Enhanced Signal Intensities Obtained by Out-of-Phase Rapid-Passage EPR for Samples with Long Electron Spin Relaxation Times

Abstract

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 SiO2 was studied. For these samples at room temperature, T1 is 200 μs and T2 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 B1 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 B1 the experimental values of T1 and T2 were used in the simulations. However, at higher B1, T2 was adjusted to match the experimental signal intensity and increased with increasing B1. At high B1, T2=T1, consistent with Redfield's and Hyde's models. For the spin concentrations examined, the out-of-phase signals at very high power (B1∼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 T2 is dominated by electron spin–spin interaction, lower spin concentration results in longer T2 and the enhancement is increased.