Spin-lattice relaxation of the quinoxaline triplet state in n-alkane matrices measured in zero magnetic field
The individual spin-lattice relaxation (SLR) rate constants (Wij) between the sublevels of the lowest triplet state of quinoxaline in n-pentane and in n-hexane Sphol'skii hosts over the temperature range of 1.2 to 4.2 K have been measured in zero applied magnetic field. The anisotropy pattern for the Wij for quinoxaline in these n-alkane hosts differs from that observed in a durene host. In durene, the SLR rate constant between the two in-plane sublevels was found to be dominant, and was suggested to occur via an Orbach relaxation process involving a localized phonon state with rotated spin axes. It is proposed here that the dominant relaxation route between the short in-plane axis sublevel (Ty) and the out-of-plane axis sublevel (Tx) observed for quinoxaline in n-alkane hosts is the result of a spin-orbit-phonon coupling. In the temperature range of 2.5 to 4.2 K, the SLR rate constant Wyx exhibits Raman relaxation with a T7 temperature dependence in an n-pentane host. In n-hexane, where the relaxation is much faster than that in n-pentane, the mechanism observed appears to be a combination of a normal direct process and a Raman process in the temperature range of 1.2 to 3.2 K. Individual decay constants for the triplet sublevels in the temperature range investigated are also presented.