Frequency dependence of electron spin relaxation for three S=1/2 species doped into diamagnetic solid hosts

Electron spin lattice relaxation rates (1/T-1) were measured as a function of temperature at two or three microwave frequencies for three S = 1/2 species in temperature ranges with different dominant relaxation processes. Between 10 and 50 K the contribution from the direct process to the relaxation rate was substantially greater at 94 than at 9.5 GHz for a vanadyl porphyrin doped into zinc tetratolylporphyrin. For bis(diethyldithiocarbamato)copper(II) doped into the diamagnetic Ni(II) analog the relaxation rate between 25 and 100 K is dominated by the Raman process and exhibits little frequency dependence between 9.2 and 94 GHz. For 4-hydroxy-2,2,6,6-tetramethylpiperidinoloxy (tempol) doped into a diamagnetic host the relaxation rate between about 40 and 100 K is dominated by the Raman process. In this temperature range, relaxation rates at 3.2, 9.2, and 94 GHz exhibit little frequency dependence. Above about 130 K, the relaxation rate for tempol decreases in the order S-band > X-band > W-band. The relaxation rates in this temperature range fit a model in which 1/T-1 is dominated by a thermally activated process that is assigned as rotation of the methyl groups on the nitroxyl ring.