Soft excitation

This experiment requires a well-separated proton resonance for excitation rectangular pulses are used for the soft excitation. 

The phenomenon of hard excitation (Minorsky, 1962) required in these conditions for the evolution to the limit cycle should be contrasted with the soft excitation that characterizes the spontaneous evolution towards such a cycle, starting from an unstable steady state (see, for example, fig. 2.13). The interest of reversible transitions between a stable steady state and an oscillatory regime stems from the fact that such behaviour is observed in some nerve cells (Best, 1979 Guttman, Lewis Rinzel, 1980). 

MRI (ID profiles) of objects with dipolar-broadened spectra was demonstrated by using soft excitation pulses. Long-lived coherent response signals... 

Proton NMR peaks produced in liquid crystals by soft excitation pulses are considerably narrower than what can be achieved with multi-pulse decoupling, and such signals can be used for very accurate measurement of coefficients of selfdiffusion. With this type of excitation, the radio-frequency power is at least six orders of magnitude smaller than an average power needed for homo- or hetero-nuclear proton decoupling. Therefore, the radio-frequency heating of the sample does not affect the measurements. 

GRADIENT-ECHO WITH SOFT EXCITATION PULSE... 

Figure 6-3. A gradient-echo pulse sequence with soft excitation pulse for measuring coefficient of self-diffusion in liquid crystals...

XPS is also often perfonned employing syncln-otron radiation as the excitation source [59]. This technique is sometimes called soft x-ray photoelectron spectroscopy (SXPS) to distinguish it from laboratory XPS. The use of syncluotron radiation has two major advantages (1) a much higher spectral resolution can be achieved and (2) the photon energy of the excitation can be adjusted which, in turn, allows for a particular electron kinetic energy to be selected. 

Figure C3.3.8. A typical trajectory for a soft collision between a hot pyrazine molecule and a CO2 bath molecule in which the CO 2 becomes vibrationally excited.

The distortion caused by the field allows an electron to pass from the molecule to the tip if the applied potential is positive or from the tip to the molecule if the potential is negative. This is called field ionization (FI), and the electron transfer occurs through quantum tunneling. Little or no vibrational excitation occurs, and the ionization is described as mild or soft. 

Precisely controllable rf pulse generation is another essential component of the spectrometer. A short, high power radio frequency pulse, referred to as the B field, is used to simultaneously excite all nuclei at the T,arm or frequencies. The B field should ideally be uniform throughout the sample region and be on the order of 10 ]ls or less for the 90° pulse. The width, in Hertz, of the irradiated spectral window is equal to the reciprocal of the 360° pulse duration. This can be used to determine the limitations of the sweep width (SW) irradiated. For example, with a 90° hard pulse of 5 ]ls, one can observe a 50-kHz window a soft pulse of 50 ms irradiates a 5-Hz window. The primary requirements for rf transmitters are high power, fast switching, sharp pulses, variable power output, and accurate control of the phase. 

Assume that we have a pendulum (Fig. 6-14) provided with a piece of soft iron P placed coaxially with a coil C carrying an alternating current that is, the axis of the coil coincides with the longitudinal axis OP of the pendulum at rest. If the coil is excited, one finds that the pendulum in due course begins to oscillate, and th oscillations finally reach a stationary amplitude. It is important to note that between the period of oscillation of the pendulum and the period of the alternating current there exists no rational ratio, so that the question of the subharmonic effect is ruled out. 

The symmetries of the lowest excited states listed in Table 1 are nothing but the symmetries to which the most soft second-order bond distortions belong. It is seen that the types of symmetry reduction predicted using the symmetry rule are in complete agreement with those obtained on the basis of the dynamic theory. 

In order to seek the most soft nuclear deformation in an excited state, the approximation is again made of replacing the sum over excited states in Eq. (17) by a dominant term corresponding to the next higher excited state. Now, the transition density p between the nth excited state corresponding to the orbital jump and the mth... 

The symmetry of the most soft distortion in the lowest excited state is given by the direct product of the symmetry of the first excited state (shown in Table 1) and that of the second excited state (shown in Table 2). These symmetries are b3g(R ) for 1 and VII 2(1 ) for XVII and IV- hi (z) for XXI and XXIII, and fli(z) for XXII. The symmetries of the lowest excited states are then predicted to be Cj, Q, and C2 , respectively. It should be noted that despite the strong vibronic coupling with the second excited state, the first excited state of sesqui-fulvalene (XXII) does not undergo a symmetry reduction. 

The study of the mechanical properties of filled elastomer systems is a chaUenging and exciting topic for both fundamental science and industrial application. It is known that the addition of hard particulates to a soft elastomer matrix results in properties that do not follow a straightforward mle of mixtures. Research efforts in this area have shown that the properties of filled elastomers are influenced by the nature of both the filler and the matrix, as well as the interactions between them. Several articles have reviewed the influence of fiUers hke sihca and carbon black on the reinforcement of elastomers.In general, the strucmre-property relationships developed for filled elastomers have evolved into the foUowing major areas FiUer structure, hydrodynamic reinforcement, and interactions between fiUers and elastomers. 

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