Spin-lattice relaxation direct process

Several types of spin-lattice relaxation processes have been described in the literature [31]. Here a brief overview of some of the most important ones is given. The simplest spin-lattice process is the direct process in which a spin transition is accompanied by the creation or annihilation of a single phonon such that the electronic spin transition energy, A, is exchanged by the phonon energy, hcoq. Using the Debye model for the phonon spectrum, one finds for k T A that... 

Ammonium alums undergo phase transitions at Tc 80 K. The phase transitions result in critical lattice fluctuations which are very slow close to Tc. The contribution to the relaxation frequency, shown by the dotted line in Fig. 6.7, was calculated using a model for direct spin-lattice relaxation processes due to interaction between the low-energy critical phonon modes and electronic spins. 

In the theory of deuteron spin-lattice relaxation we apply a simple model to describe the relaxation of the magnetizations T and (A+E), for symmetry species of four coupled deuterons in CD4 free rotators. Expressions are derived for their direct relaxation rate via the intra and external quadrupole couplings. The jump motion between the equilibrium positions averages the relaxation rate within the same symmetry species. Spin conversion transitions couple the relaxation of T and (A+E). This mixing is included in the calculations by reapplying the simple model under somewhat different conditions. The results compare favorably with the experimental data for the zeolites HY, NaA and NaMordenite [6] and NaY presented here. Incoherent tunnelling is believed to dominate the relaxation process at lowest temperatures as soon as CD4 molecules become localized. 

In order to determine the content of this noncrystalline line further, we examined in more detail the behavior of the spin-lattice relaxation. Figure 5 shows the partially relaxed spectra in the course of the inversion recovery pulse sequence (180°-t-90°-FIDdd-10s)i2o with varying x values. The magnetization that was recovered for 10 s in the z direction was turned to negative z direction by 180° pulse and the magnetization recovered in z direction for varying x was measured in the xy plane under H DD. The spectra at different steps of the longitudinal relaxation were obtained by Fourier transform and are shown in Fig. 5. In these spectra the contribution from the crystalline components with Tic s of2,560 and 263 s are eliminated due to the lack of time for recovery at each pulse sequence. Therefore, we observed preferentially the relaxation process of the noncrys-... 

Mc/s and the spontaneous emission lifetime is 10 sec. Obviously this lifetime is too long and the transitions will be saturated exceedingly easily. In other words, the populations of the two levels become essentially equal and no net transition can be observed. Fortunately there are a number of nonradiative relaxation mechanisms open to the upper spin level including interactions with other electrons, with nuclei having nuclear magnetic moments, and with the lattice. The latter process is often known as spin-lattice relaxation. The term "lattice" generally refers to the degrees of freedom of the system other than those directly related with spin. Spin... 

Interestingly, the spin-lattice relaxation time according to the direct process involving the triplet substates II and I remains unchanged within fimits of experimental error. For Pt(2-thpy-hg)2 and for Pt(2-thpy-dg)2 the sir times at T = 1.3 K are r jj. (720 10) ns and (710 10) ns, respectively (see Sect. 4.2.7.2 and Ref. [23]). Obviously, perdeuteration of the chromophore does not strongly influence the sir at low temperatures. Moreover, it is indicated that the matrix cages of the two compounds in n-octane are similar. Otherwise one would expect to observe distinctly different sir times as has been shown for Pt(phpy)2 (compare Fig. 1) in n-octane [64]. 

At low temperature, the processes of spin-lattice relaxation between the triplet substates are slow. With temperature increase, the sir rates increase strongly. For Pt(2-thpy)2, three different processes govern the sir. At a temperature below T = 3 K, the sir rate is exclusively determined by the direct process. Above T = 3 K, the Orbach process and above T = 6 K, the Raman process, become additionally important. For Pd(2-thpy)2, the processes that govern the sir have not been determined yet, but it is suggested that the Raman process is of main importance, since no real electronic state lies in the energy vicinity of the Tj state. (Figs. 19, 21, and Refs. [24,60,62,64,65].)... 

---