Electron paramagnetic resonance of lanthanides

Abstract

Many applications of lanthanides exploit their electron spin relaxation properties. Double electron-electron measurements of distances are possible because of the relatively long relaxation times of Gd³⁺. Relaxation enhancement measurements of distance are possible because of the much shorter relaxation times of other lanthanides. Magnetic resonance imaging contrast agents use the long relaxation time of the S-state Gd³⁺ ion, and NMR shift reagents use the fast relaxation of selected other lanthanides. Other than Gd³⁺ and the isoelectronic Eu²⁺ ion, spin relaxation of the lanthanides is so fast that their EPR spectra can be observed only in the liquid helium temperature range. In this chapter the EPR properties of each of the lanthanides is briefly summarized, with an emphasis on electron spin relaxation.

Keywords: Electron paramagnetic resonance; Electron spin relaxation; Kramers' ion; Non-Kramers' ions.

Figure 1.

EPR spectra of paramagnetic rare-earth ions in phosphate glass. One division of the abscissa corresponds to a magnetic field of 0.5 T = 5 kG; spectra are normalized to the maximum values in the studied range of fields. Adapted from (Antipin et al. 1982). Eu³⁺ has J = 0 ground state and is EPR silent.

Figure 2.

Integral EPR spectra for four lanthanide ions at 4.2 K for Ce³⁺ and Nd³⁺ and at 1.8 K for Er³⁺ and Yb³⁺. A) in 1:1 water:glycerol with resonance frequencies Ce³⁺, 9.251 GHz; Nd³⁺, 9.139 GHz; Er³⁺, 9.824 GHz; Yb³⁺, 9.160 GHz. B) in 1:1 water:ethanol with resonance frequencies of Ce³⁺, 9.269 GHz; Nd³⁺, 9.228 GHz; Er³⁺, 9.129 GHz; Yb³⁺, 8.940 GHz. Spectra show the amplitude of electron spin echo signals as a function of B_o. The proton nuclear modulation was removed by taking the envelope of the spectra. Reproduced with permission from (Mims and Davis 1976).

Figure 3.

Spectra of Tm³⁺ in 1:1 water:ethanol. A) Field-swept echo-detected spectrum at 4.2 K obtained with 40 and 80 ns pulses and a constant pulse spacing of 180 ns. B) Field-stepped direct-detected spectrum at 4.5 K obtained with a scan frequency of 3.6 kHz and scan segments of 25 G. C) Derivative of the spectrum in B. D) CW spectrum at 4.5 K obtained with 4 G modulation amplitude at 100 kHz. Sharp signals at about 1800 G, 3500 G, and 4000 G in the CW and direct-detected spectra are from impurities in the dielectric resonator and are not observed in the pulse experiments because the experimental parameters were not optimized for the impurity signal. Figure reproduced, with permission from (McPeak et al. 2020).

Figure 4.

Temperature dependence of relaxation rates in 1:1 water:glycerol. 1/T_m for () 1.0 mM Gd³⁺, 1/T_m for () 2.0 mM Gd(DTPA)²⁻, 1/T₁ for () 1.0 mM Gd³⁺, and 1/T₁ for () 2.0 mM Gd(DTPA)²⁻ (unpublished data).

Figure 5.

Temperature dependence of relaxation rates. 1/T_m for () 10 mM Tb³⁺ in 1:1 water:ethanol, () 1 mM Tb³⁺ in 1:1 water:ethanol, () Tb₂(oxalate)₃ in La(oxalate)₃, () 10 mM Tm³⁺ in 1:1 water:ethanol, ( ) 1 mM Tm³⁺ in 1:1 water:ethanol; 1/T₁ for () 10 mM Tb³⁺ in 1:1 water:ethanol, () 1 mM Tb³⁺ in 1:1 water:ethanol, () Tb₂(oxalate)₃ in La(oxalate)₃, () 10 mM Tm³⁺ in 1:1 water:ethanol, () 1 mM Tm³⁺ in 1:1 water:ethanol. The solid lines are fits to the temperature dependence of 1/T₁ including contributions from the direct and Raman processes. Figure reproduced, with permission, from (McPeak et al. 2020).