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Pulse with multiple selective excitation

We have implemented the principle of multiple selective excitation (pulse sequence II in fig. 1) thereby replacing the low-power CW irradiation in the preparation period of the basic ID experiment by a series of selective 180° pulses. The whole series of selective pulses at frequencies /i, /2, , / is applied for several times in the NOE build-up period to achieve sequential saturation of the selected protons. Compared with the basic heteronuclear ID experiment, in this new variant the sensitivity is improved by the combined application of sequential, selective pulses and the more efficient data accumulation scheme. Quantitation of NOEs is no longer straightforward since neither pure steady-state nor pure transient effects are measured and since cross-relaxation in a multi-spin system after perturbation of a single proton (as in the basic experiment) or of several protons (as in the proposed variant) differs. These attributes make this modified experiment most suitable for the qualitative recognition of heteronuclear dipole-dipole interactions rather than for a quantitative evaluation of the corresponding effects. [Pg.32]

We have demonstrated that the principle of multiple selective excitation with subsequent data processing appropriate to disentangle the superimposed responses may successfully be incorporated into selective ID and 2D pulse sequences. This allows to improve the overall efficiency especially of experiments with an inherent low sensitivity. In most of the presented examples a series of single selective pulses has been applied, giving rise to unwanted relaxation losses and setting an upper limit to the number of perturbed nuclear spins. With the application of single multiple selec-... [Pg.48]

It is well-known that the excitation profile by a periodic pulse also has a pattern of multiple bands in response to the multiple effective RF fields. The DANTE sequence,26 for instance, was one of the most frequently used periodic pulse in the past for selective excitation of a narrow centre band. It is constructed by a long train of hard pulses with a certain delay between two adjacent pulses. The advantage of using the DANTE sequence over the weak, soft RF pulses relies on that it is not necessary to change the RF power level in the pulse sequence. Consequently, phase distortions and certain delays accompanied by the abrupt changes of the RF power level are avoided. [Pg.22]

In the first part of this contribution the general principle of multiple frequency selective excitation is explained, followed by a short presentation of correspondingly updated selective ID and 2D pulse sequences and by a few applications and results for demonstration. The contribution concludes with a critical discussion of advantages and limitations for this kind of experiments and the perspectives for further developments. Readers interested in a more detailed description and in experimental details such as spectrometer settings are referred to the corresponding publications [2-6]. [Pg.23]

The experiments demonstrate that femtosecond laser pulses offer new opportunities for multiple-photon ionization of bioorganic molecules on surface. The fast femtosecond excitation makes it possible to produce molecular and fragmentation ions directly on the surface being irradiated. The two-photon excitation with an intense femtosecond pulse allows the selectivity of ionization of the chromophore (tryptophan in our case) in large molecules... [Pg.879]

We have so far been concerned with 2D spin-diffusion spectroscopy. There are, however, two ID experiments that are likely to be applied to catalytic problems selective excitation 72,85,861 and rotational resonance (87-93]. Selective excitation of selected resonances using the DANTE pulse trains [94] can be used to measure specific C - C connectivity in complex, multiple C-labeled solids (86). Rotational resonance can be achieved by adjusting the MAS rate to an integer fraction of the chemical shift difference between two selected carbon resonances n being a small integer. [Pg.378]

A five-period InGaN/GaN multiple quantum well (MQW) was fabricated on this high-quality GaN hornoepitaxial layer. The XRD profile of the MQW consisted of satellite peaks, where the evaluated well and barrier layer thicknesses were 3.4 and 11.3 nm, respectively. It is noteworthy that the growth rates of InGaN and GaN derived from the well and barrier thicknesses are comparable with those on the (0001) plane. As for the optical properties, we were interested in if the short radiative lifetime observed for the microfacet QWs was preserved. Therefore, TRPL measurements were conducted at 10 K. The excitation pulses were from a frequency-doubled Ti sapphire laser with a wavelength of 380 nm to selectively excite InGaN wells and a power density as low as 470 nj cm. The PL was detected by a streak camera. Figure 14.15... [Pg.403]

Figure 7.14 Pulse sequence for the HMBCS (heteronuclear multiple-bond correlation, selective) experiment, which uses advantageously a 270° Gaussian pulse for exciting the carbonyl resonances. It is also called the semisoft inverse COLOC. (Reprinted from Mag. Reson. Chem. 29, H. Kessler et al., 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)... Figure 7.14 Pulse sequence for the HMBCS (heteronuclear multiple-bond correlation, selective) experiment, which uses advantageously a 270° Gaussian pulse for exciting the carbonyl resonances. It is also called the semisoft inverse COLOC. (Reprinted from Mag. Reson. Chem. 29, H. Kessler et al., 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...
Another approach to obtain spatially selective chemical shift information is, instead of obtaining the entire image, to select only the voxel of interest of the sample and record a spectrum. This method called Volume Selective spectroscopY (VOSY) is a ID NMR method and is accordingly fast compared with a 3D sequence such as the CSI method displayed in Figure 1.25(a). In Figure 1.25(b), a VOSY sequence based on a stimulated echo sequence is displayed, where three slice selective pulses excite coherences only inside the voxel of interest. The offset frequency of the slice selective pulse defines the location of the voxel. Along the receiver axis (rx) all echoes created by a stimulated echo sequence are displayed. The echoes V2, VI, L2 and L3 can be utilized, where such multiple echoes can be employed for signal accumulation. [Pg.44]

TOPHAT-shaped 90° pulses are used in other cases as the best compromise with respect to the excitation profile, the phase homogeneity and length. Depending on the type of the detected spin-spin interaction - being either scalar or dipolar coupling - each selected spin is initially perturbed only once (ID TOCSY, ID INADEQUATE, ID C/H COSY, 2D TOCSY-COSY and 2D HMBC), or for several times (ID NOE). With each of the selected spins initially perturbed only once the inherently smaller transient NOEs would be detected in the latter case, whereas with the multiple excitation of a selected spin within the NOE build-up period the stronger steady-state NOEs are more or less approximated. [Pg.27]


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