Frequency sweep adiabatic

Adiabatic pulse A type of pulse employing a frequency sweep during the pulse. This type of pulse is particularly efficient for broadband decoupling over large sweep widths. 

Siegel et al. showed that enhancement of the CT can also be obtained using hyperbolic secant (HS) pulses to invert selectively the STs [74], Unlike the DFS waveform, whose frequency sweep is generated by a constant rf-pulse phase while modulating the amplitude, the HS pulse utilizes both amplitude and phase modulation, yielding an enhancement exceeding that obtained by DFS or RAPT [61, 74, 75]. Most recently, the pulse sequence called wideband uniform-rate smooth truncation (WURST) [76] was introduced to achieve selective adiabatic inversion using a lower power of the rf-field than that required for the HS pulses [77,78]. One of its applications involved more efficient detection of insensitive nuclei, such as 33S [79]. 

Fig. 15. Adiabatic decoupling of 13CO from 13C with a compensating pulse applied on the other side of the peaks. The compensating and decoupling pulses have the same shape but opposite frequency sweep. Due to the Bloch-Siegert effects, both the left and the right peaks are pushed towards the center while the centre peak is balanced and remains in its position. Reprinted from Ref. 47 with permission from Elsevier.

Fig. 11.5 Schematic comparison of (a) sudden and (b) adiabatic inversion of the z-component of the polarization vector. In the sudden case a n-pulse is applied while in the adiabatic case a frequency sweep is shown. The time evolution of the z-polarization as a function of the pulse duration...

Figure 9.9. The adiabatic inversion pulse. An rf frequency sweep during the pulse causes the effective rf field experienced by the spins to trace an arc from the +z-axis to the -z-axis, dragging with it the bulk magnetisation vector.

Adiabatic pulses may be studied using the Waveform analysis. These pulses can either be frequency or phase swept pulses but during the frequency sweep the adiabatic condition must always be met. For an estimation of this condition an effective B field is represented by the angles 0m and 0eff(B]), the calculation then displays the angles 0m and 0eff(Bj), a further quality factor and the frequency sweep as function of time in as a series of graphs. Adiabatic pulses are discussed in more detail in section 5.3.1. 

An adiabatic pulse is a special type of shaped pulse where either a frequency or a phase sweep occurs during the pulse duration. Adiabatic pulses are discussed in detail in section 5.3.1. So far the simulations involving the Bloch module have not considered the exact time related frequency sweep of a shaped pulse yet it is this factor that determines if each point in the pulse shape obeys the adiabatic condition. Using an adiabatic chirp pulse Check its 4.3.33 and 4.3.3.6 will examine various aspects of adiabatic pulses starting with the time evolution and the graphical representation of the amplitude and phase modulation. 

Figure 8.38. Regions of 400 ms NOESY spectra recorded (a) without and (b) with the inclusion of the zero-quantum filter shown in Fig. 8.37. The ZQC suppression employed a 20 ms adiabatic smoothed CHIRP pulse with a 40 kHz frequency sweep.

Adiabatic pulses are described by their frequency sweep and amplitude profile, which, when combined with the peak rf amplitude cc max), defines the total power of the pulse. The total frequency range, AF, over which the pulse sweeps is commonly many tens of kilohertz and pulse durations, T, are typically of the order of 1 ms, corresponding to frequency sweep rates of 10-100 MHz/s. The degree to which the adiabatic condition is satisfied for the pulse is quantified by the adiabaticity factor Q... 

The presence of zero-quantum coherence during the mixing time can substantially distort NOE intensities. This coherence can not be removed with phasecycling or gradient pulses while preserving z-magnetisation. A z-filter scheme was proposed earlier for the elimination of the coherence. It utilises an adiabatic inversion pulse with linear frequency sweep applied simultaneously with a gradient. Cano et alP demonstrated that a better suppression is achieved with a z-filter cascade that combines several filter elements. The attainable suppression ration is then equal to the multiplication of the ratios for each element. 

Garwood M and De la Barre L (2001) The return of the frequency sweep Designing adiabatic pulses for contemporary NMR. Journal of Magnetic Resonance 153 155-177. 

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