Pulse, decay

As stated before, the initial perturbation should be maximal with respect to the equilibrium state. Since we are dealing with transverse magnetization here, this maximal perturbation is obviously a 90° pulse. However, it can be immediately noticed that signals collected after a simple read-pulse, decay exponentially according to a time constant which differs from the genuine T2 by a contribution due to the static induction Bq inhomogeneity ... 

Because the induced NMR signal after switching off the 90° RF pulse decays so rapidly as mentioned above, spin echoes are often detected in many practical pulse sequences for MRI visualization. Spin echo is formed by an additional RF pulse applied to the sam-ple.The additional RF pulse is applied after an evolution period, r. This RF pulse that gives rise to inversion of the magnetization components causes the spins to rephrase and is thus referred to the 180° RF pulse as shown in Fig. 2. This rephrasing process contributes to recover the transverse magnetization which had lost in dephasing process and results in the formation of an echo . We detected the echo by the receiver coil. The time from when the 90° RF pulse is applied to when the echo forms is referred to as the echo time , TE and is equal to twice the time between the 90° and 180° pulses, i.e. 2r. 

Because of the fascination of synthetic organic chemists and molecular electronics device designers with ever-increasing charge mobilities, attention was focussed mainly on the magnitude of the end-of-pulse conductivity of PR-TRMC transients. Because of this, the after-pulse decay kinetics in discotic materials received only scant attention. The dramatic influence of the nature of the peripheral chains on the lifetime of the PR-TRMC conductivity transients, was in fact demonstrated early-on for octa-alkoxy phthalocyanine derivatives as mentioned previously in this section. This effect is illustrated with more recent data for some hexa-alkyl HBC derivatives in Fig. 7. 

Emission decays were obtained from a fs Tl sapphire laser uorescence upconversion spectrometer whose construction is reported elsewhere [8]. Here we only note that the overall temporal response used in these studies was between 112-125 fs as measured by the FWHM of the cross correlation between the pump and gate pulses. Decays (0-200 ps with a variable step size) were collected at a series of ten emission wavelengths (8 nm bandpass) which were then used to reconstruct time-evolving emission spectra in the manner described in Refs. 8 and 9. From these spectra the solvation dynamics was extracted in the form of the spectral response function,... 

Convective renewal. When normal pulse voltammetry is carried out in a convective system, as at a rotating disk, one can rely on stirring to renew the diffusion layer while the potential is held at E. This can be true even if the chemistry cannot be reversed electrolytically, as in the case where the species created in the pulse decays to an inactive product. The convection can also affect the current sampled in each pulse, so that the theoretical expectation based on diffusion theory is exceeded. However the error is often either irrelevant (as in analytical applications where calibration is possible) or fairly small (because a pulse of short... 

I rf 1 (Free Induction i Delay Pulse Decay) if used... 

Rates of Formation of Transient Species. Figure 14 shows the increase in transient absorption in N20 saturated solutions of thymine and DNA following exposure to a 40 nsec, radiation pulse. Decay of electron... 

A novel suggestion by Hoult (1979) to make the transmitter pulse decay rapidly is to reverse the phase of the end of the transmitter pulse in order to cancel the ringing. This is attractive because no additional circuitry is needed. Most modern spectrometers already have the capability of phase... 

However, ASL suffers from several intrinsic technical limitations. First, the change in spin orientation that is produced by the labeling inversion pulse decays over time as a result of T1 relaxation. Because T1 relaxation is rapid in comparison to the time it takes for blood to flow from an inverted slice to an imaged slice of the brain, the signal changes that remain present by the time the spins arrive in the brain are small in magnitude, and perfusion maps produced by ASL are noisier than those that can be achieved by techniques that use exogenous contrast. This rapid decay of spin... 

Selected scaled ks results have been presented in Table VII. A more extensive listing follows in Appendix B. Many thermal hydrogen abstraction and olefinic addition reactions have been tabulated that are too fast to be characterized using the available direct measurement methods. Based on comparisons with the scaled MNR data, the HF chemiluminescence (52), electron spin resonance (53,66), and laser pulse decay (54,67), absolute teclmiques apparently involve large systematic errors. 

For computer applications, the rate at which data can be transmitted determines the speed of computer performance. This is determined by the time constant of the electronics that describes the rate at which an electrical pulse decays that determines the possible interval between pulses. This time constant is proportional to the capacitance of the circuits that relates to the dielectric constants of their components. For this purpose, polymers are attractive, since they generally have lower dielectric constants than their non-polymeric alternatives. For this consideration, knowledge of the relationship between dielectric constant and molecular structure is essential. 

