Optical studies of vibronic bands in yellow luminescing natural diamonds

Abstract

Some brown gem-quality diamonds exhibit bright yellow luminescence when excited with a long-wave ultraviolet (365 nm) mercury lamp. The authors describe detailed absorption and luminescence measurements in the visible spectral region, and absorption measurements in the defect-induced one-phonon region, using a large number of yellow luminescing diamonds. The absorption spectrum in the visible region is dominated by a featureless vibronic band with a peak near 2.6 eV. The absence of vibronic structure on this absorption band indicates that the electron-phonon coupling is unusually strong for a colour centre in diamond. Absorption in the 2.6 eV band produces red luminescence, and structure on the emission band indicates that the phonon coupling is weaker when the centre is in its excited state. The yellow luminescence is associated with a second strongly coupled vibronic system with a zero-phonon line at 2.721 eV. The infrared measurements reveal an absorption feature at 1240 cm^-1 that appears to correlate with the 2.6 eV band, and show that the diamonds are predominantly a mixture of type IaA and type Ib material with a low total impurity concentration.