The Radio Frequency RF Pulse

RF pulse short duration, high power 3000 cycles m 

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Figure 7.2 Schematic representation of coherent oscillations between states 0) and 1) of a qubit. For an electron spin placed in a dc magnetic field, oscillations can be induced via the application of an external radio frequency (rf) pulse resonant with the Zeeman energy. The amplitude 8 of the...

Similarly to non-selective experiments, the first operation needed to perform experiments involving selective pulses is the transformation of longitudinal order (Zeeman polarization 1 ) into transverse magnetization or ly). This can be achieved by a selective excitation pulse. The first successful shaped pulse described in the literature is the Gaussian 90° pulse [1]. This analytical function has been chosen because its Fourier transform is also a Gaussian. In a first order approximation, the Fourier transform of a time-domain envelope can be considered to describe the frequency response of the shaped pulse. This amounts to say that the response of the spin system to a radio-frequency (rf) pulse is linear. An exact description of the... 

Nowadays, self-diffusion coefficients are almost exclusively measured by NMR methods, through the use of methods such as the 90-8-180-8-echo technique (Stejs-kal and Tanner sequence) [10-12]. The pulse-echo sequence, illustrated in Figure 4.4-2, can be divided into two periods of time r. After a 90° radio-frequency (RF) pulse the macroscopic magnetization is rotated from the z-axis into the x-y-plane. A gradient pulse of duration 8 and magnitude g is appHed, so that the spins dephase. 

The NMR process, then, essentially involves placing a sample in a magnetic field, applying a radio-frequency (RF) pulse, and determining the decrease in power (attenuation) of the radiation caused by the absorbing sample. This process is shown schematically in Figure 6.79. 

SateUite Transition MAS (SATRAS or STMAS), developed by Gan in 2000 [25], is an alternative approach to MQMAS for the acquisihon of high-resolution NMR spectra of quadrupolar nuclei. The principal advantage of SATRAS over MQMAS is that it is not dependent upon an efficient transfer of multiple-quantum coherences. like MQMAS, SATRAS is a 2D experiment performed under MAS conditions. The technique involves exciting the sateUite transitions in the spin manifold of quadmpolar nuclei using short radio frequency (rf) pulses. The second-order... 

The physical basis of current MRI methods has its origin in the fact that, in a strong magnetic field, the nuclear spins of water protons in different tissues relax back to equilibrium at different rates, when subject to perturbation from the resting Boltzmann distribution by the application of a short radio frequency (rf) pulse. For the most common type of spin-echo imaging, return to equilibrium takes place in accord with equation 1 and is governed by two time constants T and T2, the longitudinal and transverse relaxation times, respectively. 

The CRAMPS experiment puts a large demand on the NMR hardware (especially on NMR probes), since high-power radio frequency (rf) pulses are applied between each acquisition point. High homogeneity of the irradiation field, as well as careful setup of experimental variables, is required to avoid distortions of the proton peaks. Typical line widths afforded by the CRAMPS experiment are approximately 1 ppm, limiting the application of the CRAMPS experiment to compounds with a small number of well-resolved protons. 

Solid-state H CRAMPS NMR spectra were measured with a Chemagnetics CMX 300 spectrometer operating at 300 MHz. The internal standard was Si-rubber ((5 = 0.12) relative to (CH3)4Si (6 = 0). BR-24 and MREV-8 pulse sequences were applied. The radio frequency (RF) powers and duration windows (r) of these pulse sequences were adjusted so as to obtain the best resolution for adipic acid. 

In order to calculate the time evolution of a spin system and its manipulation by radio frequency (rf) pulses, it is conveniently described by a density operator a(t). This operator can be written in a basis of (21 -I- 1) operators Pi, which are related to the angular momentum operators 1., 1, [14]... 

The application of various multi-pulse methods in NQR requires a clear theoretical understanding of physical processes first of all behind the simplest "basic" multi-pulse sequences which include such a popular sequence as multi-pulse spin locking—MW-4, and a sequence of identically spaced coherent radio frequency (RF) pulses with intervals between them less than the time constant of free induction decay (FID) —strong off-resonance comb— SQRC. ... 

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