EXCITONIC MOLECULES IN ZNP2

We have measured luminescence spectra of a biaxial ZnP2 crystal under high density excitation at 2 K, and found two non-linear bands which are assigned as due to radiative annihilation of the excitonic molecule leaving an excition. The lower energy band is concluded to be associated with the ground state of the excitonic molecule with symmetry GAMMA-1+ and the higher energy band with the first excited rotational state with symmetry GAMMA-1- + 2-GAMMA-2-. From the time resolved spectra of these luminescences, it is assumed that a hot excitonic molecule with a large wave vector contributes to the higher energy luminescence. The binding energies of the excitonic molecules in the ground and the rotational states are estimated to be 5.0 meV and 3.6 meV, respectively.