  Recent ultrafast experiments have implicated that intrachain base-stacking
rather than base-pairing mediate the fate and transport of photoexcited species
in DNA chains. Here use 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.
