  We report on the UV laser induced fluorescence of hexagonal boron nitride
(h-BN) following nanosecond laser irradiation of the surface under vacuum and
in different environments of nitrogen gas and ambient air. The observed
fluorescence bands are tentatively ascribed to impurity and mono (VN), or
multiple (m-VN with m = 2 or 3) nitrogen vacancies. A structured fluorescence
band between 300 nm and 350 nm is assigned to impurity-band transition and its
complex lineshape is attributed to phonon replicas. An additional band at 340
nm, assigned to VN vacancies on surface, is observed under vacuum and quenched
by adsorbed molecular oxygen. UV-irradiation of h-BN under vacuum results in a
broad asymmetric fluorescence at ~400 nm assigned to m-VN vacancies; further
irradiation breaks more B-N bonds enriching the surface with elemental boron.
However, no boron deposit appears under irradiation of samples in ambient
atmosphere. This effect is explained by oxygen healing of radiation-induced
surface defects. Formation of the oxide layer prevents B-N dissociation and
preserves the bulk sample stoichiometry.
