Optical study of the secondary absorption edge in type l a diamonds

Abstract

Most natural diamonds contain A aggregates (substitutional pairs) of nitrogen atoms which give rise to intense absorption at $h\nu >30300$ cm$^{-1}$ (3.75 eV). This paper suggests that the shape of the edge is consistent with its arising in transitions localized at the A aggregates, and not, as commonly assumed, in ionizing transitions. The effects on the edge of uniaxial stresses are reported and analysed quantitatively, showing that three transitions at the foot of the edge occur from a common non-degenerate ground state to one non-degenerate excited state (at 30 307 cm$^{-1}$) and to two doubly degenerate excited states (at 31 465, 31 680 cm$^{-1}$) at the trigonal A aggregate. By regarding these transitions as the zero phonon lines of three vibronic progressions, the shape and temperature dependence of the edge may be explained semi-quantitatively. The secondary edge is also shown to be self broadened through the strain fields set up by the A aggregates themselves; the magnitude of this broadening may be understood with use of the uniaxial stress data.