  We have performed both theoretical and experimental study of optical response
of parent perovskite manganites RMnO_3 with a main goal to elucidate nature of
clearly visible optical features. Starting with a simple cluster model approach
we addressed the both one-center (p-d) and two-center (d-d) charge transfer
(CT) transitions, their polarization properties, the role played by structural
parameters, orbital mixing, and spin degree of freedom. Optical complex
dielectric function of single crystalline samples of RMnO_3 (R=La, Pr, Nd, Sm,
Eu) was measured by ellipsometric technique at room temperature in the spectral
range from 1.0 to 5.0 eV for two light polarizations: E \parallel c and E \perp
c. The comparative analysis of the spectral behavior of \varepsilon _1 and
\varepsilon _2 is believed to provide a more reliable assignment of spectral
features. We have found an overall agreement between experimental spectra and
theoretical predictions based on the theory of one-center p-d CT transitions
and inter-site d-d CT transitions. Our experimental data and theoretical
analysis evidence a dual nature of the dielectric gap in nominally
stoichiometric matrix of perovskite manganites RMnO_3, it is formed by a
superposition of forbidden or weak dipole allowed p-d CT transitions and
inter-site d-d CT transitions. In fact, the parent perovskite manganites RMnO_3
should rather be sorted neither into the CT insulator nor the Mott-Hubbard
insulator in the Zaanen, Sawatzky, Allen scheme.
