Selective pulse requirements

There are three main requirements of good-quality selective pulses ... 

Figure 7.20 Transformation of a 3D experiment into a soft 3D experiment by selective pulses, (a) A typical 3D spectrum, (b) A 3D spectrum with reduced frequency range in F, only, (c) A 3D spectrum with reduced frequency range in F and I 2, or (d) in F, F, and l. Spectra (b), (c), and (d) require selective soft pulses preceding t, <, and t, and t, l-z. and b, respectively. (Reprinted from Mag. Reson. Chem. 29, H. Ke.ssler et ai,. 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

A wide variety of ID and wD NMR techniques are available. In many applications of ID NMR spectroscopy, the modification of the spin Hamiltonian plays an essential role. Standard techniques are double resonance for spin decoupling, multipulse techniques, pulsed-field gradients, selective pulsing, sample spinning, etc. Manipulation of the Hamiltonian requires an external perturbation of the system, which may either be time-independent or time-dependent. Time-independent... 

In spite of the apparent simplicity of the method, its drawback comes from the fact that a two-spin system has been assumed. It provides merely global information spanning all protons prone two interact by dipolar coupling with the considered carbon. Selective information requires pulsed experiments stemming from the general solution of Equation (14) given below. 

It has been mentioned in Section 7.3, and it was implicit all over Chapter 7, that a finite time is required to achieve selective saturation or inversion of a signal by a soft pulse, during which time polarization starts to be exchanged, causing non-linearity of the response (see also Section 9.3). It should be stressed that this is not the case in all common 2D experiments based on non-selective pulses, which have durations of the order of microseconds instead of milliseconds, as required for selectivity. Selectivity in 2D experiments is intrinsic because of the double frequency labeling along f and /2. 

In a shift register, a selection pulse is shifted from one stage to the next at every clock pulse. The frame rate and the number of rows in the display determine the requirements for the clock frequency. As the image update time of the display is 600 ms, a frame rate of approximately 10 Hz is sufficient for black and white images. This enables e-reading applications. The corresponding minimum clock fre-... 

POMA A complete mathematica implementation of the NMR product-operator formalism—Guntert et al. JMR 101A, 103-105 (1993). POMA is a flexible implementation of the product operator formalism for spin-1/2 nuclei written for Mathematica. It provides analytical results for the time evolution of weakly coupled spin systems under the influence of free precession, selective and non-selective pulses, and phase cycling. As part of Mathematica, it requires a license, but the source code is free. Mathematica also provides a framework for visualizing and storing results. 

The constant amplitude pulses are usually easier to implement and they do not require special hardware. A typical example and also the most frequently used kind of constant amplitude, frequency selective pulse is the DANTE pulse train [4]. In the simplest case it consists of a sequence of square pulses interleaved with periods of free precession. Unfortimately, the excitation profile of the DANTE sequence has extensive sidelobes. It also produces sidebands at frequencies ... 

A single RF pulse is sufficient to create MQC in half-integer quadrupolar spin systems, as long as the strength of the RF pulse Pj < vq, which is often the case. Similarly, a single RF pulse can also convert the MQC to observable SQC. We denote such a basic sequence as Ph- h which is sketched in Fig. 5a, where Ph denotes a hard unmodulated rectangular pulse often called CW pulse. The first pulse is optimized to excite MQC and has a phase ]. The second pulse with a phase converts MQC to SQC which is detected by the receiver. The receiver phase ec is cycled to select the required coherence pathways and to eliminate the undesired ones. The value of the receiver phase equals — [, Ap,,] where Ap,- is the difference between the coherence order after and... 

To close this paper, we believe that both the theoretical and experimental aspects of excited-state relaxation in aromatic polymers will continue to be the subject of lively debate in the near future. Thus, the analyses of non-equilibrium transport based upon asymmetric energy-space master equations (43., 53) as well as theoretical models for a description of EET and rotational sampling are challenging many-partlole problems in polymer photo-physios. From an experimental standpoint of view, the time resolution of fluorescence system-configurations requires further Improvement in order to test these concepts. Moreover, site-selective pulse-and-probe techniques should help to reveal transient excited-state distributions, energy relaxation and trapping on sub-ps time scales in forthcoming measurements. 

Although spectrometer transmitters are frequently used at full power when applying single pulses, there are many instances when lower transmitter powers are required, for example the application of decoupling sequences or of selective pulses. These lower powers are derived by attenuating the transmitter output, and the units used for defining the level of attenuation are the deciBel or dB. This is, in fact, a measure of the ratio between two power levels. Pi and P2, as defined by ... 

A shaped pulse must fulfil two main criteria it must be selective and it must generate the required tilt angle e.g. 90° or 180°. The selectivity of a shaped pulse, which is related to the excitation range, is inversely proportional to the pulse length. Selectivity also depends upon the shape of the pulse. Unlike hard pulses, the pulse length and the pulse shape are pre-determined by the desired excitation profile. Thus, the pulse power must be adjusted to the desired title angle of the selective pulse. The relationship between these parameters is complex but may be broken down into three steps and examined using either a spectral representation or the tools of the Bloch module of NMR-SIM. 

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