The Effect of Light on the Esr Spectra of Humic Acids

Summary Photo‐induced Electron Spin Resonance signals have been detected in basic aqueous solutions of humic acids from soil and from synthetic and commercial sources. Parameters of these signals under atmospheres of air, O 2 and N 2 were the same as or very similar to those of the already known ‘dark’ stable ESR‐signals of humic acids and melanins. The maximum increase of signal amplitude under the influence of 395–780 nm wavelength irradiation was one order of magnitude. The influence of microwave power on ESR‐signal amplitude was examined under different atmospheres. It has been found that oxygen strongly diminishes the saturability of signals. This indicates that oxygen has an influence upon relaxation properties of paramagnetic centres of humic acid. Photoinduced ESR signal is a newly discovered feature of humic acids, similar to that of melanin and may be expected to affect physico‐chemical processes occurring in soils.