Intersystem conversion between singlet and triplet exciton states in ZnP2

Detailed excitation spectra for singlet and triplet luminescence in ZnP2 are observed at 2 K by using a Ti:sapphire laser. When a singlet exciton is excited, conversion to a triplet exciton occurs over a wide excitation energy range. The spin memory of the exciton is easily lost under the band-to-band excitation condition. When the triplet exciton is excited, the excitation spectrum for the triplet luminescence shows a clear hydrogen-like series up to n = 9, which reflects the absorption spectrum of a spin triplet exciton series with s-d doublet structure for n greater than or equal to 3. The excitation spectrum for the singlet luminescence under the excitation of the triplet exciton is observed for the first time as a new exciton series appearing only for n greater than or equal to 3. The new exciton series results from the contribution of the d-envelope function of the exciton, and the s-envelope function alone does not give rise to the singlet luminescence.