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 spec ies in temperature ranges with different dominant relaxation processes. Bet ween 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 porphyr in doped into zinc tetratolylporphyrin. For bis(diethyldithiocarbamato)copp er(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 frequen cy dependence between 9.2 and 94 GHz. For 4-hydroxy-2,2,6,6-tetramethylpipe ridinoloxy (tempol) doped into a diamagnetic host the relaxation rate betwe en about 40 and 100 K is dominated by the Raman process. In this temperatur e 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 temperatur e range fit a model in which 1/T-1 is dominated by a thermally activated pr ocess that is assigned as rotation of the methyl groups on the nitroxyl rin g.