In situ ESR studies were made as a function of exposure time to HNO3. Detailed measurements performed at the very beginning of the intercalation process indicated a dramatic variation of the A/B ratio and intensity of the ESR signal associated with bulk HOPG. This could be interpreted by the formation of a macroscopic 'intercalation' layer and the advance of the interface separating this layer and the non-intercalated bulk HOPG. Using the theory of Zevin for the ESR of layer-substrate systems, the results yield the thickness of the 'intercalation' layer, d, as a function of the exposure time, t. The relationship, d = (2Dt)1/2, was deduced, where D was the diffusion constant of the intercalated species. ESR measurements made throughout the entire intercalation process revealed the formation of a new electron spin resonance line and a continuous shift of this line as a function of exposure time. This continuous variation may be interpreted as being due to charge transfer according to McClure and Yafet and Elliot, as well as to the formation of strain fields. The data strongly suggested that at the beginning of the intercalation process, the HNO3 species do not form large clusters and that the diffusion fronts were probably not sharp.

An in Situ ESR Study of the Intercalation of HNO3 into Highly Oriented Pyrolytic Graphite: Diffusion, Charge Transfer and Strain Fields. Palchan, I., Davidov, D., Zevin, V., Polatsek, G., Selig, H.: Synthetic Metals, 1986, 16[2], 215-26