Electron spin-echo studies of British coals at S-Band

Abstract

Three-pulse electron spin-echo studies have been made, at S-Band (ca. 3.8 GHz), of a number of samples from the British Coal Bank and of samples derived by heat or chemical treatment of these coals. Samples, whether natural coals or those prepared by chemical treatment or by heating up to 400 °C, were found to have phase memory times that were sufficiently long to enable spin echoes to be observed. Some high-carbon anthracites and all the high temperature chars were exceptions. Although two-pulse echoes at X-band (9.4 GHz) indicate the possibility of proton hyperfine modulation, the intense proton modulation seen in many samples at S-band is a much clearer indication that the free radicals in the samples contain these nuclei. Extracted free radicals also contain protons. The problems of modelling such centres are discussed.