Ultrafast Dynamics in Na-Doped Water Clusters and the Solvated Electron

The lifetimes of the first electronically excited state of (H(2)O)(n)...Na and (D(2)O)(n)...Na clusters up to n = 40 have been measured by two-color pump-probe spectroscopy (800 and 400 nm) with 35 fs laser pulses. The excited-state lifetime decreases rapidly from 1.2 ps at n = 2 to approximately 100 fs at n > or = 10. For (D(2)O)(n)...Na, the average lifetime is about 3.6 times longer. The fast energy redistribution is explained by conversion of the electronic excitation into vibrations of the ground state. A simple model based on Fermi's Golden Rule predicts the observed trends but fails to reproduce the observed lifetimes quantitatively. The longer lifetimes for deuterated clusters are discussed in the framework of the famous energy gap law and indicate that the stretching modes of water play an important role in the energy-transfer process.