Coherent pulse sequences shape

Selected entries from Methods in Enzymology [vol, page(s)] Acquisition of frequency-discriminated spectrum, 239, 162-166, 170 sensitivity, 239, 169-173 constant-time, 239, 23-26 doublequantum filtered, 239, 236 gradient pulse experiments, 239, 185-189 protein structural information, 239, 377-379 pulse sequence and coherence transfer pathway, 239, 148-149 paramagnetic metalloprotein, 239, 494-497 data recording, SWAT method, 239, 166-169, 172 line shapes, effects of gradient pulses, 239, 162-166 identification of protein amino acid resonances, 232, 100 cyclosporin A, 239, 240-241. 

Figure 6. Pulse sequences and coherence transfer pathway diagrams for (a) a 2D CRAMPS experiment incorporating a z-filter to ensure that pure absorption-mode line shapes are obtained and (b) a 2D constant-time CRAMPS experiment. The relative performance of the two experiments with respect to yielding high-resolution H NMR spectra in the indirect (F ) dimension is illustrated by Figure 5c,d and is discussed in the text. (Adapted with permission from Figure 2 of ref 78. Copyright 2001 American Chemical Society.)...

In solution-state NMR, many important experiments incorporate the creation and evolution of MQ coherence (MQC).5,6,84-86 Since MQC cannot be directly detected, experiments that follow the evolution of a MQC are inherently at least two-dimensional. This is the case with H- H DQ MAS spectroscopy. Figure 7 shows a corresponding pulse sequence and coherence transfer pathway diagram first, a DQC is excited, which subsequently evolves during an incremented time period q the DQC is then converted into observable single-quantum (SQ) coherence (SQC), which is detected in the acquisition period, q. To select the desired coherence transfer pathways, e.g., only DQC during q, a phase cycling scheme is employed.79,80 Pure absorption-mode two-dimensional line shapes are ensured by the selection of symmetric pathways such that the time-domain... 

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. 

A number of investigators are now developing pulse-shaping and modulation techniques that are useful with ultrashort laser pulses. These methods will permit preparation of precisely timed and phased multipulse sequences of arbitrary complexity for use in nonlinear spectroscopy. In addition, rather than just exploiting pulse sequences to project coherences into echo intensities and time shifts for spectroscopic purposes, as in the methods discussed above, several investigators are devising pulse sequences to focus wavefimctions onto... 

The technique employs a specially designed 2D NMR pulse sequence [ 100], which forces nuclear spins to act collectively via their dipolar couplings, thereby creating unobservable multiple-quantum (MQ) coherences. The MQ coherences then evolve in the /, time domain and, after conversion into observable singlequantum coherences, are indirectly detected over the acquisition time t2- The multiple-quantum information is contained in the F dimension of the 2D spectrum (Fig. 13). The modified phase-incremented experiment [ 105] proceeds with the evolution held fixed and offers ease of operation and more accurate intensity distributions in return for loss of information contained in the fine structure and shapes of the MQ peaks. 

The above method could be expanded to N-level system, every time we choose two lower levels which couple with a common exited state by two pulses, by choosing the pulse sequences and shapes we can realize the coherence transfer and contribution in a N-level system. 

Figure 6.10 Ultrafast efficient switching in the five-state system via SPODS based on multipulse sequences from sinusoidal phase modulation (PL). The shaped laser pulse shown in (a) results from complete forward design of the control field. Frame (b) shows die induced bare state population dynamics. After preparation of the resonant subsystem in a state of maximum electronic coherence by the pre-pulse, the optical phase jump of = —7r/2 shifts die main pulse in-phase with the induced charge oscillation. Therefore, the interaction energy is minimized, resulting in the selective population of the lower dressed state /), as seen in the dressed state population dynamics in (d) around t = —50 fs. Due to the efficient energy splitting of the dressed states, induced in the resonant subsystem by the main pulse, the lower dressed state is shifted into resonance widi die lower target state 3) (see frame (c) around t = 0). As a result, 100% of the population is transferred nonadiabatically to this particular target state, which is selectively populated by the end of the pulse.

If pulse shaping devices and linear amplifiers are available, then rapid, phase-coherent changes of the rf amplitude can be conveniently implemented. In this case, the Hartmann-Hahn mismatch that is created by the additional pulse can be further reduced by increasing the rf amplitude of the additional pulse (see Fig. 24D and D ). The offset dependence of the coherence-transfer efficiency of a boosted MLEV-17 sequence (MLEV-17b) with I f = 10 kHz and v = 20 kHz is shown in Fig. 24D. 

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