Coherent pulse sequences duration

We first examine how this works for the case of coherent flow. A typical pulse sequence is shown in figure Bl.14.9. This sequence creates a spin echo using two unipolar gradient pulses on either side of a 180° pulse. The duration of each gradient pulse of strength G, is . The centres of the gradient pulses are separated by A. 

Several such gradients G, all of equal duration, applied at different times in a particular pulse sequence would each contribute to the total dephasing of all coherence transfer pathways, except those for which O ... 

Comparison of the results of the one-dimensional gradient supported 31P/15N 1H -se-HSQC experiment with phase-cycled HSQC and HMQC experiments gave relative S/N-ratios of 0.75 0.92 1 which was, under consideration of the suppression of one of the two possible coherence transfer pathways by the field gradients and the longer duration of the pulse sequence, interpreted in terms of a very good performance.25 The main benefits of the PFG-es-HSQC sequence were seen, however, in the excellent level of artefact suppression which allowed one to observe correlations via very small couplings even in cases where the active isotopomer is present in low natural abundance and its lines are normally obscured by residual parent signals (Fig. 3). 

The electronic absorption spectra of complex molecules at elevated temperatures in condensed matter are generally very broad and virtually featureless. In contrast, vibrational spectra of complex molecules, even in room-temperature liquids, can display sharp, well-defined peaks, many of which can be assigned to specific vibrational modes. The inverse of the line width sets a time scale for the dynamics associated with a transition. The relatively narrow line widths associated with many vibrational transitions make it possible to use pulse durations with correspondingly narrow bandwidths to extract information. For a vibration with sufficiently large anharmonicity or a sufficiently narrow absorption line, the system behaves as a two-level transition coupled to its environment. In this respect, time domain vibrational spectroscopy of internal molecular modes is more akin to NMR than to electronic spectroscopy. The potential has already been demonstrated, as described in some of the chapters in this book, to perform pulse sequences that are, in many respects, analogous to those used in NMR. Commercial equipment is available that can produce the necessary infrared (IR) pulses for such experiments, and the equipment is rapidly becoming less expensive, more compact, and more reliable. It is possible, even likely, that coherent IR pulse-sequence vibrational spectrometers will... 

Time-domain spectroscopies entail a major shift in emphasis from traditional spectroscopies, since the experimenter can control, in principle, the duration, shape, and sequence of pulses. One may say that traditional, CW spectroscopy, is passive—the experimenter attempts to study static properties of a particular molecule. Coherent pulse experiments are active in that, given a set of molecular properties (which may in fact be known from various spectroscopies), one tries to arrange for a desired chemical product, or to design a pulse sequence that will probe new molecular properties. The time-dependent quantum mechanics-wavepacket dynamics approach developed here is a natural framework for formulating and interpreting new multiple pulse experiments. Femtosecond experiments yield to a particularly simple interpretation within our approach. 

Figure 1 Schematic representation of the different steps involved in a standard 2D HSQC pulse sequence. Thin and thick vertical rectangles represent 90° and 180° hard pulses, respectively. The delay should be set to 1/(2 /(CH)), and S represents the duration of the PFG and its recovery delay. In this scheme, coherence selection is performed by the gradient pair G1/G2 using the echo-antiecho protocol.

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