Enhanced Signal Intensities Obtained by Out-of-Phase Rapid-Passage EPR for Samples with Long Electron Spin Relaxation Times
✍ Scribed by James R. Harbridge; George A. Rinard; Richard W. Quine; Sandra S. Eaton; Gareth R. Eaton
- Book ID
- 102597428
- Publisher
- Elsevier Science
- Year
- 2002
- Tongue
- English
- Weight
- 175 KB
- Volume
- 156
- Category
- Article
- ISSN
- 1090-7807
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✦ Synopsis
To understand the signals that are observed under rapid-passage conditions for samples with long electron spin relaxation times, the E defect in irradiated vitreous SiO 2 was studied. For these samples at room temperature, T 1 is 200 µs and T 2 ranged from 35 to 200 µs, depending on spin concentration. At X band with 100-kHz modulation frequency and 1-G modulation amplitude there was minimal lineshape difference between the low-power, in-phase spectra and high-power spectra detected 90 • out-of-phase with respect to the magnetic field modulation. Signal enhancement, defined as the ratio of the intensities of the out-of-phase to the in-phase signals when B 1 for both observation modes is adjusted to give maximum signal, was 3.4 to 9.5 at room temperature. The origin of the out-of-phase signal was modeled by numerical integration of the Bloch equations including magnetic field modulation. The waveforms for the E signal, prior to phase sensitive detection, were simulated by summing the contributions of many individual spin packets. Good agreement was obtained between experimental and calculated waveforms. At low B 1 the experimental values of T 1 and T 2 were used in the simulations. However, at higher B 1 , T 2 was adjusted to match the experimental signal intensity and increased with increasing B 1 . At high B 1 , T 2 = T 1 , consistent with Redfield's and Hyde's models. For the spin concentrations examined, the out-of-phase signals at very high power (B 1 ∼ 0.33 G) displayed a linear relationship between peak-to-peak signal amplitude and spin concentration. Under the conditions used for spin quantitation the signal-to-noise for these spectra was up to 5 times higher than for the in-phase signal, which greatly facilitates quantitation for these types of samples. For samples in which T 2 is dominated by electron spin-spin interaction, lower spin concentration results in longer T 2 and the enhancement is increased.