  Charge Transfer (CT) has enjoyed continuous interest due to increasing
experimental control over molecular structure leading to applications in, for
example, photovoltaics and hydrogen production. In this paper, we investigate
the effect of CT states on the absorption spectrum of linear molecular
aggregates using a scattering matrix technique that allows us to deal with
arbitrarily large systems. The presented theory performs well for both strong
and weak mixing of exciton and CT states, bridging the gap between previously
employed methods which are applicable in only one of these limits. In
experimental spectra the homogeneous linewidth is often too large to resolve
all optically allowed transitions individually, resulting in a characteristic
two-peak absorption spectrum in both the weak- and strong-coupling regime.
Using the scattering matrix technique we examine the contributions of free and
bound states in detail. We conclude that the skewness of the high-frequency
peak may be used as a new way to identify the exciton-CT-state coupling
strength.
