Detection of undistorted continuous wave (CW) electron paramagnetic resonance (EPR) spectra with non-adiabatic rapid sweep (NARS) of the magnetic field

Abstract

A continuous wave (CW) electron paramagnetic resonance (EPR) spectrum is typically displayed as the first harmonic response to the application of 100 kHz magnetic field modulation, which is used to enhance sensitivity by reducing the level of 1/f noise. However, magnetic field modulation of any amplitude causes spectral broadening and sacrifices EPR spectral intensity by at least a factor of two. In the work presented here, a CW rapid-scan spectroscopic technique that avoids these compromises and also provides a means of avoiding 1/f noise is developed. This technique, termed non-adiabatic rapid sweep (NARS) EPR, consists of repetitively sweeping the polarizing magnetic field in a linear manner over a spectral fragment with a small coil at a repetition rate that is sufficiently high that receiver noise, microwave phase noise, and environmental microphonics, each of which has 1/f characteristics, are overcome. Nevertheless, the rate of sweep is sufficiently slow that adiabatic responses are avoided and the spin system is always close to thermal equilibrium. The repetitively acquired spectra from the spectral fragment are averaged. Under these conditions, undistorted pure absorption spectra are obtained without broadening or loss of signal intensity. A digital filter such as a moving average is applied to remove high frequency noise, which is approximately equivalent in bandwidth to use of an integrating time constant in conventional field modulation with lock-in detection. Nitroxide spectra at L- and X-band are presented.

Fig. 1

(a) Simulated absorption spectrum of a nitroxide at 9.5 GHz where g_x = 2.0085, g_y = 2.0061, g_z = 2.0023; A_x = 6.2 G, A_y = 4.3 G, A_z = 36.9 G; and τ_R = 8 × 10⁻¹⁰ s. (b) Pseudomodulation of absorption spectrum with amplitudes of 0.25, 0.5, 1.0, and 2.0 G, resulting in progressively greater signal intensity. Note the change in vertical scale. (c) Integration of the pseudomodulated spectra. Spectra have been normalized to one for comparison. Relative peak heights of the three lines are fairly well preserved. (d) Residual of the integrated pseudomodulated spectrum subtracted from the unmodulated spectrum in (a). Field modulation results in line-broadening under all conditions.

Fig. 2

Instrumental setup utilized to perform the NARS experiment.

Fig. 3

EPR spectra of 200 µM pdMTSL in degassed sec-butylbenzene collected using magnetic field modulation (dashed line) and NARS (solid line).

Fig. 4

Comparison of (a) NARS and (b) field modulation spectra of 10 µM TEMPOL in water at L-band. A factor of five improvement in SNR was observed with NARS detection. Data were acquired in the same acquisition time.