Abstract: Five single crystals of the natural diamonds,four Synthetic diamonds and two diamond-like films (PCVD film) have been investigated by means of ESR method. The results show that there are four kinds of N-center free radicals and one kind of C-center free radical in the I_b type natural diamonds, and the unpaired electron is located on an antibonding orbital of the N3 impurity center in the I_aB3 type natural diamonds.The ESR spectra of the I_b type synthetic diamonds are similar to the natural diamonds,except that the linewidthes are broader since they contain some paramagnetic elements,such as Ni,Fe,etc.For diamond films,the concentration of the C-center free radicals varies greatly. however,the concentration of the N-center free radicals,which are usually quite low in diamond-like films, varies relative slightly.Finally,the two satellite peaks of the C-center free radicals are found to be the direct evidence of the existence of hydrogen atoms in the PCVD films.

Key words: Diamond, Diamond-like material, ESR