The symmetry of an N centre in sodium chloride

The symmetry of the colour centre giving rise to an absorption line at 8373 Å in the N-band region of irradiated sodium chloride has been determined by the application of uniaxial stress to the crystal. The stress splitting patterns indicate that the centre has monoclinic symmetry, and the results are compared with those obtained previously on similar zero-phonon lines in lithium fluoride and sodium fluoride. The phonon-assisted transitions accompanying the line are compared with recent calculations of the vibrational spectrum of sodium chloride.