The two-pulse echo decay is sometimes too fast to obtain a satisfactory frequency spectrum after Fourier transformation. In this case the three-pulse sequence shown in Fig. 2.21(a) is an alternative. It gives rise to a stimulated echo at time r after the third k/2 pulse. The decay rate is limited by the electron spin-lattice relaxation time Ti, which is usually longer than the phase memory relaxation time Tm for the two-pulse decay. 

It can be seen that the pulse decay strongly depends on cycle time (tperi d) and reactor bed length. If the periodic time is very long the system reaches a so-called quasisteady state in periodic operation and very short periods (see curve (a) in Fig. 4.9) lead to a merging of pulses. The choice of pulsing frequency can be influenced, e.g. by pulse attenuation characteristic, mass storage in catalyst particles and reaction kinetics. 

Muon beams used in pSR can be distinguished by their time structure (pulsed vs. continuous) and also by the muon momentum or energy (surface vs. decay-channel beam lines). The two differing features, time structure and muon energy, are completely independent of each other. There exist, for example, pulsed surface beams and pulsed decay-chaimel beams. Their different properties render the various beam types advantageous for different applications. We discuss first the time structure. 

Temperature Pulse Decay (TPD) Technique. Temperature pulse decay (TPD) technique is based on the approach described and developed by Arkin, Chen, and Holmes [Arkins et al., 1986 1987], This method needs no insulation, in contrast to some of the methods described above, since testing times are short, usually on the order of seconds. However, the determination of the thermal conductivity or the blood flow rate requires the solution of the transient bioheat transfer equation. 

The temperature pulse decay technique has been used to measure both the in vivo and in vitro thermal conductivity and blood flow rate in various tissues (Xu et al., 1991 1998). The specimen does not need to be cut from the body, and this method minimizes the trauma by sensing the temperature with a very small thermistor bead. For the in vitro experimental measurement, the measurement of thermal conductivity is simple and relatively accurate. The infinitively large tissue area surrounding the probe implies that the area affected by the pulse heating is very small in comparison with the tissue region. This technique also requires that the temperature distribution before the pulse heating should reach steady state in the surrounding area of the probe. 

Temperature Pulse Decay Technique. As described in Sec. 2.4 under Temperature Pulse Decay (TPD) Technique, local blood perfusion rate can be derived from the comparison between the dieoretically predicted and experimentally measured temperature decay of a thermistor bead probe. The details of the measurement mechanism have been described in that section. The temperature pulse decay technique has been used to measure the in vivo blood perfusion rates of different physical or physiological conditions in varimis tissues (Xu et al., 1991 1998). The advantages of this technique are that it is fast and induces little trauma. Using the Pennes bioheat transfer equation, the intrinsic thermal conductivity and blood perfusion rate can be simultaneously measured. In some of the applications, a two-parameter least-square residual fit was first performed to obtain the intrinsic therm conductivity of the tissue. This calculated value of thermal conductivity was then used to perform a one-parameter curve fit for the TPD measurements to obtain the local blood perfusion... 

Arkin, H., Holmes, K. R., Chen, M. M., and Bottje, W. G., 1986, Thetmal Pulse Decay Method for Simultaneous Measurement of Local Thermal Conductivity and Blood Perfusion a Theoretical Analysis, ASME Journal of... 

In contrast to the case of Se, the hole transit pulse in As2Se3 shows a long tail and there is often no discontinuity in the pulse discernable. Scharfe (1970) observed a break in the transient pulse only when he used Au contacts with evaporated As Seg. Al contacts in contrast yielded a transit pulse decaying without structure. Using Au contacts and taking the breakpoint as a measure of the transit time T Scharfe obtained a drift mobility for holes which increases linearly with field. A hole drift mobility of 4 X... 

Pulse decay time The time required for the trailing edge of a pulse to decrease from 90 percent to 10 percent of its peak amplitude. 

Fig. 8.18 Bond order flux (arrows) and bond order density (contour lines) plotted on xz plane induced by a pulse laser circularly polarized on this plane. The central frequency tc and field strength Eg shined are 0.057 and 0.03, respectively. The peak time and the characteristic time of pulse decay tn, are 4.84 fs and 2.42, respectively. The solid and dotted contour lines denote the positive and negative bond order densities, respectively. The increment of the contour lines is 0.01. Snapshot times are (a) 3.25 fs (b) 4.00 fs (c) 4.50 fs (d) 5.25 fs. The positions of all atoms are projected onto the the xz plane. The overlapped H atoms outside the B-B bonds are not shown. The flux vectors are multiplied by 20 for presentation. (Reprinted with permission from T. Yonehara et al, Chem. Phys. 366, 115 (2009)).

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