Investigation of paraexcitons in KI

The lowest paraexciton resonance in KI is measured for the first time. The narrow exciton line at 5.8279 eV is split into four components in a magnetic field. A $g$ value of 1.35 is deduced from the Zeeman splitting of the paraexciton. In addition, a reliable value for the exchange splitting ($\Delta =45\mathrm{}$ meV) is gained from the three-photon spectra, which is much lower than that obtained by one-photon spectroscopy.