The luminescence of nanocrystalline ZnS : Mn2+ indeed shows a brief decay time part (?a hundred ns), but also a long (ms) decay time is observed. The brief decay time is ascribed to some defect-associated emission of ZnS, and is not with the decay on the transition of your Mn2+ impurity. The transition of the Mn2+ has a `standard??decay of about 1.