Multiple quantum generation

The value of the delay A in the double-quantum experiment affects the amount of multiple-quantum generated and hence the intensity in the spectrum. All of the couplings present in the spin system affect the intensity and as couplings cover a wide range, no single optimum value for A can be given. An unfortunate choice for A will result in low intensity, and it is then possible that correlations will be missed. No such problems occur with COSY. 

Phase cycling is widely employed in multipulse NMR experiments. It is also required in quadrature detection. Phase cycling is used to prevent the introduction of constant voltage generated by the electronics into the signal of the sample, to suppress artifact peaks, to correct pulse imperfections, and to select particular responses in 2D or multiple-quantum spectra. 

Nozik AJ (2008) Multiple exciton generation in semiconductor quantum dots. Chem Phys Lett 457 3-11... 

New Solar Cells Quantum Dot (QD) Structures and Multiple Exciton Generation (MEG)... 

Luque, A. Marti, A. Nozik, A. J. 2007. Quantum dots Multiple exciton generation and intermediate bands. MRS Bull. 32 236-241. 

This scheme is applied in the so-called X-half-filter technique (Fig. 17.4a, c), with the only difference of an additional 90 (X) pulse with constant phase [16, 17]. Instead of generating heteronuclear multiple quantum coherence 2 Iy Sy, which cannot readily be detected (in case of selecting the 1H-X pairs), one now always ends up with proton antiphase coherence, but with the same phase alternation ... 

Heterocorrelations can be detected both in direct and reverse modes. In the latter mode, dramatic enhancements of sensitivity can be achieved owing to the larger sensitivity of protons with respect to heteronuclei. In the most common heterocorrelation pulse sequences for reverse detection, called heteronuclear multiple quantum coherence (HMQC) (Fig. 8.2G) [25,26], H-I3C MQ (multiple quantum) coherence is generated by first applying a 90° pulse on protons and, after a time t chosen equal to 1/2 J[j, by applying a 90° pulse on carbon (Fig. 8.19). 

Multiple quantum experiments in dipolar coupled systems Although single quantum coherence is the only magnetisation that can be directly observed, for a collection of N strongly dipolar coupled spins, it is possible to induce multiple quantum coherences. A number of pulse sequences have been developed for this purpose (Gleason 1993), for which one of the most common generators is... 

High conversion efficiency via multiple exciton generation in quantum dots... 

Ellingson R. J., Beard M. C., Johnson J. C., Yu P., Micic 0.1., Nozik A. J., Shabaev A. and Efros A. L. (2005), Highly efficient multiple exciton generation in colloidal PbSe and PbS quantum dots , Nano Lett. 5, 865-871. 

So far, biexponential relaxation has been the only aspect of the behaviour of quadrupolar nuclei which has distinguished them from the classical model of nuclear magnetism. This is because all the density matrices generated have contained only single-quantum elements, which behave classically in response to rf pulses. The order of multiple-quantum coherence for a density matrix element ernm is n — m for the special case of a single spin. The coherence pattern for a spin- density matrix is thus,... 

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