The C center isolated nitrogen-related infrared absorption at 2688 cm-1: perfect harmony in diamond

Seventy-six diamonds with detectable C centers were selected based on the presence of a 1344 and/or 2688 cm(-1) absorption in their infrared (IR) spectra. Such diamonds are always distinctly colored, often in yellow to orange with various modifications of colours such as brown and green. Fifty-seven of them had an IR spectrum exhibiting the well-known C center-related 1344.4 cm(-1) (hereinafter 1344 cm(-1)) IR absorption peak with an absorption coefficient of more than 0.1. The IR spectra of 15 diamonds had a 1344 cm(-1) peak with an absorption coefficient below 0.1 cm(-1). Finally, in the spectra of 4 samples the 1344 cm(-1) peak was undetectable because of strong aggregated nitrogen absorption but the 2688 cm(-1) line was present. When the 1344 cm(-1) absorption is superior to 0.1 cm(-1), this fairly sharp line (FWHM = 3.3-4.2 cm(-1)) at 2688 cm(-1) is detected, which correlates in intensity with the 1344 cm(-1) feature.