Femtosecond Decay Dynamics of Intact Adenine and Thymine Base Pairs in a Supersonic Jet

We investigated the decay dynamics of the DNA base pairs adenine-adenine (A(2)), adenine-thymine (AT), and thymine-thymine (T(2)) produced in a supersonic jet by femtosecond (fs) time-resolved photoionization spectroscopy. The base pair was excited by a fs pump pulse at 267 nm and the population change of its excited state was monitored by non-resonant three-photon ionization using a fs probe pulse at 800 nm after a certain time delay. All of the transients recorded in the mass channel of the parent ion exhibited a tri-exponential decay, with time constants ranging from 100 fs to longer than 100 ps. Most of these time constants coincide well with the previous values deduced indirectly from the transients of protonated adenine (AH(+)) and thymine (TH(+)), which were assumed to be produced by fragmentation of the base-pair ions. Notably, for the transient of T(2), we observed a new decay component with a time constant of 2.3 ps, which was absent in the transient of TH(+). We suggest that the new decay component arises from the decay of stacked T(2) dimers that are mostly ionized to T(2)(+), whereas the decay signal recorded in the mass channel of TH(+) is merely from the relaxation of hydrogen-bonded T(2) dimers. From the amplitude of the new decay component, the population of the stacked T(2) dimers relative to the hydrogen-bonded dimers was estimated to be similar to 2% in the supersonic jet, which is about fifteen times higher than the theoretical value.