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The excited-state dynamics of adenine and thymine dimers and the adenine-thymine base pair were investigated by femtosecond pump-probe ionization spectroscopy with excitation wavelengths of 250-272 nm. The base pairs showed a characteristic ultrafast decay of the initially excited pi pi* state to an n pi* state (lifetime tau(pi pi*) approximately 100 fs) followed by a slower decay of the latter with tau(n pi*) approximately 0.9 ps for (adenine)2, tau(n pi*) = 6-9 ps for (thymine)2, and tau(n pi*) approximately 2.4 ps for the adenine-thymine base pair. In the adenine dimer, a competing decay of the pi pi* state via the pi sigma* state greatly suppressed the n pi* state signals. Similarities of the excited-state decay parameters in the isolated bases and the base pairs suggest an intramonomer relaxation mechanism in the base pairs.

Ab initio calculations and time-resolved photoionization spectroscopy were carried out to characterize the role of the lowest two pi sigma* excited states for the photoinduced processes in the adenine monomer, adenine dimer, and adenine-water clusters. The calculations show--with respect to the monomer--a stabilization of 0.11-0.14 eV for the pi sigma* states in different isomers of adenine dimer and an even bigger stabilization of 0.14-0.36 eV for isomers of adenine-(H2O)1 and adenine-(H2O)3. Hence, the stabilized pi sigma* states should play an important role in the excited-state relaxation of partially or fully solvated adenine. This conclusion is supported by experimental results: In the adenine monomer, strong n pi* state signals are observed. Those signals are reduced in adenine dimer and vanish in water clusters due to the competing relaxation via the pi sigma* states.

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The photoinduced H-atom-transfer reaction in indole(NH3) n clusters has been analyzed by femtosecond time-resolved photoelectron−photoion coincidence spectroscopy. The different contributions to the measured time-dependent ion and electron signals resulting from ionization by one and two probe photons can be discriminated and analyzed separately. In particular, the distinctively different dynamical behavior observed for clusters with small (n = 1−3) and larger (n ≥ 4) numbers of ammonia molecules is elucidated. For the small clusters an ultrafast process with a time constant of about 150 fs is identified and attributed to internal conversion from the initially excited ππ* state to the πσ* state. In contrast, for the larger clusters (n ≥ 4) such an initial ultrafast process is not observable probably for Franck−Condon reasons, while a structural rearrangement mechanism after the H transfer on a time scale of 10 ps is clearly recognized.