Interaction with the rf field

Very slow exchange. The lifetime at each site, 2r, is sufficiently long that a given nucleus enters site A and precesses many times at frequency (vA — v0) before leaving site A and entering site B. The result is that interaction with the rf field occurs, and in the fixed laboratory frame of reference a resonance line appears at vA. A similar situation occurs for the nucleus at site B. Thus the spectrum consists of two sharp lines at vA and vB, just as it would in the absence of exchange. (See Fig. 2.14[Pg.40]

Line broadening in solid-state NMR arises from spin interactions which can be described in first order by coupling tensors of rank two (cf. Section 3.1) [Hael, Mehl, Schl], The spin interactions are either linear or bilinear in the spin operator. Linear interactions are the Zeeman interaction, the chemical shielding, and the interaction with the rf field. Bilinear interactions are the J coupling, the dipole-dipole coupling, and the quadrupolar interaction. In isotropic materials like powders, glasses, and undrawn polymers, wide lines are observed as a result of an isotropic orientational distribution of coupling tensors. 

When considering the study of redistribution reactions, one must remember that NMR spectra are extremely sensitive to the rates of the processes that cause the exchanges. For very slow exchange, the lifetime at each site is long with reference to the interaction with the rf field thus the spectrum will consist of two sharp lines as it would in the absence of exchange. For moderate... 

Nonoptical methods can be used to probe ground and metastable excited states as well as short-lived levels. One of these techniques, for example, utilizes the backward part, i.e., the B magnet of a classical Rabi apparatus, to deflect molecules after laser excitation and interaction with the rf field, and leaves the other particles unaffected, or vice versa. Another method, which has been applied rather frequently since it provides a high sensitivity, is the detection of ions or photoelectrons after state selective ionization of atoms or molecules. This can be accomplished by photoionization or by field ionization, where the latter applies particularly well to very highly excited states, i.e., to Rydberg atoms. 

Many numerical simulations have been performed for understanding in more detail the process of collisional relaxation in rf traps. A summary can be found in Ref. 7. In principle, an ion can always gain energy if it undergoes a collision during its interaction with the rf field. This effect depends strongly on the mass ratio. For example it is much easier to cool heavy ions in a light buffer gas than the other way around. Very problematic... 

Fig. 2 (A) Macroscopic equilibrium magnetization Mq from the vector sum of the individual nuclear moments fi (B) magnetization interacting with Bi rf field in the laboratory reference frame and (C) magnetization interacting with Bi field in the rotating reference frame.

If the carrier frequency of the RF pulses is close to one of the resonance transitions of the irradiated sample, the effect of the multi-pulse sequence leads to the establishment of a new time independent effective Hamiltonian Heff, now also containing the interaction of nuclear-spins with the RF field (that the effective Hamiltonian is time independent should be imderstood that it is independent of the number of the pulse interval but not independent of the time inside the definite pulse interval). A part of the dipole Hamiltonian Ha no longer commutes with the new effective Hamiltonian, which after the T2 time results in heat mixing of the quadrupole reservoir and the reservoir of the components of... 

The principle of the ENDOR method is illustrated in Fig. 1. It refers to the most simple spin system with an electron spin S = 1/2 and a nuclear spin I = 1/2 for which an isotropic hf interaction, aiso, is considered. In a steady state ENDOR experiment4, an EPR transition (A, D), called the observer, is partly saturated by microwave radiation of amplitude B while a driving rf field of amplitude B2, called the pump, induces nuclear transitions. At frequencies vj and v2, the rf field tends to equalize the populations within the ms-states. This alters the degree of saturation of the observer so that, in the display of the EPR signal height versus the radio frequency, two ENDOR lines at transition frequencies vj = aiso/2 - vn (A, B) and v2 = ais0/2 + v (C, D) will be observed (v = / NgnBo denotes the nuclear Zeeman frequency for a static field B0). 

The NMR phenomenon is based on the magnetic properties of nuclei and their interactions with applied magnetic fields either from static fields or alfemaling RF fields. Quanfum mechanically subatomic particles (protons and neutrons) have spin. In some nuclei these spins are paired and cancel each other out so that the nucleus of the atom has no overall spin that is, when the number of protons and neutrons is equal. However, in many cases the sum of the number of protons and neutrons is an odd number, giving rise to... 

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