Free induction decay-detected high-field electron spin resonance for accurate monitoring highly polarized spin dynamics
Year: 2026
Type: preprint
Abstract: Understanding the dynamics of highly polarized spins using electron spin resonance (ESR) is increasingly relevant to dynamic nuclear polarization (DNP), polarized targets, and spin-based quantum technologies.In this regime, however, conventional echo-detected ESR is no longer a linear reporter of spin polarization, because the electron spin coherence time T2 itself depends on polarization through suppression of electronelectron flip-flops.Here, we present a dual ESR/nuclear magnetic resonance instrument operating at 7 T and temperatures down to ~1.3 K, where the sample is immersed in a liquid helium bath and the electron spin polarization reaches ~99.8%.The system combines a resonator-free quasi-optical 190-196 GHz spectrometer with a low-heat-load smooth-wall waveguide, and a compact dual NMR coil/ESR waveguide.Resonator-free operation eliminates ring-down and enables free induction decay (FID) detection.Using 40 mM TEMPOL in DNP juice as a model system, we show that frequency-swept ESR spectra from echo and FID detection agree, whereas saturation-recovery measurements of the longitudinal relaxation time T1S diverge strongly.This discrepancy between the two approaches arises due to the nonlinearity of echo-detection at high polarization, which we show to be difficult to correct without the introduction of severe bias.FID detection is thus a more robust and quantitative probe of electron spin dynamics at near-unity polarization.This work paves the way towards the accurate characterization of the dynamics within networks of coupled, highly polarized electron spins relevant to DNP at 1 K and the monitoring of short-lived photo-excited spin systems.
Source: ChemRxiv
Institution: Northwestern University
Language: English
Subfield: Spectroscopy
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7221564440