Magnetic nonresonant

For the case of muonium, nonresonant spin precession in a magnetic field provides a copious source of information about its crystallographic sites and the associated unpaired electron distribution around them (see Chapter 15). Here, the concentration of muons is always too low for molecule formation, and migration to impurities and implantation defects can be kept small by the short muon lifetime and use of pure material and low temperature. 

The quantum theory of molecular collisions in external fields described in this chapter is based on the solutions of the time-independent Schrodinger equation. The scattering formalism considered here can be used to calculate the collision properties of molecules in the presence of static electric or magnetic fields as well as in nonresonant AC fields. In the latter case, the time-dependent problem can be reduced to the time-independent one by means of the Floquet theory, discussed in the previous section. We will consider elastic or inelastic but chemically nonreac-tive collisions of molecules in an external field. The extension of the formalism to reactive scattering problems for molecules in external fields has been described in Ref. [12]. 

Derivative Detection of EPR Transition. The EPR spectrum is usually displayed as the first derivative of the absorption y"(H), because the nonresonant low-frequency and low-amplitude RF modulation (co1/ 2% = typically 100 kHz) applied to the coils near the magnet is detected by a rectifier in addition to the drop in microwave power level due to the RF resonant absorption (typically co0/27c = 9.1 GHz if H0 = 0.34T) The signal is processed by a phase-sensitive circuit, which detects a back-and-forth sweep across resonance in small magnetic field increments (relative to the DC field and to the width of the measured spectrum), thus generating a response df /dH (see Fig. 11.60). 

The detection of energy at this transition frequency is the basis of NMR spectroscopy. The actual detection of NMR signals, however, is made possible through the bulk magnetization (AT) of the nuclear system that arises from the resultant of the individual nuclear magnetic moments that are distributed between the various energy levels. The rotating components (x and y) of p transverse to the direction (z) of B0 at nonresonant equilibrium have no phase coherence and A7x = My = 0,... 

Hess [77] prepared this polymer and studied its photoluminescence, optically detected magnetic resonance (ODMR) and nonresonant magnetic field effects. 

From early on it has been recognized that low-field nonresonant microwave absorption can be used to characterize the high-temperature superconductors. Type-II superconductors can be penetrated by the magnetic field via flux vortices, and thermal excitations of these vortices are responsible for microwave absorption. Hence, this kind of nonresonant EPR spectroscopy can provide some information on the vortex dynamics and on the lower critical field Hc A number of EPR experiments focusing on the low-field signals have been reported on HTSC. The nonresonant absorption yields a broad peak in the field derivative of the absorption speetrum, sometimes an additional fine structure has been observed, and in all experiments strong hysteresis effects were noted. 

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