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We present a comprehensive ellipsometric study of an untwinned, nearlystoichiometric LaMnO_3 crystal in the spectral range 1.2-6.0 eV at temperatures20 K < T < 300 K. The complex dielectric response along the b and c axes of thePbnm orthorhombic unit cell, \epsilon^b(\nu) and \epsilon^c(\nu), is highlyanisotropic over the spectral range covered in the experiment. The differencebetween \epsilon^b(\nu) and \epsilon^c(\nu) increases with decreasingtemperature, and the gradual evolution observed in the paramagnetic state isstrongly enhanced by the onset of A-type antiferromagnetic long-range order atT_N = 139.6 K. In addition to the temperature changes in the lowest-energy gapexcitation at 2 eV, there are opposite changes observed at higher energy at 4 -5 eV, appearing on a broad-band background due to the strongly dipole-allowed O2p -- Mn 3d transition around the charge-transfer energy 4.7 eV. Based on theobservation of a pronounced spectral-weight transfer between low- andhigh-energy features upon magnetic ordering, they are assigned to high-spin andlow-spin intersite d^4d^4 - d^3d^5 transitions by Mn electrons. The anisotropyof the lowest-energy optical band and the spectral weight shifts induced byantiferromagnetic spin correlations are quantitatively described by aneffective spin-orbital superexchange model. An analysis of the multipletstructure of the intersite transitions by Mn e_g electrons allowed us toestimate the effective intra-atomic Coulomb interaction, the Hund exchangecoupling, and the Jahn-Teller splitting energy between e_g orbitals in LaMnO_3.This study identifies the lowest-energy optical transition at 2 eV as anintersite d-d transition, whose energy is substantially reduced compared tothat obtained from the bare intra-atomic Coulomb interaction.

Low-energy Mott-Hubbard excitations in LaMnO_3 probed by optical ellipsometry