Spin-lattice relaxation Raman process

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].)... 

Indeed, both the exponential temperature dependence that characterize the Orbach process and the T behavior associated with the Raman type process have been observed in spin lattice relaxation ... 

The ESR of Ce ", Nd ", Dy ", Er ", and Yb in the hydrated trichloride, YCI3 6H2O, has been investigated by Schulz and Jeffries (1967). In these materials the lanthanide site symmetry is only C2, however the ESR results indicate an effective CEF of nearly Ce or C h symmetry. Spin-lattice relaxation time measurements were also made on these systems (except for Yb " ) as indicated by the data tabulation in table 18.28. Usually a Raman process term was found, of order (Ti) " lO" T sec . An exponential Orbach process was... 

Raman Process of Spin-Lattice Relaxation. In addition to the one-phonon process described above, a two-phonon scattering process according to Fig. 14 b can also occur. The temperature dependence of this Raman process of sir can be approximated (for the non-Kramers situation) at low temperature and for AE phonon energies to A(Raman) - T [97,224,227]. Also, this process has been observed experimentally [224, 227, 230, 231]. At very low temperature and in the presence of a fast direct process, Raman scattering can usually be neglected. 

---