Terahertz frequency electron paramagnetic resonance

Electron paramagnetic resonance (EPR) can be used as a non-perturbative probe to study the electronic structure and dynamics of paramagnetic compounds and materials. Such systems represent a large body of transition metal, rare-earth, and radical species, with properties that underlie catalytic processes, molecular magnetism, spin-crossover and other molecular and collective spin phenomena. In many cases, however, spin transitions occur in the terahertz (THz) frequency range. In other instances, dynamical processes involving spin occur on picosecond and sub-picosecond time scales. Standard pulsed EPR instrumentation operates in the microwave frequency range, with current research efforts up to several hundred GHz [1] and has nanosecond time resolution limited by the speed of the electronics used to detect the signals. Furthermore, the ability to synchronize an EPR measurement with an initiation event (e.g. a light-induced excitation) without significant timing jitter is difficult.

Recently, our group used ultrafast terahertz (THz) radiation generated by optical rectification of 100-fs 800-nm laser pulses to measure the zero-field EPR spectra of several transition metal compounds [2]. This new approach employs an optical step-wise detection scheme (electro-optic sampling), enabling access to the entire THz frequency range with sub-picosecond time resolution. The method offers inherent time-synchronization with optical pulses that may generate electronic excited states by virtue of all time delays being encoded optically relative to the 800-nm laser pulse that is used for THz generation. It also offers prospects for nonlinear EPR measurements including 2D EPR on molecular compounds, as demonstrated already for collective (magnon) systems [3]. We will describe the methodology and current efforts using it, as well as efforts to extend the capabilities further.

References

[1] J. Tesler et al. J. Biol. Inorg. Chem. (2014)

[2] J. Lu et al. Chem. Sci. (2017)

[3] J. Lu et al. Phys. Rev. Lett. (2017)