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 N1 centre with C2h symmetry is not consistent with these results. Possible alternative defect structures are considered.