Search Results - (Author, Cooperation:H. W. Lee)

2013-02-01

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

2012-07-28

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2011-11-05

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2018-07-14

American Association for the Advancement of Science (AAAS)

2375-2548

Natural Sciences in General

2011-06-18

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

1089-7550

AIP Digital Archive

Physics

The multi-shot evolution of laser-induced periodic structure was studied by the time-resolved optical diffraction and reflection from the surface of polycrystalline silicon-on-insulator during the accumulation of 20 ns ruby laser pulses. In contrast to the results previously reported for the bulk sample, it was observed that the channel of the periodic structure development changed during the multiple irradiations even with a fixed fluence. The observed change occurs because the energy absorption is enhanced by the presence of the periodic structure preformed by the preceding pulses, and the absorbed laser energy at the polycrystalline layer is insulated by the underlying SiO2 layer. A previously unappreciated dip structure was observed in the time-resolved diffraction and a possible explanation is suggested postulating the preferential energy deposition in the valleys of the periodic structure.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Optical damage thresholds of submicron-thick, electron beam deposited HfO2 and SiO2 films on BK-7 substrates have been measured by monitoring the emission of neutral constituents during excitation with time-delayed pairs of 70-ps laser pulses at a wavelength of 1064 nm. The dependence of the optical damage threshold on time delay provides evidence of the optical damage mechanism. For SiO2, linear absorption is the mechanism for energy deposition into the films by the laser beams. The data for HfO2 are less definitive, although linear absorption is the most likely damage mechanism. The behavior of the single-layer films is compared to multilayer HfO2-SiO2 high-reflector coatings, for which a "conditioning'' effect causes an increased optical damage threshold due to multiple pulse laser excitation at fluences below the single-pulse optical damage threshold.

Electronic Resource

1365-2133

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

Ultraviolet (UV) irradiation is known to induce serious oxidative damage in the skin via lipid peroxidation. Nitric oxide (NO) synthesized by keratinocytes, melanocytes and endothelial cells in response to proinflammatory cytokines and UV radiation, has been reported to prevent UV-induced apoptosis in the skin. We have examined the effects of NO on UVB-induced lipid peroxidation in murine skin in vivo. UVB induced a dose-dependent increase in lipid peroxidation of skin extracts in vitro; however, lipid peroxidation in the skin in vivo remained unaffected at irradiation doses of less than 1·0 J cm−2 and decreased significantly at doses over 1·5 J cm−2 (P 〈 0·01). Time-delayed inhibition of lipid peroxidation in the skin in vivo was observed after irradiation at 1·5 J cm−2. Administration of N G-nitro- l-arginine methyl ester ( L-NAME), an inhibitor of NO synthesis, enhanced lipid peroxidation (P 〈 0·05), while it suppressed the ear-swelling response (ESR), a biological marker of inflammation. By contrast, administration of sodium nitroprusside, an NO enhancer, suppressed lipid peroxidation (P 〈 0·01), while it enhanced the ESR. Expression of inducible nitric oxide synthase (iNOS) was observed from 12 to 48 h postirradiation at doses of 0·4–1·6 J cm−2. The UVB-induced iNOS expression was markedly inhibited by L-NAME, suggesting that iNOS is a major enzyme in the production of NO. These results suggest that NO acts as a mediator of the inflammatory response in UVB-irradiated skin, and that lipid peroxidation is inversely regulated with the NO-mediated inflammatory response in vivo.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

It is demonstrated that laser-surface interactions that cause optical surface damage of nominally transparent materials can be investigated by observing the effects of excitation by pairs of picosecond pulses separated by a variable time delay. Laser-induced emission of neutrals is used as the detection mechanism in the present experiments.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report, for the first time, the direct write laser patterning of highly conductive Al from a liquid precursor, triisobutylaluminum (TIBA). Al wires were written on Si with a scanned Ar+ laser from liquid TIBA at speeds of up to several mm/s. Wires 3 μm wide by 1 μm high with a resistivity of 5.6 μΩ cm were routinely achievable.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The total charge of the emitted ions from the planar metal targets irradiated by the mode-locked ruby laser was measured. The dependence of the total emitted ions on the laser fluence is consistent with the theory of the multiphoton ionization in the low fluence region. However, it shows saturation phenomena at the higher fluence regions. These saturations could be explained by the depletion of the neutral atoms and the level shifting of the intermediate state due to the strong Stark effect.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Nanocrystals of gallium nitride were synthesized in silica aerogel host matrices by pyrolysis of dimeric dimethylgallium-diphenylamide precursor sequestered in the nanometer scale aerogel pores. Powder x-ray diffraction (XRD) and selected area electron-diffraction results verify the formation of hexagonal gallium nitride material in the aerogels, and Scherrer-broadening analysis of the XRD data indicate the gallium nitride particle size is ∼20 nm. Transmission electron microscopy results show that the gallium nitride nanoparticles have diameters between 10 and 40 nm with an average diameter of 23 nm. The synthesis parameters for the gallium nitride nanocrystals in the aerogel hosts are discussed. © 1996 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The C 1s and O 1s core-level photoabsorption spectra of poly[2-methoxy,5-(2′-ethyl- hexoxy)-1,4-phenylene vinylene] (MEH-PPV) before and after exposure O2 and broadband visible light were recorded to determine the degradation pathway for this polymer. The change in the O 1s spectra as a function of exposure demonstrates that the O adds to the polymer chain to form a carbonyl group. Exposure to only O2 or only light causes no change in the C 1s or O 1s spectra. In the C 1s spectra, the change in the dependence on the photon angle of incidence after exposure demonstrates that O attacks the polymer at the double bond in the vinyl group thereby altering the extended conjugation of the polymer. © 1996 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Collision of hyperthermal Cs+ ion beams with a Si(111) surface partially covered with water gives rise to emission of CsX+ cluster ions (X is a surface atom or molecule) even when the monomer X+ ions are not produced. The yield for atomic and cluster ion emission is examined as a function of Cs+ collision energy, based on which, we propose that CsX+ species are formed by recombination of the scattered, low kinetic energy Cs+ ions and the gaseous neutral species emanating from a surface. It is also demonstrated that under this condition the secondary neutral flux contains a large fraction of molecular units. © 1995 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

