Adiabatic Sweep Research Articles

Compensation for Spin–Spin Coupling Effects during Adiabatic Pulses

It is widely recognized that adiabatic fast passage is the most effective method for inverting nuclear spins over a wide range of chemical shifts in situations where radiofrequency power is limited. One possible drawback of an adiabatic sweep is that different groups of spins are inverted at slightly different times, and this can perturb the symmetry of refocusing experiments by appreciable amounts (milliseconds), introducing a serious phase dispersion. We demonstrate how a pair of adiabatic pulses can be used to compensate such refocusing errors. Usually, but not always, this requires the use of two adiabatic pulses with opposite directions of frequency sweep. One important application is the “isotope filter,” a scheme for suppressing all proton responses except those from directly bound13CH groups; a pair of opposed adiabatic pulses allows compensation of refocusing errors over a range of different spin inversion times. In an extension of this technique, a range of13CH coupling constants can also be accommodated by exploiting the rough linearity between the13C chemical shift and the magnitude of1JCH. Heteronuclear two-dimensional experiments that involve inverting13C spins during the evolution period can also benefit from a compensating pair of adiabatic pulses, in this case with the same sense of frequency sweep. Finally, a pair of opposed adiabatic pulses is used in a new scheme (“ECHO-WURST”) for suppressing cycling sidebands in broadband heteronuclear decoupling; we show a 500-MHz proton spectrum decoupled from13C, where the residual cycling sidebands are below 0.06%.

Suppression of Cycling Sidebands Using Bi-level Adiabatic Decoupling

Repetitive heteronuclear decoupling schemes are suscepti- spins interchanges theaand blabels. For I spins at reso-ble to the problem of cycling sidebands (1). Viewed in the nance near the center of the sweep, the adiabatic pulse causestime domain, decoupling is basically a repeated refocusing reconvergence after a time period of approximately 0.5T,of the divergence of magnetization vectors due to the spin– i.e., near the end of the sweep. They reach full divergencespin coupling, and unless the focusing is precise and the again near the center of the next adiabatic sweep, ready forsampling of the observed signal is exactly synchronized with refocusing. Consequently, these principal sidebands are mostthe focus points, spurious modulation is normally introduced intense for spins with chemical shifts near the middle of theinto the free induction decay. The Fourier transform of these sweep.oscillatory artifacts consists of pairs of modulation sidebands The subharmonic sidebands also have a phase gradientflanking the resonance frequency and separated from it by that is a function of the decoupler sweep, and which isthe cycling frequency. These cycling sidebands are undesir- canceled by alternating the sweep direction (Fig. 2). How-able because they can eventually interfere with the detection ever, these sidebands are most prominent near the edges ofof meaningful NMR signals, for example, the cross peaks the sweep. For a chemical shift near one edge, the corre-in two-dimensional NOESY spectra. In general, the cycling sponding vectors aand bdiverge for almost the entire sweepsideband problem is more serious at low decoupler power, duration T and reconverge to a focus near 2T, at which timewhich is the preferred operating condition. the next adiabatic sweep can have little effect. We must waitNow that efficient adiabatic decoupling schemes have until 3T for full divergence before spin inversion is oncebeen developed (2–14), it is possible to cover chemical- again effective. The resulting modulation frequency isshift ranges that far exceed even those required for carbon- 1/(2T) hertz.13 decoupling in a field of 18.8 T. Consequently, radiofre- The principal and subharmonic sidebands cannot be can-quency heating is no longer a serious problem, but some celed by simply desynchronizing the decoupling and sam-care must be taken to minimize cycling sidebands. We distin- pling operations; this only spreads the sidebands across theguish two main types— principal cycling sidebands that oc- entire decoupling bandwidth. [The asynchronous decouplingcur at frequency offsets of {1/(T) hertz, where T is the proposed for homonuclear systems (15) addresses a differentadiabatic pulse duration, and subharmonic sidebands which problem, i.e., the artifacts created by the presence of a modu-appear at {1/(2T) hertz. lated radiofrequency field (16).] Two different proceduresThe variation of the principal cycling sidebands as a func- are recommended for suppression of the two types of side-tion of the decoupler sweep (Fig. 1) shows a progressive bands. Subharmonic sidebands are greatly attenuated by en-change of phase through almost {1807 from one edge of suring that spins at all chemical shifts experience the adia-the sweep to the other, but if the sweep direction is reversed, batic spin inversion as nearly as possible at the same time.the dispersion contributions are inverted. Averaging over This suggests the use of shorter adiabatic sweeps inter-both sweep directions leaves these sidebands everywhere in spersed with ‘‘windows’’ of zero decoupler intensity, im-pure absorption (Fig. 1c). Clearly, the principal sidebands plying operation at a higher peak decoupler level, B

