Abstract
We present femtosecond pump-probe mass and photoelectron spectra for adenine (A) and microhydrated A(m)(H(2)O)(n) clusters. Three distinct relaxation processes of photoexcited electronic states were distinguished: in unhydrated A, relaxation of the optically bright pipi* state occurred via the dark npi* state with respective lifetimes of <0.1 and 1.3 ps. In microhydrated clusters A(H(2)O)(n), relaxation via the npi* state is quenched by a faster relaxation process, probably involving pisigma* states. For the predominantly hydrogen-bonded adenine dimer (A(2)), excited state relaxation is dominated by monomer-like processes. When the adenine dimer is clustered with several water molecules, we observe a nanosecond lifetime from excimer states in pi-stacked clusters. From the electron spectra we estimate adiabatic ionization potentials of 8.32 eV (A), 8.27 eV (A(H(2)O)(1)), 8.19 eV (A(H(2)O)(2)), 8.10 eV (A(H(2)O)(3)), 8.18 eV (A(2)), and 8.0 eV (A(2)(H(2)O)(3-5)).
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