SPREAD pulse

Fig, 3.3.9 INil 1) SPREAD pulses for saturation of longitudinal magnetization, (a) Saturation profile. Magnetization in the selected slice is to be preserved, (b) Efficiency of the DIGGER pulse. The response to a nonselcctive pulse applied in a c gradient is. shown as well as the same response preceded by a DIGGER pulse, (c) Time-domain profile of SPREAD pulse. 

Flow would the width of a spreading pulse change if the diffusion coeffieient doubled ... 

The fascinating aspect of this apparatus is the way in which the diffusion coefficient appears. Equation 5.6-8 has the same mathematical form as Eq. 2.4-14, but the dispersion coefficient E replaces the diffusion coefficient. So far, as good. However, E varies inversely with D, as explained in Section 4.4. Consequently, a widely spread pulse means a large E and a small D. A very sharp pulse indicates small dispersion and hence fast diffusion. 

CN] —> I + CN. Wavepacket moves and spreads in time, with its centre evolving about 5 A in 200 fs. Wavepacket dynamics refers to motion on the intennediate potential energy surface B. Reprinted from Williams S O and lime D G 1988 J. Phys. Chem.. 92 6648. (c) Calculated FTS signal (total fluorescence from state C) as a fiinction of the time delay between the first excitation pulse (A B) and the second excitation pulse (B -> C). Reprinted from Williams S O and Imre D G, as above. 

The critical requirements for the ion source are that the ions have a small energy spread, there are no fast neutrals in the beam and the available energy is 1-10 keV. Both noble gas and alkali ion sources are conunon. Por TOP experunents, it is necessary to pulse the ion beam by deflecting it past an aperture. A beam line for such experiments is shown in figure B1.23.5 it is capable of producing ion pulse widths of 15 ns. 

The technique just described requires the porous medium to be sealed in a cell, so It cannot be used with pellets of irregular shape or granular material. For such materials an alternative technique Introduced by Eberly [64] is attractive. In Eberly s method the porous pellets or granules are packed into a tube through which the carrier gas flows steadily. A sharp pulse of tracer gas is then injected at the entry to the tube, and Its transit time through the tube and spreading at the exit are observed. A "chromatographic" system of this sort is very attractive to the experimenter,... 

From the Heisenberg uncertainty principle as stated in Equation (1.16) estimate, in cm and Hz, the wavenumber and frequency spread of pulsed radiation with a pulse length of 30 fs, typical of a very short pulse from a visible laser, and of 6 ps, typical of pulsed radiofrequency radiation used in a pulsed Fourier transform NMR experiment. 

An important consequence of shortening a laser pulse is that the line width is increased as a result of the uncertainty principle as stated in Equation (1.16). When the width of the pulse is very small there is difficulty in measuring the energy precisely because of the rather small number of wavelengths in the pulse. For example, for a pulse width of 40 ps there is a frequency spread of the laser, given approximately by (2 iAt), of about 4.0 GFIz (0.13 cm ). 

In most ultrasonic tests, the significant echo signal often is the one having the maximum ampHtude. This ampHtude is affected by the selection of the beam angle, and the position and direction from which it interrogates the flaw. The depth of flaws is often deterrnined to considerable precision by the transit time of the pulses within the test material. The relative reflecting power of discontinuities is deterrnined by comparison of the test signal with echoes from artificial discontinuities such as flat-bottomed holes, side-drilled holes, and notches in reference test blocks. This technique provides some standardized tests for sound beam attenuation and ultrasonic equipment beam spread. 

Were these spreads not so likely to remain large, one could look forward to a replacement of crystals by pulse-height selectors as a means... 

The second term corresponds to a simple delay and can be generated by air propagation. The third and higher terms are specific of propagation through material. They induce pulse spreading for optical telecommunications and degradation of the interferences contrast in the frame of interferometry. 

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