Application of pulsed heating in time-resolved EPR spectroscopy for longitudinal relaxation measurements

Transient or time-resolved electron paramagnetic resonance spectroscopy (TR EPR) is a powerful method for studying various photogenerated paramagnetic species. The use of low-energy quanta, such as terahertz (THz) radiation, as an external stimulus in TR EPR allows the initiation of spin dynamics without generating new paramagnetic species other than those already present in the system. This spin dynamic reflects the return of the system to thermodynamic equilibrium, governed by a spin-lattice relaxation time, T1. The latter, together with a phase memory time, is of paramount importance for the practical implementation of single-molecule magnets and molecular spin qubits. In this work, we present TR EPR spectroscopy with pulsed heating by THz pulses as a versatile spectroscopic method for determining T1 in a wide range of paramagnetic systems. To define the scope of the method, we developed a numerical model based on the Liouville-von Neumann equation, with the equilibrium density matrix defined by the temperature profile of the lattice. Using experimental data obtained for [CoTp2] (cobalt(II) bis[tris(pyrazolyl)borate]) with S = 3/2, we compared the proposed method with two other commonly used techniques: alternating current (AC) magnetometry and pulsed EPR. All three methods were found to be in qualitative agreement and provided complementary information about the relaxation properties. TR EPR spectroscopy showed the orientation dependence of T1. AC magnetometry revealed the dependence of T1 on the value of the external magnetic field, which was attributed in the literature to a field-induced Raman process. Finally, pulsed EPR spectroscopy was found to be biased by strong spectral diffusion.