Two-pulse sequences

Figure Al.6.30. (a) Two pulse sequence used in the Tannor-Rice pump-dump scheme, (b) The Husuni time-frequency distribution corresponding to the two pump sequence in (a), constmcted by taking the overlap of the pulse sequence with a two-parameter family of Gaussians, characterized by different centres in time and carrier frequency, and plotting the overlap as a fiinction of these two parameters. Note that the Husimi distribution allows one to visualize both the time delay and the frequency offset of pump and dump simultaneously (after [52a]).

The most popular, and also a very accurate, experimental method for measuring nonselective spin-lattice relaxation-rates is the inversion recovery (180°-r-90°-AT-PD)NT pulse sequence. Here, t is the variable parameter, the little t between pulses, AT is the acquisition time, PD is the pulse delay, set such that AT-I- PD s 5 x T, and NT is the total number of transients required for an acceptable signal-to-noise ratio. Sequential application of a series of two-pulse sequences, each using a different pulsespacing, t, gives a series of partially relaxed spectra. Values of Rj can... 

Figure 1. Schematic illustration of two-pathway control in the (a) frequency and (b) time domains. In case (a) the ground state is excited to a coupled continuum by either one photon of frequency CO3 or three photons of frequency C >i. Control is achieved by introducing a phase lag between the two fields. In case (b) a two-pulse sequence has sufficient bandwidth to excite a superposition of two intermediate states. Control is achieved by introducing a delay, At, between the pulses, resulting in a phase difference of to At.

A solution of cinnamic aldehyde 4 was used to test the two pulse sequences, the results of which are shown in fig. 6. Three frequencies were selected to simultaneously perturb Cl, C3 and Cl for experiment IVa and Cl, C4 and Cl for experiment IVb to correlate directly and long-range coupled spins respectively. To minimize a decrease in sensitivity due to relaxation effects during the pulse cascade, the frequencies in the pulse train were ordered according to their estimated spin-lattice relaxation times. The... 

In general, the results of the calculations establish that it is possible to guide the reaction to preferentially form one or the other product with high yield. Note that, unlike the original Tannor-Rice pump-dump scheme, in which the pulse sequences that favor the different products have different temporal separations, the complex optimal pulses occupy about the same time window. Indeed, the optimal pulse shape that generates one product is very crudely like a two-pulse sequence, which suggests that the mechanism of the enhancement of product formation in this case is that the time delay between the pulses is such that the wavepacket on the excited-state... 

Finally, we want to emphasize an interesting result of the numerical calculation that has been proven experimentally. As shown in Fig. 6, there exist two pulse sequences (b, c) leading to a small population of level 2. In case (c) most of the population is transferred to level 3 while in case (b) nearly all of the population remains in the initial level. In coherent ion dip experiments case (b) is used as it provides deeper dips due to the more effective suppression of ionization. Using higher laser intensities would allow us to achieve nearly 100% ion dips also in case (c) however, for off-resonant conditions the ion current would be smaller by an order of magnitude than in the pulse sequence of case (b) and dips are more difficult to detect. 

From a frequency domain point of view, a femtosecond pump-probe experiment, shown schematically in Fig. 1, is a sum of coherent two-photon transition amplitudes constrained by the pump and probe laser bandwidths. The measured signal is proportional to the population in the final state Tf) at the end of the two-pulse sequence. As these two-photon transitions are coherent, we must therefore add the transition amplitudes and then square in order to obtain the probability. As discussed below, the signal contains interferences between all degenerate two-photon transitions. When the time delay between the two laser fields is varied, the... 

The ESE method was used to identify the overlapping ESR spectra of the irradiated PMMA more definitely. The measurements of the ESE-detected ESR were made at 77 K with the 90° — t — 180 two-pulse sequence, at various fixed times of longitudinal relaxation, tl5 while the external magnetic field was swept slowly. The t( -resolved ESR spectrum was obtained from the difference between the echo intensity at a fixed x of 0.5 ps with and without a 90° saturation pulse... 

Several pulse methods were developed for estimation distances between two slowly-relaxing spins. In a pulse electron-electron double resonance (PELDOR) technique a spin echo is created by a two-pulse sequence at one microwave frequency. The timing of a pulse at a second microwave frequency is varied (Milov et al., 1998). This method is suitable for analysis of weak dipolar interactions. 3-pulse PELDOR with all three pulses at the same microwave frequency ( 2 + 1 sequence) was proposed by Raitsimling and his co-workers (2000). A specific feature of the 2 + 1 technique is suppression of dipolar interaction of randomly distributed spins, which allows the selection of a dipolar interaction between radicals. Using a 4- pulse experiments it was possible to eliminate an inherent dead experimental deadtime that limits the magnitude of the dipolar interaction in 2 + 1 sequence and in 3-pulse ELDOR experiments (Pannier et al., 2000). 

Fig. 11 a-d Pulse sequences and coherence transfer pathways for the 2D STMAS experiment a two-pulse sequence as described by Gan b amplitude modulated z-filter acquisition scheme c phase-modulated shifted-echo experiment d split-ti shifted-echo experiment. The values of k, k and k" are chosen to refocus the second-order quadrupolar broadening at the end of the period... 

The spins are then left to relax during the pulse delay time (PD) back to full thermal equilibrium with the lattice and then the same two-pulse sequence is applied, but with a different value for the interval(t) between them. This process, depicted schematically below, is then repeated for an appropriate number of... 

Stage 1 involves first the determination of the Ri-value of the protons (A) of interest by the two-pulse sequence (10) referred to earlier, in which the perturbing, 180°-pulse is applied in a non-selective fashion to all the proton resonances (we shall refer to this value as the non-selective relaxation rate (ns)). Then the two-pulse sequence is repeated, except that now the 180°-pulse is applied selectively (11), (12) to just the A-resonance, and the recovery back to thermaT equTTibrium of the magnetisation of A is monitored with the usual non-selective 90° pulse this gives a new relaxation rate constant, the single-selective relaxation rate (R ()C) where Vindicates the nucleus which has been subjected to the single-selective 180° pulse). 

Fig. 9.2.8 Tz parameter imaging with a surface coil, (a) The phantom consists of three sheets from poly(urethane) rubber. A magnetic-field gradient was applied perpendicular to the surface coil for space encoding along the depth axis y. (b) A two-pulse sequence with flip angles a and 2a, respectively, was used for excitation, (c) Stack of ID images for different echo times fg = 2r. (d) 7) parameter image extracted from the signal decays in c.

Two pulse sequences were used to characterize hydrogen cyanide (HCN) polymers. Characterization of polymerization products of HCN is of great... 

The two-pulse sequence is not the only sequence which leads to the formation of an echo. A pulse... 

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