# Uniaxial Stress Studies of the 2.498 eV (H4), 2.417 eV and 2.536 eV Vibronic Bands in Diamond

E. S. De Sa, G. Davies

## Abstract

The H4 vibronic band is the dominant end product of annealing irradiated diamonds which contain impurity nitrogen in one specific aggregated form, the 'B' form. This paper reports uniaxial stress experiments on the H4 zero phonon line, and establishes it unambiguously as a $\langle 110\rangle$ electric dipole transition at a monoclinic I centre. Two independent and previously ignored zero phonon lines at 2.417 and 2.536 eV are also shown to be $\langle 110\rangle$ dipole transitions at monoclinic I centres. The uniaxial stress and vibronic properties of the three centres suggest that their molecular structures are all similar to the commonly observed H3 centre, implying that a family of closely related, but different, optical centres may be produced by irradiating and annealing type I a diamonds. The lack of mirror symmetry in the H4 absorption and luminescence bands is shown to be consistent with the existence of two nearly degenerate excited electronic states at the H4 centre, which may interact through modes of vibration of the appropriate symmetry. The model assumes that transitions from the ground state occur readily to one excited electronic state and negligibly to the other. It is found that substantial breakdown of mirror symmetry may occur without any observable complications in the uniaxial stress spectra, as at the H4 centre, or with the appearance of stress induced absorption, as at the H3 centre, with only small differences in the properties of the two centres. Uniaxial stress measurements on a prominent absorption line in the H4 vibronic sideband are consistent with the model.