  Recent ultrafast experiments have implicated intrachain base-stacking rather
than base-pairing as the crucial factor in determining the fate and transport
of photoexcited species in DNA chains. An important issue that has emerged
concerns whether or not a Frenkel excitons is sufficient one needs
charge-transfer states to fully account for the dynamics. In we present an
$SU(2)\otimes SU(2)$ lattice model which incorporates both intrachain and
interchain electronic interactions to study the quantum mechanical evolution of
an initial excitonic state placed on either the adenosine or thymidine side of
a model B DNA poly(dA).poly(dT) duplex. Our calculations indicate that over
several hundred femtoseconds, the adenosine exciton remains a cohesive
excitonic wave packet on the adenosine side of the chain where as the thymidine
exciton rapidly decomposes into mobile electron/hole pairs along the thymidine
side of the chain. In both cases, the very little transfer to the other chain
is seen over the time-scale of our calculations. We attribute the difference in
these dynamics to the roughly 4:1 ratio of hole vs. electron mobility along the
thymidine chain. We also show that this difference is is robust even when
structural fluctuations are introduced in the form of static off-diagonal
disorder.
