Abstract
The authors report measurements of the ultraviolet absorption ascribed to single-substitutional nitrogen in diamond; it is suggested that the ultraviolet absorption arises due to electric dipole transitions localised at three different defects. Uniaxial stress measurements for lines at 4.059 and 4.567 eV occurring at two of these defects show that they are trigonal centres. These results together with published data suggest that the defect where the 4.059 eV zero phonon line and vibronic bands occur is the single-substitutional nitrogen atom.