High-resolution Stark spectroscopy of the 6070 AA colour centre in NaF using spectral hole-burning

The nature of the 6070 AA colour centre in NaF has been investigated using high-resolution spectral hole-burning. Well resolved Stark splittings were observed in seven experimental geometries and it was found that the centre lacks inversion and has a plane of symmetry perpendicular to (110). The transition dipole is perpendicular to this symmetry plane and the difference of ground and excited-state permanent dipole moments lies in this plane along (1.000(5), 1.000(5), 1.445(10)). The previous assignment of this line to an N 1 centre with C 2h symmetry is not consistent with these results. Possible alternative defect structures are considered.