Electron-spin-lattice relaxation of photoexcited triplet states in disordered organic solids

We measured the zero-field electron-spin-lattice relaxation in the photoexcited triplet state of several aromatic molecules in organic glasses. In the whole temperature range 1.335 K, the relaxation rates follow a power law with a small exponent. In 3-methyl-pentane glass this exponent is 2.20.2 for the whole temperature range. In alcohol glass, the exponent changes from 2.2 to 3.2 (0.2) for temperatures greater than 5 K. We found that the spin-lattice relaxation is not affected by glass deuteration. The results are interpreted by considering a coupling of the electron spin to the disorder modes [two-level systems (TLSs)] of the doped glass. It seems that the special type of TLS which relax the spin is related to the dopant molecule and is, most probably, a rotational tunneling motion. The data can be well described by assuming a direct TLS phonon coupling and a superposition of a direct and a Raman-type process.