. The cases of two and four counterpropagating pulses having different carrier frequencies are considered. In the case of adiabatic atom-field interaction, it is shown that the atom’s momentum can change by an integer multiple of the photon momentum and the corresponding standard deviation is small as compared to the photon momentum squared.     

Improved Broadband Inversion Performance for NMR in Liquids

New NMR broadband inversion pulses that compensate both for resonance offset and radiofrequency (RF) inhomogeneity are described. The approach described is a straightforward computer optimization of an initial digitized waveform generated from either a constant-amplitude frequency sweep or from an existing composite inversion pulse. Problems with convergence to local minima are alleviated by the way the optimization is carried out. For a given duration and maximum allowable RF field strength B₁ (but not necessarily given RMS power deposition), the resultant broadband inversion pulse (BIP) shows superior inversion compared to inversion pulses obtained from previous methods, including adiabatic inversion pulses. Any existing BIP can be systematically elaborated to build up longer inversion pulses that perform over larger and larger bandwidths. The resulting pulse need not be adiabatic throughout its duration or across the entire operational bandwidth.     

Inversion in an extended three-level medium produced by adiabatic population transfer

Features of the adiabatic population transfer are studied with the spatial evolution of interacting pulses propagating in an optically dense medium of three-level Λ-atoms taken into account. A self-consistent analytical solution describing the spatial-temporal dynamics of interacting short pulses under the conditions of adiabatic population transfer is constructed in the adiabatic approximation with consideration for the first nonadiabatic correction. Practically complete inversion on a forbidden transition determined by coherent (adiabatic) population transfer is shown to take place over a length of the medium, which may exceed the absorption length of a weak probing pulse in the absence of control radiation on the adjacent transition by several orders of magnitude.