A variety of nonphotochemical hole burning experiments are performed on the system pentacene (PC) in benzoic acid (BZA) crystals. These experiments and previously reported fluorescence line narrowing experiments are used to probe the nature of the inhomogeneous broadening of electronic transitions of chromophores in crystals and glasses. A microscopic model is presented which explains detailed results from both types of experiments. Two fundamental assumptions are made in formulating the model. First, there is a large degree of accidental degeneracy in the absolute energy distributions of the chromophores; i.e., molecules in different environments can have the same absolute energy. Second, this degeneracy is lifted when the electronic state is changed. Narrow band excitation of PC causes a well-defined change in the PC's local environment, producing a narrow spectral hole in the inhomogeneously broadened PC absorption origin and an antihole which is spectrally well-separated from the PC origin. The microscopic model is successful in explaining experimental results on correlations between hole widths and antihole widths, hole positions and antihole positions, and in explaining fluorescence line narrowing experiments on the correlation between the frequencies of resonant and nonresonant fluorescence and the broadening of nonresonant fluorescence. The result is a microscopic picture which demonstrates that narrow band excitation of an inhomogeneously broadened electronic transition involves molecules with a very wide distribution of absolute energies.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

We report the observation of 3–4 MeV beams of several species of small doubly charged molecules (10B11B++, 10B++2, 11B12C++, 9Be14N++, 12C13C++, 12C13CH++, 12C14N++, 12C2H++, 12CH++2, 9Be16O++, 10B16O++) emerging from a tandem accelerator. These observations significantly increase the number of such molecules known. All are probably produced in metastable states with lifetimes longer than 1 μs. Substantial fluxes (〉108 molecules per second was shown for CN++) of these doubly charged molecules were obtained and they were identified by observing the fragmentation of molecular ions after passing through a carbon foil. In some cases the molecules are observed to decay in flight. An efficiency of 10−4 in producing mass 25 molecules (12C13C++ +12C2H++) has been measured. Advantage is taken of the existence of "atomic interference free'' mass regions, where no atomic negative ions are formed, therefore allowing the production of pure molecular beams. Some implications of this work for accelerator mass spectrometry (AMS) are discussed. An application of doubly positive charged molecules in a search for doubly negative charged molecules was made, establishing an upper limit of 1:1010 of 10B11B−−/10B11B− and of 1:109 of 12C13C−−/12C13C−.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

We present a theoretical study of the optical dephasing of the electronic transitions of delocalized molecular dimer states. The dephasing arises from the dynamical interactions of the dimer states with the phonon bath of the host crystal. We introduce a general dephasing model for dimers with strong intermolecular interactions and analyze the consequences of the model with optical Redfield theory. A quantum mechanical microscopic model is developed in which the interaction Hamiltonian of the dimer states and the phonon bath states is clearly delineated. We treat both linear and quadratic excitation-phonon coupling in which the two molecules comprising the dimer may be coupled differently to the phonons. Couplings to acoustic, optical, and pseudolocal phonons (librations) are considered explicitly. The temperature dependence of the dephasing rates are obtained for scattering between delocalized dimer states, scattering to dimer librations and phonon-induced pure dephasing. These results are used to analyze the photon echo studies on pentacene dimers and tetracene dimers in p-terphenyl and on naphthalene dimers in perdeuteronaphthalene. In the pentacene dimers and tetracene dimers, we conclude that the dephasing results from scattering to the dimer librations via a one-phonon process. In addition, a quantitative value for the libron–phonon coupling is determined. In the naphthalene dimer system, we find scattering between delocalized dimer states, induced by linear excitation-phonon coupling, to be the predominant dephasing mechanism. A quantitative value is determined for the phonon coupling matrix element responsible for scattering between the delocalized states.

Electronic Resource

1439-0264

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

In this immunocytochemical study on the constitutive expression of Pax-7 protein in the postnatal chicken brain, Pax-7 showed region and cell type specific expression. In the optic tectum, only cells in grey matter showed positive immunoreactivities (IRs), whereas those in the white matters did not show any IRs. In thalamic nuclei and several pontine nuclei, we also localized Pax-7 positive IRs. On the contrary, in the cerebellum, Pax-7 was mainly localized within the Bergmann glia, whereas Purkinje cells did not show any IRs. In double immunolabelling studies, most of the Pax-7 IRs did not originate from neuroglial cells such as oligodendrocytes, microglia or astrocytes, but from neurons, with the exception of Bergmann glia in the cerebellum. The presence of Pax-7 IRs in the adult chicken brain could suggest that Pax-7 might play a role in maintaining normal physiological function in some postnatal chicken brain cells.

Electronic Resource

1365-2230

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

Electronic Resource

0022-1325

Psychology