Articles you may be interested inElectronic spectra of the jet-cooled DNA base adenine were obtained by the resonant two-photon ionization ͑R2PI͒ and the laser induced fluorescence ͑LIF͒ techniques. The 0-0 band to the lowest electronically excited state was found to be located at 35 503 cm Ϫ1 . Well-resolved vibronic structures were observed up to 1100 cm Ϫ1 above the 0-0 level, followed by a slow rise of broad structureless absorption. The lowest electronic state was proposed to be of n * character, which lies ϳ600 cm Ϫ1 below the onset of the * state. The broad absorption was attributed to the extensive vibronic mixing between the n * state and the high-lying * state.

A multiphoton ionization study was carried out on hydrated clusters of DNA base adenine in supersonic molecular beams. Resonant two-photon ionization at 266 nm showed that the relative ion intensity between the hydrated clusters A m (H2O) n and the unhydrated ones was anomalously small, particularly for m = 1, in contrast to the case of electron impact ionization. The ratio was of the order of 10-2 for m = 1, 10-1 for m = 2, but about 1 for m ≥ 3. One-photon excitation to the first electronically excited state was found to be responsible for the extensive fragmentation of adenine monomer hydrates A1(H2O) n . The water acts as a proton-donating solvent whose hydrogen bonding with the solute becomes weakened in the n π* excited state, thereby giving the excited state its repulsive nature. Hydrates of adenine complex A m (H2O) n (m ≥ 2) tend to better survive the rupture of the water cage at higher m, probably because the energy transfer between adenine molecules is not sufficiently fast. The fragmentation was found to be less extensive at higher levels of excitation with a much weaker n π* character. Change of solvent to those of less proton-donating or even proton-accepting character systematically reduced the tendency of fragmentation.

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Tunneling spectra of a polycrystalline bulk HgBa&Ca2Cu308+& with T, = 132 K have been measured by point-contact spectroscopy using a low-temperature scanning tunneling microscope. Unlike other high-T, superconductors the conductance of this material shows a BCS-like gap structure with a relatively small smearing parameter I'. Conductances with a linear density of states for IeVI ( b, and a small cusp at the gap are also observed. From a simple calculation based on the assumption that the surface is covered with a normal layer, we explained the linear dI/dV for IeVI ( 4 and obtained two distinctively different energy gaps 6 = 48 meV and 22 meV with the resultant 2A/k&T, = 8.5 and 3.9, respectively.

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