Two-photon absorption of P excitons in ZnP2.

The fine structure of the P excitons in \ensuremath{\beta}-${\mathrm{ZnP}}_{2}$ is studied by two-photon absorption. Due to the anisotropy of the crystal a giant energy splitting of the 2P exciton states of up to 7 meV is observed. This splitting allows the determination of the reduced masses without an external field. The following values are obtained: ${\mathrm{\ensuremath{\mu}}}_{\mathit{a}}$=0.47${\mathit{m}}_{0}$, ${\mathrm{\ensuremath{\mu}}}_{\mathit{b}}$=0.53${\mathit{m}}_{0}$, and ${\mathrm{\ensuremath{\mu}}}_{\mathit{c}}$=0.088${\mathit{m}}_{0}$.