Electron spin lattice relaxation rates for S=1/2 molecular species in glassy matrices or magnetically dilute solids at temperatures between 10 and 300 K
Y. Zhou et al., Electron spin lattice relaxation rates for S=1/2 molecular species in glassy matrices or magnetically dilute solids at temperatures between 10 and 300 K, J MAGN RES, 139(1), 1999, pp. 165-174
The temperature dependence of X-band electron spin-lattice relaxation betwe
en about 10 and 300 K in magnetically dilute solids and up to the softening
temperature in glassy solvents was analyzed for three organic radicals and
14 S = 1/2 transition metal complexes. Contributions from the direct, Rama
n, local vibrational mode, thermally activated, and Orbach processes were c
onsidered. For most samples it was necessary to include more than one proce
ss to fit the experimental data. Debye temperatures were between 50 and 135
K. For small molecules the Debye temperature required to fit the relaxatio
n data was higher in 1:1 water:glycerol than in organic solvents. For large
r molecules the Debye temperature was less dependent upon solvent and more
dependent upon the characteristics of the molecule. The coefficients of the
Raman process increased with increasing g anisotropy and decreasing rigidi
ty of the molecule. For the transition metal complexes the data are consist
ent with major contributions from local modes with energies in the range of
185 to 350 K (130 to 240 cm(-1)). The coefficient for this contribution in
creases in the order 3d < 4d transition metal. For C-60(-) anions there is
a major contribution from a thermally activated process with an activation
energy of about 240 cm(-1). For low-spin hemes the dominant contribution at
higher temperatures is from a local mode or thermally activated process wi
th a characteristic energy of about 175 cm(-1). (C) 1999 Academic Press.