Infrared optical excitations inLa2NiO4

Measurements are reported of the transmission of single crystals of ${\mathrm{La}}_2$ ${\mathrm{NiO}}_4$ containing no excess oxygen. Several weak absorption bands are observed in the range 0.15–2 eV arising from magnetic and crystal-field excitations. The lowest-energy band, at 0.25 eV, is quantitatively described by a recent theory for phonon-assisted creation of a virtual bound state of two magnons. However, unlike ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$, no higher-energy sidebands are detected, suggesting that the broad, structured absorption seen clearly in ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$ between ∼0.4 and 1 eV results from sidebands to crystal-field excitons as previously proposed.