Magnetic amplitude-modulated

The integral describes the spatial amplitude modulation of the excited magnetization. It represents the excitation or slice profile, g(z), of the pulse in real space. As drops to zero for t outside the pulse, the integration limits can be extended to infinity whereupon it is seen that the excitation profile is the Fourier transfonn of the pulse shape envelope ... 

Fig. 10. The ESR signal produced at various points on the resonant line in a magnetic field modulated spectrometer. The vertical magnetic field modulation interacts with the bell-shaped adsorption curve [F(H)1 to produce the horizontal ESR signal. Here AH is the half amplitude line width and Hu is the center of resonance (S3).

Assume a 180° PIP is applied at the centre of the 13CO in the middle of the 13C evolution time for homonuclear decoupling and at the same time a compensating PIP is applied on the other side of the l3C (Fig. 11) to minimize the disturbance to the 13C that may have a transverse or longitudinal magnetization. The two simultaneous PIPs become an amplitude modulated pulse described by 2/i cos (27iAft)Ix, where f is the pulse strength of each 180°... 

In order to conveniently detect and display the resonance signal, audio modulation coils are used to modulate the static magnetic field. Close to a resonance the rf voltage will be audio amplitude modulated in the same way that a carrier wave is audio amplitude modulated in radio broadcasting. In this way by modulating with an amplitude large compared to the line width, the entire resonance signal may be displayed on a cathode-ray oscilloscope. 

ESEEM is a pulsed EPR technique which is complementary to both conventional EPR and ENDOR spectroscopy(74.75). In the ESEEM experiment, one selects a field (effective g value) in the EPR spectrum and through a sequence of microwave pulses generates a spin echo whose intensity is monitored as a function of the delay time between the pulses. This resulting echo envelope decay pattern is amplitude modulated due to the magnetic interaction of nuclear spins that are coupled to the electron spin. Cosine Fourier transformation of this envelope yields an ENDOR-like spectrum from which nuclear hyperfine and quadrupole splittings can be determined. 

Figure 11.13 shows the energy levels involved and the transitions studied for H2 in the N = 1 rotational level of the G 1 + state. The experiments were performed using a fixed microwave frequency, typically 9204 MHz, and the resonances detected by scanning the magnetic field amplitude modulation of the microwave power at 100 kHz and lock-in amplifier detection were employed. Polarising filters were used to detect the fluorescence, so that changes in polarisation could be observed. 

Figure 11.24. Experimental arrangement used by Ernst and Kindt [44] in their pump/probe microwave/optical double resonance study of a rotational transition (18.2 GHz) in the ground state of CaCl. The photomultiplier tubes which monitor fluorescence are situated on the axis perpendicular to both the laser beam and the molecular beam. The C region, where the molecular beam is exposed to microwave radiation, is magnetically shielded to minimise stray Zeeman effects. The microwave power was amplitude modulated at 160 Hz and the modulated fluorescence detected by photomultiplier B. 

In highly nonisotropic Hartmann-Hahn mixing sequences, the selection of a single magnetization component is a built-in property. Pure amplitude modulation can therefore be obtained in the evolution period using the pulse sequence of Fig. 37A without further modifications. For example. 

The amplitude modulation of the antiphase proton magnetization 2/,-5 2, generated prior to the generation of zero and double quantum coherences by the first Sn 90° pulse, obeys a sine law of the type sin[/7cA] with j=V( Sn, H). This amplitude modulation as a function of the coupling constant varying from 0 to 200 Hz is presented in Figure S for two fixed delays, A = 3.8 and 62 ms. 

ESR spectra at X-band were measured with a RE-1006 spectrometer operating at 9,6 GHz (empty cavity at ambient temperature) and 50-lcHz magnetic field modulation. Spectra were recorded at a microwave power of 1 mW and a modulation amplitude 0.02 mT to avoid line shape distortions, which could arise from experimental conditions such as microwave saturation and overmodulation. Scan range was 150-420 mT, Calibration of g-values is based on diphenylpicrylhydracyl (DPPH, g=2.0036) and Cr (g= 1.9796) standards. The amounts of the paramagnetic species (PMS) were calculated by double integration of the resonance line areas. 

S. Hediger, B. H. Meier and R. R. Ernst, Rotor-synchronized amplitude-modulated nuclear magnetic resonance spin-lock sequences for improved cross polarization under fast magic angle sample spinning. J. Chem. Phys., 1995, 102, 4000-4011. 

Multi-quantum transitions can only be observed indirectly by a modulation of the detected signal with the phase of the multi-quantum coherence. This modulation is achieved in an experiment by variation of an evolution time prior to detection. Repetitive detection of the signal for different evolution times provides the information about the evolution of the multi-quantum coherence. The indirect detection of spectroscopic information based on phase or amplitude modulation of the detected signal is the principle of multi-dimensional NMR spectroscopy [Eml]. Thus multi-quantum NMR is a special form of 2D NMR. Also, NMR imaging can be viewed as a special form of multi-dimensional NMR spectroscopy, where the frequency axes have been coded by the use of magnetic field gradients to provide spatial information. 

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