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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Multiphoton ionization of thymine and uracil clusters generated by a supersonic molecular beam gave rise to a remarkable alternation of mass spectral intensities between even-and odd-numbered clusters. Such alternation was observed in clusters of up to 30 molecules. Excitation to the two lowest electronically excited states seemed to be a strong prerequisite. In view of the well known photodimerization reaction of thymine and uracil in the bulk phase, it is proposed that such alternation in the mass spectral intensity resulted from formation of photodimer units within the cluster on intense UV irradiation. Several analogues of thymine with no known propensity for photodimerization in the bulk phase did not exhibit any sign of such alternation in the cluster mass spectrum. The intrinsic UV window for photodimerization, and hence photoinduced mammalian mutagenesis, was estimated to be approximately 210 -280 nm, significantly narrower than the previously reported bulk values of 150 -300 nm. S olar radiation is known to cause mutagenesis and carcinogenesis in mammalian cells (1-4). It is now widely accepted that DNA is the central target for such sunlight-induced lethality. In particular, pyrimidine base dimers are believed to play a key role. The adjacent bases of thymine in a DNA strand tend to undergo facile photodimerization on irradiation of UV light (1-5), and these photodimers cause a miscarriage of genetic information during replication of DNA. Despite many studies (6-11) and a recent heightened awareness of the ozone-hole problem, direct information on the intrinsic features of this process is lacking, mainly because it has never been investigated at an isolated molecular level. Here we generated molecular clusters of pyrimidine bases and found that strong UV irradiation of these cold clusters leads to a mass spectrum with a striking alternation of intensities between even-and odd-numbered clusters. Several derivatives of thymine and uracil with no known propensity for photodimerization did not yield such mass distribution when they were subject to the same irradiation condition. We therefore seemed to have found a strong correlation between the occurrence of the alternation pattern in cluster mass spectrum and the known tendency of the same molecule to undergo photodimerization in the bulk phase. By using the alternation as the criterion for photodimerization in an isolated molecular system, we estimated the intrinsic wavelength range of UV light causing photodimerization, a leading cause of photoinduced mammalian mutagenesis. It covers a range of 210-280 nm, which is significantly narrower than the previously reported bulk value of 150-300 nm.The experimental methods used in the present study have been described elsewhere (12, 13). Briefly, we used a molecular beam machine equipped with a photoionization time-of-flight (TOF) MS (Fig. 1). Clusters of thymine or uracil were generated by supersonic expansion of the sample with 2 atm (1 atm ϭ 101.3 kPa) argon carrier gas through a high-temperature pulsed nozz...

Articles you may be interested inClusters of thymine were found to undergo facile and extensive photodimerization upon strong UV irradiation. Multiphoton excitation/ionization yields cluster ions with excess energy, which undergo thermal fragmentation during flight in time-of-flight mass spectrometer. Reflectron mass spectrometry was employed to investigate the fragmentation dynamics. Remarkably size-specific modes of fragmentation were found: a cluster ion that consists of an even number of molecules tends to lose two molecules, while one that consists of an odd number of molecules loses one molecule. The strong alternation of intensity in the cluster mass spectrum was found to be due to such distinct fragmentation modes. The two-molecule loss was found to be a single fission process of a dimeric unit, which was in accord with a model of the cluster ion that consists of mostly dimeric units. The culprit was extensive intracluster photodimerization, which was shown to occur through absorption of anomalously large numbers of photons by the cluster system during excitation/ ionization.

We observed the surface degradation of MgO single-crystal substrates stored in humid air after cleavage by atomic force microscope. Annealing at 1000 °C in dry oxygen removed local defects though residual subgrain structure remains. The morphology of YBa2Cu3O7−δ (YBCO) films on the annealed and degraded substrates reflects these changes of the substrates. We confirmed that the annealing process of degraded substrate improves the film quality by measuring transition temperature Tc and critical current Jc of each film.