  Atactic polystyrene of two different molecular weights (560900 g.mol-1 and
212400 g.mol-1) have been studied as spin-coated films of thickness varying
from ~30 nm to ~250 nm, i.e., ~2Rg to ~12Rg where Rg is the unperturbed radius
of gyration of polystyrene, using x-ray reflectivity and transmission
UV-spectroscopy. Electron density profiles along depth of the films show
formation of layers parallel to the substrate surface, when the film thickness
is below 4Rg, with layer spacing ~2Rg. The pure electronic singlet 1A1g -> 1E1u
indicates exciton interaction throughout film thickness, as evidenced by the
lineshape, drop in extinction coefficient and size-dependent blue shift of the
corresponding UV peak. Analysis of the total energy of the exciton as a
function of film thickness shows that above the thickness of 4Rg the exciton is
delocalized over the film thickness through linear, molecular J-aggregates. The
separations between these molecules in the two polystyrene samples match with
the respective layer spacing obtained from x-ray reflectivity. Transition
dipole moments of the molecules in this aggregate are parallel and each dipole
is arranged at an angle of ~43 degrees with the axis of the linear chain. Below
a film thickness of 4Rg, as the molecular layers are formed, the exciton
becomes localized within these layers. This localization is clearly indicated
by the lineshape becoming comparable to isolated molecular bands, and by both
the extinction coefficient and blue shift reaching maximum values and becoming
size independent. Localization of excitons due to layer formation has been
explained as caused by decrease of cohesion between layers.
