The line-widths and spin-spin relaxation times of P1 centers in synthetic Ib and natural Ia diamonds, with P1 and P2 center concentrations ranging from 0.03 to 400ppm[at], were measured. It was found that, at concentrations which were higher than about 10ppm[at], the line-width depended linearly upon the concentration. At lower concentrations, the electron-13C dipolar contribution to the line-width predominated and the width of the line remained constant.

J.A.Van Wyk, E.C.Reynhardt, G.L.High, I.Kiflawi: Journal of Physics D, 1997, 30[12], 1790-3