Korringa spin lattice relaxation

The spin-lattice relaxation rate, T -1, is due to modulation of the hy-perfine interaction by the electronic spin motion of the conduction electrons and therefore probes the electron dynamics. The (/+S, + I-Si+) term in Hc induces transitions of the nucleus, giving the Korringa relation [21] for a noninteracting electron gas ... 

This is clear for a macroscopic (even if not semi-infinite) crystal, but we may wonder about the limit in size to which it will remain valid What is the work function of a nanocrystal For a molecule, there is a difference between the amount of work required to pull an electron off (the ionization energy) and the work gained by pushing an extra electron on (the electron affinity). As a condition for the application of the work function concept we may require that the ionization energy of the system be reasonably nearly equal to its electron affinity (it is of course easier to propose the criterion than to verify it experimentally). In this chapter, I frequently use the existence of a Korringa-type of spin lattice relaxation, or that of a conspicuous Knight shift, as a criterion for metalhc behavior, but this is not quite the same as the ionization/affinity criterion. 

The temperature independence of the Korringa product T T is a better indicator of the metallic character of an NMR signal than the value of its shift. In most metal hydrides, the spin lattice relaxation at room temperature contains important contributions from the diffusive motion of the proton in the hydride lattice. To investigate the Korringa product one must work at relatively low temperatures for the palladium hydrides only values of x > 0.65 are accessible (Fig. 20). For x = 1, 7 T = 46 sK (67), but no values have been given for the shift. Supposing that the total shift 5(1) can be derived from Eq. (21) with the parameters given previously for the palladium black, we obtain 5(1) 51 ppm. From the Ti T value... 

Fk . 22. Spin lattice relaxation rate Tl of II in bulk Pdll, with x in the 0.7 to 0.8 range as a function of temperature and for several Larmor frequencies vo The straight line indicates a temperature-independent Korringa product TiT. characteristic of metallic behavior there is a nonzero LDOS on the H at the Fermi energy, in qualitative agreement with Fig. 21b. [Reproduced with permission from Schoep el al (68). Copyright 1974 Filsevier Science.]... 

This result, seems to indicate the only Korringa-typc spin lattice relaxation for adsorbed hydrogen reported so far. If we assume that it is due to contact interaction with the spins of s-like electrons (by using reasoning similar to that apphed for PdH in Section lll.B), its value gives 200 ppm and places the zero of the Knight shift scale at 115 ppm. Whereas such values cannot be ruled out, it is more hkely that this one-component analysis of shift and relaxation is insufficient. 

Fig. 45. Time/temperature scaling of spin lattice relaxation curves in a saturation-recovery experiment. At each temperature the raw amplitude data arc first normalized by the amplitude of the fully relaxed signal so that all values fall between zero (saturation at short times) and 1 (lull recovery at long times). The time points t are multiplied by the temperature at which the relaxation curve was obtained. If the relaxation is governed by the Korringa process, the scaled points fall on a temperature-independent curve, even if the relaxation is not simply exponential, as in the cases shown here. The sample is Pt/Ti02 of dispersion 0.60 (determined by electron microscopy) at several hydrogen coverages (calculated from the dispersion) 0.1, 0.5, and 1.0 monolayers. The squares in c show data at 110 K for another Pt/TiOi catalyst of dispersion 0.36.

Fig. 54, Korringa relationship for the Pt spin lattice relaxation in the surface peak of the spectrum. The straight lines show that the platinum surface keeps its metallic character,...

Fig, 59. Kffect of alkali impregnation on Pt spin lattice relaxation for Pt/TiOj. (a) Spin lattice relaxation time across the NMR spectrum for several clean-surface (open symbols) and alkali-impregnated (solid symbols) samples at 80 K. The changes are important near f.lO G/kFFz (the surface signal) and undetectable at 1.13 G/kHz. (b) Korringa relationship for the spin lattice relaxation at the surface peak of the spectrum. The alkali impregnation does not change the metallic character but increases the Ef 1. DOS on the metal surface. ( An extrapolation of earlier clean-surface data, obtained at lower temperature, is shown by the dashed line.)... 

The Korringa relationship [21] indicates thatl/Ti oc T, where T is the spin—lattice relaxation time and T is the absolute temperature of the sample. This unique temperature dependence of l/Ti is the NMR fingerprint of a metallic state. It results from the fact that only conduction electrons around the Fermi level can satisfy energy conservation for the electron-nuclear spin flip-flop relaxation process, and the fraction of these electrons is proportional to bT. When all relaxation mechanisms other than the first term in Eq. (1) can be neglected, and there are only x-like electrons at the Fermi level (such as in the alkali metals), the Korringa relationship takes its simplest form ... 

Fig. 13 Temperature dependence of spin-lattice relaxation rates for chemisorbed CH3 CN and CHsCN. The clear absence ofa Korringa relationship for CHsCN (solid circles) indicates that the carbon atom in the methyl group does not acquire any metallic character. (Adapted from Ref [38].)...

Unlike CO adsorbed onto supported Ru nanopartieles, CO adsorbed on Ru-black showed a large isotropic shift and a symmetric broadening of the NMR spectrum. In all these catalysts, the spin-lattice relaxation time (Tl) has followed the Korringa behavior characteristic of metallic systems [179]. Thus, CO adsorbed on Pt/Ru catalysts attains metallic properties due to the mixing of CO molecular orbitals with the conduction electron states of the transition metal. This observation strongly suggests that the electronic influence of Ru on surface Pt atoms is only a local effect, and Pt atoms situated away from Ru sites retain their original electronic band structure properties. 

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