Magneto-optical study of paraexcitons in NaI

Effects of an external magnetic field on the free-exciton luminescence (FEL) and the paraexciton resonance have been studied for NaI. The Zeeman splitting of the paraexciton is observed with use of the technique of magnetic-electric dipole two-photon absorption, from which the effective g value is derived to be 1.15. It is also found that the field-induced redshift of the zero-phonon luminescence line from free excitons follows exactly the Zeeman shift of the lowest level of the paraexciton. This fact provides clear evidence that the FEL of NaI originates from the radiative annihilation of paraexcitons, and not of orthoexcitons, which were proposed in previous works.