Peculiarities of the reflection-absorption and transmission spectra of ultrathin films under normal incidence of light

This paper analyzes the prospects of studying optically anisotropic monolayers on a liquid substrate using linearly polarized light. For quantification of the electronic transmission spectra of the studied monolayers on the transparent subphase and Langmuir-Blodgett films transferred onto transparent plates as well, we obtained formulas for optical densities of transmission spectra in a triple system: surrounding medium-ultrathin film-substrate (liquid or solid). The expressions derived in this work are compared with previously obtained formulas for optical densities of spectra measured in the reflection mode for an analogous system. It is shown that, with all other conditions being equal, employment of this measurement technique allows one to obtain spectra the peak intensities of which (measured in absorbance units) are almost an order of magnitude higher than the corresponding values for transmission spectra. The regularities revealed in this work are confirmed by experimental measurements carried out for monolayers of certain conjugated organic compounds of various structures: amphiphilic chromoionofores, photosensitive oligomeric organic conductors, and electrochemically multistable rigid macrocyclic compounds. A good quantitative agreement between theory and experiment was obtained.