Infrared optical excitations in La2NiO4.

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 \ensuremath{\sim}0.4 and 1 eV results from sidebands to crystal-field excitons as previously proposed.