First-Satellite Spectroscopy, a New Method for Quadrupolar Spins

A new spectroscopic method that detects only the ±1 12 (double-arrow) ±12 transitions (i.e., first satellites) of half-integer quadrupolar nuclei in solids is presented. The technique is based on a central-transition signal enhancement method that makes use of adiabatic radiofrequency sweeps. A plot of the enhanced central-transition signal amplitude versus the stop frequency of the sweep is an integrated version of the first-satellite spectrum. To find the quadrupole parameters, one can fit this integrated spectrum directly or differentiate the data to reveal the traditional spectrum (of the first satellites). The technique is demonstrated for27Al in Al2O3and93Nb in LiNbO3. First-satellite spectroscopy is more sensitive than other methods for studying wide, quadrupolar broadened spectra. The technique is easier to perform experimentally. Since the resulting spectra contain only the innermost satellites, the experimental data are considerably simpler and easier to interpret in cases of two or more inequivalent sites.

Single- and Double-Resonance Experiments of Quadrupolar Nuclei in Solids Using Sensitivity Enhancement of the Central Transition

Abstract Population enhancement for NMR of the central transition of quadrupolar nuclei in solids is performed by using adiabatic radiofrequency sweeps of the satellite transitions. Single-resonance experiments are reported for 17 O, 23 Na, 27 Al, and 93 Nb nuclei in powders of Al 2 O 3 , NaNbO 3 , and LiNbO 3 , and in Pyrex glass. Experimental techniques using single-coil probe circuits capable of both wideband frequency sweeps and narrowband detection are presented. The combination of population enhancement with cross polarization from 27 Al to 17 O in α-Al 2 O 3 (corundum) powder is demonstrated. The sensitivity-enhancement technique is robust, with no critical parameters.

Broadband and adiabatic inversion of a two-level system by phase-modulated pulses.

We describe a class of continuously phase-modulated radiation pulses that result in coherent population inversion on resonance as well as over a large range of transition frequencies and radiation field strengths. This is a population-inversion analogy to self-induced transparency. Simulations of the inversion properties of the modulated inversion pulse (MIP) are presented. It is shown that the inversion behavior can be explained by treating the MIP as a highly efficient adiabatic sweep. Criteria for establishing adiabaticity are discussed in detail. Finally, a method is presented for generating a sequence of phase-shifted radio-frequency pulses, from the continuously modulated pulse, which can be implemented on modern NMR and coherent optical spectrometers; experimental confirmation is given.

Specific Heat and Magnetocaloric Effects in TbPO4 1. Experiments

AbstractAt low temperatures TbPO4 undergoes two successive phase transitions. Antiferromagnetic ordering along the tetragonal c‐axis at TN1 = 2.28 K is followed below TN2 = 2.13 K by a structure of lower symmetry where the moments are tilted off the c‐axis by about 20°. The B‐T phase diagram (B along c) is determined down to 1 K by means of specific heat, adiabatic and isothermal field sweeps. In applied field there is an additional spin‐flop phase (between Bc1 = 0.61T and Bc2 = 1.06T in the limit T → 0).

Effects of modulation on magnetic resonance

This study is concerned with the effects of modulation on magnetic resonance which are due to changes in the longitudinal magnetization of a specimen during modulation and adiabatic sweep through a steady magnetic field. The analysis covers various amplitudes and rates of harmonic modulation of the field. Formulas are derived for the corrections to the form functions of resonance curves.