  We present a comprehensive ellipsometric study of an untwinned, nearly
stoichiometric LaMnO_3 crystal in the spectral range 1.2-6.0 eV at temperatures
20 K < T < 300 K. The complex dielectric response along the b and c axes of the
Pbnm orthorhombic unit cell, \epsilon^b(\nu) and \epsilon^c(\nu), is highly
anisotropic over the spectral range covered in the experiment. The difference
between \epsilon^b(\nu) and \epsilon^c(\nu) increases with decreasing
temperature, and the gradual evolution observed in the paramagnetic state is
strongly enhanced by the onset of A-type antiferromagnetic long-range order at
T_N = 139.6 K. In addition to the temperature changes in the lowest-energy gap
excitation 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 O
2p -- Mn 3d transition around the charge-transfer energy 4.7 eV. Based on the
observation of a pronounced spectral-weight transfer between low- and
high-energy features upon magnetic ordering, they are assigned to high-spin and
low-spin intersite d^4d^4 - d^3d^5 transitions by Mn electrons. The anisotropy
of the lowest-energy optical band and the spectral weight shifts induced by
antiferromagnetic spin correlations are quantitatively described by an
effective spin-orbital superexchange model. An analysis of the multiplet
structure of the intersite transitions by Mn e_g electrons allowed us to
estimate the effective intra-atomic Coulomb interaction, the Hund exchange
coupling, 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 an
intersite d-d transition, whose energy is substantially reduced compared to
that obtained from the bare intra-atomic Coulomb interaction.
