Relaxation Time Measurements by Longitudinally Modulated ENDOR Spectroscopy on Irradiated L-Alanine Single Crystal

Longitudinally modulated electron-nuclear double-resonance (LOMENDOR) investigations of CH3(COOH)CH· radicals in an irradiated single crystal of L-alanine have been performed over a temperature range from 203 to 313 K. First- and second-harmonic LOMENDOR, LOMESR, and variable-frequency LOMENDOR spectra have confirmed that cross relaxation due to random isotropic hyperfine coupling modulation via methyl group rotation is the dominant nuclear relaxation mechanism. The electron and nuclear relaxation times have been measured and the corresponding relaxation probabilities obtained. A comparison of the experimental and theoretical magnitudes of the cross-relaxation probability leads to the determination of the activation energy and correlation time for the rotation of the methyl group, which are in good agreement with values previously obtained by other methods.