Hahn probe pulse

Within the two-pulse EOM-PMA, the signals are calculated without resort to several commonly used simplifications like the doorway-window approximation or the neglect of dissipation effects during the pulses [1]. The two-pulse EOM-PMA leads to a A/c scaling for the computation of time- and frequency-resolved spectra, in contrast to the Nt x No, scaling for the a posteriori decomposition of the total polarization in ffie nonperturbative approach [11] or for the method developed by Tanimura and Mukamel [25,26]. The method of Hahn and Stock [27] also exhibits a Noj scaling, but it is efficient only for temporarily well separated pump-probe pulses... 

The basic components of the solid state spectrometer are the same as the solution-phase instrument data system, pulse programmer, observe and decoupler transmitters, magnetic system, and probes. In addition, high-power amplifiers are required for the two transmitters and a pneumatic spinning unit to achieve the necessary spin rates for MAS. Normally, the observe transmitter for 13C work requires broadband amplification of approximately 400 W of power for a 5.87-T, 250-MHz instrument. The amplifier should have triggering capabilities so that only the radiofrequency (rf) pulse is amplified. This will minimize noise contributions to the measured spectrum. So that the Hartmann-Hahn condition may be achieved, the decoupler amplifier must produce an rf signal at one-fourth the power level of the observe channel for carbon work. 

NMR. The CP/MAS spectra were obtained on a single coil, double-tuned probe similar in design to that reported earlier (15) but modified (16) for the electromagnet of the Varian XL-100-15 system. A homebuilt receiver system allows single coil operation. The probe uses a D2O external lock and a special rotor and stator assembly design to provide MAS. The carbon-13 (25.16 MHz) and the proton (100.06 MHz) radio frequency fields are 17 G and 12 G, respectively for 90 watts of power. The H spin locking pulse of 90° may be varied in terms of length and amplitude while the amplitude of the - C irradiation can be controlled to within 0.1 db to match the Hartmann-Hahn (17) condition. The isolation between the two channels is in excess of... 

In this section, aspects of Hartmann-Hahn experiments are discussed that are important for practical applications. There are obvious instrumental differences between heteronuclear and homonuclear Hartmann-Hahn experiments, such as the necessity for one or several heteronuclear rf channels and double- or triple-resonance probes. In addition, the rf amplitude of the channels must be matched, that is, the duration of the respective 90° pulses must be carefully adjusted such that the difference is not larger than a few percent. A detailed discussion of setup experiments for the calibration of hetero pulses has been given, for example, by Griesinger et al. (1994). 

The fact that the echo decays very nq>idly in the time domain leads to a problem common in solid-state NMR work the appearance of a pulse feedthrong . This artifact results from the recovery of the receiver circuits and a ringdown of the transmitter and probe circuits after the high-power excitation pulses. To reduce this problem a pulse sequence used to eliminate ringdown in quadrupolar echo NMR was employed [10]. In this sequence, the phase of the tc/2 and the n pulses of the Hahn-echo sequence are varied in such a way that the arti cts add destructively and are essentially eliminated. 

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