Comparison of Continuous Wave, Spin Echo, and Rapid Scan EPR of Irradiated Fused Quartz

Abstract

The E' defect in irradiated fused quartz has spin lattice relaxation times (T(1)) about 100 to 300 μs and spin-spin relaxation times (T(2)) up to about 200 μs, depending on the concentration of defects and other species in the sample. These long relaxation times make it difficult to record an unsaturated continuous wave (CW) electron paramagnetic resonance (EPR) signal that is free of passage effects. Signals measured at X-band (~9.5 GHz) by three EPR methods: conventional slow-scan field modulated EPR, rapid scan EPR, and pulsed EPR, were compared. To acquire spectra with comparable signal-to-noise, both pulsed and rapid scan EPR require less time than conventional CW EPR. Rapid scan spectroscopy does not require the high power amplifiers that are needed for pulsed EPR. The pulsed spectra, and rapid scan spectra obtained by deconvolution of the experimental data, are free of passage effects.

Figure 1

X-band CW EPR spectra of the E′ defects in irradiated fused quartz collected with an E580 spectrometer with 0.1 G modulation amplitude at 100 KHz at 50 dB attenuation (2.0 μW) ( solid line) and 21 dB attenuation (1.6 mW) (dashed line). Signal acquisition time was 40 s.

Figure 2

Comparison of rapid scan and conventional CW EPR spectra of irradiated quartz. Magnetic field scans were from low field to high field. a) As-recorded sinusoidal rapid scan signal obtained with a scan rate of 4.7 MG/s. 1024 averages were recorded in about 5 sec. The incident microwave power was about 3.3 mW. b) Slow-scan absorption spectrum obtained by deconvolution of signal in a. c) First derivative spectrum obtained by pseudomodulation of the signal in b. d) Single scan of a conventional field-modulated first-derivative CW EPR spectrum of the same sample, obtained in 1 minute using about 0.02 mW incident microwave power, 10 kHz modulation frequency and 0.05 G modulation amplitude.

Figure 3

Rapid scan spectra of the irradiated quartz sample were obtained as a function of incident microwave power while scanning the magnetic field through resonance at several scan rates. The maximum in the power saturation curve increases with increasing scan rate. The point that corresponds to the acquisition conditions for the spectra shown in Figure 2a–c is circled in red.

Figure 4

a) As-recorded X-band field-swept, echo-detected EPR spectrum of irradiated quartz, obtained with π/2 = 1024 ns, π = 2048 ns, 1 scans with 4 step phase cycling, power = 15 mW, 1024 points, 10 G sweep width, 2 ms pulse repetition time. Data were acquired in 20 seconds. b) First derivative spectrum obtained by pseudomodulation of the signal in a.