Superposition, double-shifting

The double-resonance data (Fig. 40a) show that the signal from Pt in the surface of the particles is symmetric around a center position of approximately 1.096 G/kHz. This is an inhomogeneous linewidth there are many different types of surface Pt, each resonating at a slightly different frequency, and the observed signal is an unresolved superposition of all these elementary resonances. In the NMR layer model (Fig. 48), this idea is expanded further it is supposed that the difference in resonance frequency, and therefore also in LDOS, between all elementary surface resonances is less than the difference between typical surface and subsurface resonances. The important distribution of surface LDOS values has its equivalent in the MAS-NMR of adsorbed CO the MAS does not narrow the signal (Figs. 35c and 35d) because its width is due to a distribution of isotropic shifts rather than to shift anisotropies. 

The sum is as a pure sine wave at the fundamental frequency, phase-shifted, with double amplitude when (p = 0° and zero amplitude with

[Pg.265]

---