Electron spin-lattice relaxation times from the decay of E.S.R. emission spectra

Abstract

Chemically induced electron polarization provides a new method for the direct determination of electron spin-lattice relaxation times of radicals in liquids. A rapid-response time-resolved E.S.R. spectrometer has been used to observe the decay of an emission spectrum into the corresponding absorption, and the effective relaxation time extracted by fitting an exponential curve to the early part of the decay, and then obtaining the true T ₁ at any point of the spectrum by extrapolation to zero microwave power. The decay is monitored on resonance in the dispersion mode and at the points of maximum slope in the absorption mode : both results converge to T ₁ at zero power with a gradient determined by T ₂. Results for T ₁ and T ₂ on the benzophenone ketyl radical in liquid paraffin over a viscosity range 0·3 poise to 30 poise are reported. In this preliminary investigation the deuterated radical was used, and no attempt was made to resolve the hyperfine lines. The values of T ₁ and T ₂ for the modulation broadened spectrum are in the region of 10^-4 s and 10^-8 s respectively, and are supported by saturation data. The effect of relaxation on steady-state polarized spectra is described in the light of some of the results obtained.