Electron spin relaxation in pseudo-Jahn-Teller low-symmetry Cu(II) complexes in diaqua(L-aspartate)Zn(II) center dot H2O crystals

Low-temperature (4-55 K) pulsed EPR measurements were performed with the ma gnetic field directed along the z-axis of the g-factor of the low-symmetry octahedral complex [Cu-63(L-aspartate)(2)(H2O)(2)] undergoing dynamic Jahn- Teller effect in diaqua(L-aspartate)Zn(II) hydrate single crystals. Spin-la ttice relaxation time T-1 and phase memory time Tm were determined by the e lectron spin echo (ESE) method. The relaxation rate 1/T-1 increases strongl y over 5 decades in the temperature range 4-55 K. Various processes and mec hanisms of T-1-relaxation are discussed, and it is shown that the relaxatio n is governed mainly by Raman relaxation processes with the Debye temperatu re Theta (D) = 204 K, with a detectable contribution from disorder in the d oped Cu2+ ions system below 12 K. An analytical approximation of the transp ort integral I-8 is given in temperature range T = 0.025-10 Theta (D) and a pplied for computer fitting procedures. Since the Jahn-Teller distorted con figurations differ strongly in energy (delta (12) = 240 cm(-1)), there is n o influence of the classical vibronic dynamics mechanism on T-1. Dephasing of the ESE (phase relaxation) is governed by instantaneous diffusion and sp ectral diffusion below 20 K with resulting rigid lattice value 1/T-M(0) = 1 .88 MHz. Above this temperature the relaxation rate 1/T-M increases upon he ating due to two mechanisms. The first is the phonon-controlled excitation to the first excited vibronic level of energy Delta = 243 cm(-1), with subs equent tunneling to the neighbor potential well. This vibronic-type dynamic s also produces a temperature-dependent broadening of lines in the ESEEM sp ectra. The second mechanism is produced by the spin-lattice relaxation. The increase in T-M is described in terms of the spin packets forming inhomoge neously broadened EPR lines. (C) 2001 Academic Press.