Xenon relaxation, mechanism

The xenon relaxation mechanism in aqueous solution was examined in detail recently and determined to result essentially from dipolar interactions, as... 

H, and relaxation times have been measured for Sn[ri-C5H3(1,3- Bu2)]2 in toluene and xenon at 300 K (see Table 4, Figure 6). For the most part, relaxation times were longer in xenon (e.g. the values for Sn were 2.6 and 8.4 s in toluene and Xe respectively). However for the quaternary carbons the T values were lower in Xe. Presumably this is because those carbons with no attached protons have no efficient relaxation mechanism in conventional solvents, but undergo efficient spin-rotation relaxation in supercritical solvents. 

Experiments on the T of xenon in the solid state showed that it was highly temperature- and field-dependent. A mechanism for the xenon relaxation was described in which a two-photon Raman process was involved that increased the Xe relaxation. At colder temperatures, Xe is also relaxed by its coupling to... 

Dipole-dipole interactions occurring during collisions provide a relaxation mechanism for atoms in the gas phase. Xenon-129 atoms in the gas phase have spin-lattice relaxation times on the order of minutes which are dependent on gas density, temperature and the amount of other gases in the sample. For xenon-129 adsorbed in cavities inside zeolite matrices relaxation occurs via dipole-dipole interactions with hydrogen atoms that are bound to the sides of the cavities. 

For a harmonic crystal the phonon lifetime is infinite and there is no scattering of thermal phonons.To understand the mechanism on how the guest-host interactions lead to the anomalous temperature dependence of the thermal conductivity, the lifetimes were calculated for phonon-phonon scatterings as a result of the anharmonic terms in the xenon-water potential of xenon hydrate in the small and large cage. The inverse relaxation time (lifetime), of a lattice vibration with frequency C0j q) (/ is the branch index and q is the direction of the momentum transfer) is related to the transition rate, W, of the lattice wave scattered from state qj q f by a defect according to, ... 

Results of the calculated thermal conductivity for ice Ih, S-I methane hydrate and empty hydrate are depicted in Figure 14. The thermal conductivity of ice Ih has improved, but the absolute value is still slightly smaller than the experiment. The calculations reproduced previous observation that the thermal conductivity of the hydrate is lower than ice Ih and the empty hydrate. Even though the empty hydrate has a lower thermal conductivity than ice Ih, the crystalline temperature profile is similar. A surprising finding is the reversal in the thermal conductivity of methane hydrate at low temperature. From 250 to 100 K, the thermal conductivity decreases slightly. When the hydrate is cooled below 100 K, the conductivity increases and follows the trend as a crystal. This unusual temperature profile has indeed been observed in methane and xenon hydrates,details of which will be deferred to a later part of this chapter. To unravel the thermal transport mechanism, various correlation functions were computed and the relaxation times analyzed.The HCACF can be fitted to... 

Hyperpolarized noble gas atoms, like xenon, can interact with protons and other nuclei via dipolar cross-relaxation, a mechanism which is supported by stochastic processes of motion. By means of this interaction, hyperpolarized xenon can transfer its polarization to protons and other nuclei of interest. An efficient way to make use of this SPINOE mechanism is to freeze hyperpolarized xenon onto the surface of the sample. During the defrosting process large amounts of xenon can penetrate into the liquid sample, resulting in a large SPINOE enhancement. 

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