Free induction decay-detected high-field electron spin resonance for accurate monitoring highly polarized spin dynamics
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.
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BibTeX
@misc{stern2026free,
title = {{Free induction decay-detected high-field electron spin resonance for accurate monitoring highly polarized spin dynamics}},
author = {Quentin Stern and Hoang M. Le and Martyna Judd and Joshua S. Straub and Songi Han},
howpublished = {ChemRxiv},
year = {2026},
month = oct,
doi = {10.26434/chemrxiv.15010273/v1},
url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15010273/v1},
note = {CC BY},
}
RIS
TY - UNPB
TI - Free induction decay-detected high-field electron spin resonance for accurate monitoring highly polarized spin dynamics
AU - Quentin Stern
AU - Hoang M. Le
AU - Martyna Judd
AU - Joshua S. Straub
AU - Songi Han
PY - 2026
DA - 2026/10/12
PB - ChemRxiv
DO - 10.26434/chemrxiv.15010273/v1
UR - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15010273/v1
AB - 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.
ER -