Field-stepped direct detection electron paramagnetic resonance

Abstract

The widest scan that had been demonstrated previously for rapid scan EPR was a 155 G sinusoidal scan. As the scan width increases, the voltage requirement across the resonating capacitor and scan coils increases dramatically and the background signal induced by the rapidly changing field increases. An alternate approach is needed to achieve wider scans. A field-stepped direct detection EPR method that is based on rapid-scan technology is now reported, and scan widths up to 6200 G have been demonstrated. A linear scan frequency of 5.12 kHz was generated with the scan driver described previously. The field was stepped at intervals of 0.01 to 1 G, depending on the linewidths in the spectra. At each field data for triangular scans with widths up to 11.5 G were acquired. Data from the triangular scans were combined by matching DC offsets for overlapping regions of successive scans. This approach has the following advantages relative to CW, several of which are similar to the advantages of rapid scan. (i) In CW if the modulation amplitude is too large, the signal is broadened. In direct detection field modulation is not used. (ii) In CW the small modulation amplitude detects only a small fraction of the signal amplitude. In direct detection each scan detects a larger fraction of the signal, which improves the signal-to-noise ratio. (iii) If the scan rate is fast enough to cause rapid scan oscillations, the slow scan spectrum can be recovered by deconvolution after the combination of segments. (iv) The data are acquired with quadrature detection, which permits phase correction in the post processing. (v) In the direct detection method the signal typically is oversampled in the field direction. The number of points to be averaged, thereby improving the signal-to-noise ratio, is determined in post processing based on the desired field resolution. A degased lithium phthalocyanine sample was used to demonstrate that the linear deconvolution procedure can be employed with field-stepped direct detection EPR signals. Field-stepped direct detection EPR spectra were obtained for Cu2+ doped in Ni(diethyldithiocarbamate)2, Cu2+ doped in Zn tetratolylporphyrin, perdeuterated tempone in sucrose octaacetate, vanadyl ion doped in a parasubstituted Zn tetratolylporphyrin, Mn2+ impurity in CaO, and an oriented crystal of Mn2+ doped in Mg(acetylacetonate)2(H2O)2.

250 MHz Rapid Scan Cross Loop Resonator

“…The improved signal-to-noise for rapid scan is consistent with prior observations for other samples and resonators [18]. Xband rapid scan of nitroxides and other broad-line species were reported in many prior studies, both at low microwave frequencies [3,12,19] and at X-band with small reflection resonators [20,21].…”

“…[31] was used to acquire the imaging data. The data were acquired in quadrature and the two channels were combined prior to image reconstruction [20,21]. The resulting image is shown in Figure 10.…”

Rapid-scan EPR imaging

“…The Hyde lab has collected Cu(II) spectra in several segments of about 20 G each and assembled them post-acquisition [18, 19]. Yu et al collected Cu(II), vanadyl, and Mn(II) spectra over as much as 6200 G in a method called field-stepped direct detection rapid scan [32] (Fig. 6).…”

Rapid-Scan EPR of Nitroxide Spin Labels and Semiquinones

“…A field-stepped direct detection EPR method that is based on rapid-scan technology has been developed with scan widths up to 6200 G (Yu et al, 2015). A scan frequency of 5.12 kHz was generated with a linear scan driver.…”