Resonance condition frequency changes

If the sample is balhed in electromagnetic radiation of frequency v, then a transition between the (wo spin states, corresponding to the electron changing its component of spin, can take place providing the resonance condition ... 

In the adiabatic experiment, the density operator is prepared such that it is initially oriented along the starting effective Hamiltonian prepared to be far away from the resonance condition (Fig. 11.5 b). The direction of the effective Hamiltonian is then slowly changed (e.g. by a change in rf amplitude or frequency) to pass through the resonance condition to the final position, again far away from the resonance condition. If the change of the effective Hamiltonian is carried out adiabatically, the density operator will follow the trajectory of the Hamiltonian, and a (complete) polarization transfer can occur. The variation in the size of the effective Hamiltonian at the resonance condition only influences the condition for adiabaticity. Therefore, it is in principle possible to obtain si-... 

After the on-resonant imaging, the frequency of t 2 was changed snch that the frequency difference corresponds to none of the Raman-active vibrations ( off-resonant ). Figure 10.11 shows a normal Raman spectrum of the DNA in a part of the fingerprint region. The solid arrows on the spectrum denote the frequencies adopted for the on-resonant and off-resonant conditions in CARS imaging. 

ENDOR spectroscopy has proven to be a valuable technique to provide information on both free and protein bound flavin radicals. Since flavin radical ESR spectra can be partially saturated at moderate microwave power, ENDOR spectra may be observed as nuclear spin transitions by detection of changes in the partially saturated ESR signal as a function of nuclear radio frequency. The resonance condition for nuclei (when I = Vz) is described by the following equation ... 

The individual vectors p, before irradiation, are out of phase with one another and this can be represented by the vector M0 aligned in the Oz direction (Fig. 9.4). As the resonance condition is reached, all the vectors pack together and rotate in phase with B[. Hence, M0 changes direction and finally reaches an angle a with the Oz axis, which is controlled by the time and power of irradiation (Fig. 9.7). Thus M0 acquires an Mxv component in the horizontal plane that is maximum when a — 7t/2, while maintaining a component Mz in the direction of the Oz axis (except if a = tt/2). The frequency of rotation of the magnetisation vector is equal to that of the precession movement. Under these conditions, some nuclei will proceed to the second orientation allowed (in the case where I = 1/2). The system will slowly return to its original state after the irradiation is stopped. A coil is used to detect the component in the Oy direction (Fig. 9.8). 

The stabilization of the field-frequency ratio, which is required for HDMR experiments in the INDOR mode, can be obtained by locking the NMR spectrometer field-frequency ratio to the resonance condition of a strong and sharp signal in the sample under investigation. (14) Additional stabilization can be achieved by locking together the main frequency of the spectrometer and master oscillator of the synthesizer. (15, 16) In this case, the drift in the frequency of the synthesizer is balanced by the change in the main frequency of the spectrometer. 

The Raman intensity plotted against the exciting laser wavelength is called an excitation profile. Excitation profiles such as that shown in Fig. 3-21 of Chap. 3 provide important information about electronic excited states as well as symmetry of molecular vibrations. The intensity of a Raman line is maximized if strict resonance conditions are met (Section 1.15). When constructing excitation profiles, the frequency dependence of /(v) is of interest. It is difficult, however, to determine the / dependence on v from intensity changes because K and A also vary with v. 

CW experiments (sometimes called stationary or steady state ) are ones in which either no modulations are used, or they are so low in frequency that no spectral complications ensue. (This is only approximately the case if 100 kHz field modulation is employed. This frequency gives rise to modulation sidebands and, under saturating conditions, rapid passage effects.) Time-domain ESR involves monitoring the spin system response as a function of time. Pulse ESR can be divided into two broad categories the response of spin systems to sequences of microwave pulses (spin echo) and the response of spin systems to step changes in resonance conditions (saturation recovery). 

The method can only be used for product gases that are condensed quantitatively under reaction conditions these include water, carbon dioxide, and ammonia. Reactions can be studied only at low pressures and significant investigations of dehydrations using this technique have been published (23-25). This technique can alternatively use a quartz crystal with the reactant sample deposited on or otherwise attached to it and the mass loss monitored through resonance frequency changes as described earlier. 

The first procedure for locking is to establish the resonance condition for the deuterium signal, which involves altering either the field or the frequency of the lock transmitter. Of these two options the latter is preferred since it avoids the need for changing transmitter frequencies and is now standard on modem instruments. Beyond this, there are three fundamental probe-dependent parameters that need to be considered for optimal lock performance. The first of these is the lock transmitter power used to excite the deuterium resonance. This needs to be set to the highest usable level to maximise the signal-to-noise ratio but must not be set so high that it leads to lock saturation. This is the... 

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