Off-resonance pulses

We want also to stress that (as shown by Chiao and co-workers [32]) there is still another situation in which one expects e.m. wavepacket propagation in a medium at speed higher than c. This occurs for off-resonance pulses through a medium with inverted atomic populations. Experiments aimed at detecting superluminal propagation in such a kind of medium are presently being performed at Berkeley [4]. 

In impulsive multidimensional (1VD) Raman spectroscopy a sample is excited by a train of N pairs of optical pulses, which prepare a wavepacket of quantum states. This wavepacket is probed by the scattering of the probe pulse. The electronically off-resonant pulses interact with the electronic polarizability, which depends parametrically on the vibrational coordinates (19), and the signal is related to the 2N + I order nonlinear response (18). Seventh-order three-dimensional (3D) coherent Raman scattering, technique has been proposed by Loring and Mukamel (20) and reported in Refs. 12 and 21. Fifth-order two-dimensional (2D) Raman spectroscopy, proposed later by Tanimura and Mukamel (22), had triggered extensive experimental (23-28) and theoretical (13,25,29-38) activity. Raman techniques have been reviewed recently (12,13) and will not be discussed here. 

To confirm the demagnetization field hypothesis, we repeated the same experiments with presaturation pulses both on and off resonance for both samples (off-resonance pulses had no effect), and then proceeded to repeat all tests on a 500 MHz spectrometer equipped with a room temperature probe. Finally we moved to a different 500 MHz spectrometer located in a different building equipped with a 5 mm cold probe and repeated the experiments (data not shovm). The tests on the 500 MHz spectrometers confirmed that the lock perturbations were proportional to the spectrometer magnetic field strength, the presence of a high Q cooled-probe, and the concentration of H in the sample. Future examinations will... 

The case of off-resonance pulses is a little bit more involved, since now the axis of nutation is given by the effective magnetic field in Equation (2.3.6) and, therefore, is not contained in the transverse plane. The extent to which this axis is tilted out of the transverse plane and the sense of the tilt is determined by the frequency offset ( >l — 2). Also the nutation frequency is no longer given by co = y Bi, but depends on the offset. It can be shown that the rotation operator describing such off-resonance pulse can be written as a product of five rotations about orthogonal axes [5] ... 

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. 

Finally, we would like to point out that in the off-resonance region, the response time of the nonlinearity is limited only by the optical pulse width r, as long as (Ea -Tiaj)/h >>2ir(x ). (8) This is no longer true when collisions (or phonons in solids) are present. For optical frequencies close enough to the absorption edge, the collision induced transitions to the excited state will cause the x s response time to be limited by the relaxation time of the excited states. (8)... 

The energy imparted to the ions depends on the energy of the rf pulse and the duration of the pulse. The energy does not have to be raised in one event but may be absorbed by the ion in small portions. A technique called sustained off-resonance excitation (SORT) (82) uses a low-amplitude rf pulse that is off-resonance to the ion cyclotron frequency. The difference of the cyclotron frequency and the excitation frequency (-500 Hz) causes the ion to experience in- and out-of-phase excitation that has the effect of a repeated expansion and shrinkage of the cyclotron orbit. In this process, the ion undergoes a large number of low-energy collisions and the Ecom slowly increases until the ion dissociates. 

Bloch-Siegert phase shift is usually introduced in homonuclear nuclear decoupling with an inversion pulse that is far off-resonance to the spins in precession. To minimize the disturbance, the strength of the inversion pulse... 

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. 

Fig. 4. The HNCO-TROSY experiment for recording solely interresidual 1HN, 15N, 13C correlations in 13C/15N/2H labelled proteins. All 90° (180°) pulses for the 13C and 13C spins are applied with a strength of 2/ /l5 (p/ /3), where 2 is the frequency difference between the centres of the 13C and 13Ca regions. All 13Ca pulses are applied off-resonance with phase modulation by Q. A = 1/(4/hn) Tn = l/(4/NC ) S = gradient + field recovery delay 0 < k < TN/z2,max- Phase cycling i = y 4>2 = x, — x + States-TPPI 03 = x 0rec = x, — x.

Fig. 16. Cross-relaxation or Z-spectra derived from the solvent proton spectra of cross-linked bovine serum albumin gel supported in a ternary solvent system consisting of 8.8% H2O, 8.7% acetone, and 8.8% methanol and 63% D20.The offset axis represents the frequency offset of the off-resonance preparation pulse which partly saturates the immobilized spin. In this case the 3 s preparation had an amplitude of 880 Hz at a Larmor frequency of 500 MHz (87).

In this experiment a series of selective 180° pulses serves to individually label selected carbon spins prior to the series of C- and H-pulses used for refocusing and polarization transfer. Therefore the frequency of any of these initial 180° pulses is set either on-resonance to the resonance of carbon i (fi), or set off-resonance (/off-res.) which simply inverts (label —) or not inverts (label -f) the corresponding spin polarization respectively. In the example below three target spins with resonance frequencies f, fi and /3 are chosen. A series of three selective pulses has to be applied and at least four experiments have to be performed with the frequencies of the selected pulses set as shown in the acquisition scheme below and four separately... 

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