On-resonance pulses

For a spin whose chemical shift is exactly at the center of the spectral window, we call the pulse an on-resonance pulse because the pulse (or carrier ) frequency is exactly equal to the resonant frequency (precession frequency or Larmor frequency vG) of the spin. During the pulse, we can use the vector model to show the B field (the pulse) as stationary in the rotating frame of reference, because the x and y axes are rotating about the z axis at exactly the frequency of the pulse. The position of the B field in the x -y plane depends on the phase of the pulse, which is just the place in the sine function (0-360°) where the radio frequency oscillation starts at the beginning of the pulse. This can be controlled by the spectrometer and is written into the pulse sequence by the user ... 

Thus for an on-resonance pulse, the B0 field does not exist in the rotating frame and the effective field experienced by the spins is just the B field. The magnitude of the effective... 

The amplitude of an RF pulse can be expressed in units of telsa (Bi). This corresponds to the magnitude (length) of the B vector in a rotating-frame vector diagram. Pulse amplitude is most commonly expressed in terms of the frequency of rotation of sample magnetization as it precesses around the B vector (for on-resonance pulses) during the pulse. 

What value of is required to provide a 90° on-resonance pulse of 20 fJLs for 13C ... 

This is the usual magnetic resonance lineshape for transitions in a two-level system without damping. At resonance the population oscillates sinusoidally between the two states (this is known as Rabi oscillation). A n-pulse is an on-resonance pulse with 2bx = Tt, which transfers all the population from state (0) to state (c). In Section 15.4.3 we will discuss how this can be used in a molecular beam to map out the fields along the beamline. An on-resonance 7r/2-pulse 2bz = n/2) drives the transition only half-way, creating an equal superposition of states (0) and (c) with a definite relative phase. The density matrix element describing this coherence at the end of... 

Figure B2.4.6. Results of an offset-saturation expermient for measuring the spin-spin relaxation time, T. In this experiment, the signal is irradiated at some offset from resonance until a steady state is achieved. The partially saturated z magnetization is then measured with a kH pulse. This figure shows a plot of the z magnetization as a fiinction of the offset of the saturating field from resonance. Circles represent measured data the line is a non-linear least-squares fit. The signal is nonnal when the saturation is far away, and dips to a minimum on resonance. The width of this dip gives T, independent of magnetic field inliomogeneity.

Isotope shifts for most elements are small in comparison with the bandwidth of the pulsed lasers used in resonance ionization experiments, and thus all the isotopes of the analyte will be essentially resonant with the laser. In this case, isotopic analysis is achieved with a mass spectrometer. Time-of flight mass spectrometers are especially well-suited for isotopic analysis of ions produced by pulsed resonance ionization lasers, because all the ions are detected on each pulse. 

In our tip-enhanced near-field CARS microscopy, two mode-locked pulsed lasers (pulse duration 5ps, spectral width 4cm ) were used for excitation of CARS polarization [21]. The sample was a DNA network nanostructure of poly(dA-dT)-poly(dA-dT) [24]. The frequency difference of the two excitation lasers (cOi — CO2) was set at 1337 cm, corresponding to the ring stretching mode of diazole. After the on-resonant imaging, CO2 was changed such that the frequency difference corresponded to none of the Raman-active vibration of the sample ( off-resonant ). The CARS images at the on- and off- resonant frequencies are illustrated in Figure 2.8a and b, respectively. 

W. J. Goux, L. A. Verkruyse, S. J. Salter 1990, (The impact of Rayleigh-Benard convection on NMR pulsed-field-gra-dient diffusion measurements), J. Mag. Reson. 88, 609. 

The 13CO inversion pulse can be achieved by a PIP with the carrier placed at the centre of the 13C . This inversion PIP causes an additional procession of the 13C spins. The amount of the phase shift in the rotating frame of the 13CO, where the centre of the 13CO spins is on-resonance to the inversion PIP, can be calculated... 

The spectrum at the bottom of Fig. 16 is obtained with the double adiabatic decoupling pulse, one located at —23.2 kHz and the other at 23.2 kHz. The BSFS is compensated and sidebands are eliminated by the compensating pulse. In addition, the amplitude of the peak is higher than that in the middle, showing a better decoupling effect. Similar results were obtained for 13C off-resonance <5 ranging from —3 to 3 kHz, where < /A/<0.13 can be treated as close to on-resonance. 

To overcome these problems, a compensating PIP can be applied on the other side of the 13C region immediately after the first inversion pulse. The first pulse is a 180° pulse with a x phase and is on-resonance to the centre of the 13CO. As mentioned above, it can still be denoted as a PIP O0, 0°, 0.6 ps,... 

Irradiation of a sequence of very short resonant RF-pulses with a sum of pulse angles of 0° for on-resonant spins. Spin ensembles with long Tj show nearly full longitudinal magnetization after the pulse sequence. Spin ensembles with short T2 get severe transverse magnetization loss between and during the pulses and therefore is clearly reduced after application of the pulse sequence (e.g., Ref 44). 

The peak rf amplitude required to achieve optimum excitation with a selective excitation pulse is given in comparison to the rf amplitude required to achieve an on-resonance 90° flip-angle with a selective rectangular pulse, the simplest conceivable shape. 

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