Rapid frequency scan EPR

Abstract

In rapid frequency scan EPR with triangular scans, sufficient time must be allowed to insure that the magnetization in the x, y plane decays to baseline at the end of the scan, which typically is about 5T(2) after the spins are excited. To permit relaxation of signals excited toward the extremes of the scan the total scan time required may be much longer than 5T(2). However, with periodic, saw-tooth excitation, the slow-scan EPR spectrum can be recovered by Fourier deconvolution of data recorded with a total scan period of 5T(2), even if some spins are excited later in the scan. This scan time is similar to polyphase excitation methods. The peak power required for either polyphase excitation or rapid frequency scans is substantially smaller than for pulsed EPR. The use of an arbitrary waveform generator (AWG) and cross loop resonator facilitated implementation of the rapid frequency scan experiments reported here. The use of constant continuous low B(1), periodic excitation waveform, and constant external magnetic field is similar to polyphase excitation, but could be implemented without the AWG that is required for polyphase excitation.

Fig. 1

Comparison of | Xk |for triangular (a) and saw-tooth (b) excitation. The periods for the triangular and saw-tooth waveforms were 30 and 15 μs, respectively, and the scan ranges were ± 8.5 MHz.

Fig. 2

Comparison of ${\mid \text{X}}_{\text{k}}^{\text{p}}\mid$ for polyphase excitation (green dashed line) with N = 256, and | Xk |for rapid frequency scan (blue solid line) of ± 8.5 MHz. Calculations were done with B₁=1 and T=15 μs. The y axis scales linearly with B₁. The parameters were selected to correspond with experiment. The red dotted line is the limiting value for an infinitely wide scan. Resonator Q = 250 for the experiments corresponds to a 3 dB bandwidth of ±2.5 MHz which limits the bandwidth of frequencies that impact the spins. Since the signal bandwidth is much narrower than the bandwidth of the resonator, the impact of resonator Q on the excitation profile was not included in the calculation.

Fig. 3

Polyphase and fast frequency scan EPR signals for LiPc (a) digitized after amplification of the output of the detection resonator. There are 4 cycles of polyphase excitation (0–60 μs) and 4 cycles of the saw-tooth frequency scan (60–120 μs). The EPR signal (blue) was obtained by subtraction of the off-resonance background (red). (b) M_x and M_y components of the frequency-scan EPR signal y(t) obtained by digital phase-sensitive detection at the constant carrier (1.0378 GHz) for the time segment of the frequency scan from 80 to 100 μs

Fig. 4

Comparison of signals for LiPc obtained by rapid frequency scan (green points and dashed lines) and polyphase continuous excitation (blue points and solid lines). (a) Relative signal amplitudes as a function of B₁, (b) lineshape obtained at B₁ = 0.085 G, (c) FWHM line widths as a function of B₁, and (d) lineshape obtained at B₁ = 0.17 G. The simulated dependence of signal amplitude and linewidths on B₁ is shown for polyphase excitation (solid blue line) and fast frequency scans (dashed green line).