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ZnO nanoparticles with different shapes and sizes were prepared by changing coordinating ligands. Hexagonal cones presented UV and green (∼500 nm) emissions, which originated from excitons and defects, respectively. Oxygen vacancies were found to be major defects in the hexagonal cones. Blue emission at ∼440 nm was observed for hexagonal plates, and analyses of time-resolved photoluminescence spectra showed that two transitions were responsible for this blue emission, because transitions from Zni to the valence band (430 nm) and from Zni to VZn (480 nm) were distinguished by emission wavelengths and lifetimes. The visible emissions from defects were related to the roles of coordinating ligands. [ABSTRACT FROM AUTHOR]

This study investigated the effects of the post-O2-annealing process on light emission from silicon nanocrystals. Using pulsed laser ablation in an O2-filled atmosphere, the SiOx (x<2) thin layers were synthesized prior to O2-annealing. We observed a strong photoluminescence from the silicon nanocrystals embedded in the annealed layers at 500 °C, which depended on the post-O2-annealing time. We also found that the interface property between the silicon nanocrystal and stoichiometric SiO2 matrix is critical for efficient light emission. The electroluminescence spectra in the visible range were obtained above the on-current density of 0.7 A/cm2 under several forward-bias voltages. The blueshift in the electroluminescence was attributed to the band-filling effect under high current injection mode. The external emission efficiency was estimated to be about 0.3% by using the band-filling model. [ABSTRACT FROM AUTHOR]

Photofragmentation of mass-selected aniline+(water)n (An+Wn, n=4–20) clusters is investigated over photon energies ranging from 1.65 to 4.66 eV by linear tandem time-of-flight mass spectrometry. The aniline ring turns out to survive irradiation of photons, and most of the absorbed photon energy flows to the hydrogen-bonding networks to be used up for liberation of water molecules. The average number of ejected water molecules measured as a function of photon energy reveals that the loss of water molecules is a photoevaporation process. The distributions of internal energies for parent ions and binding energies of water molecules are estimated from the plots of photofragment branching ratio versus photon energy, which give nice Gaussian fits. Also, density functional theory calculations are performed to obtain optimized structures of isomers for An+Wn clusters and binding energies. The authors find that the An+W6 cluster has a highly symmetric structure and its binding energy in An+W6→An+W5+W stands out. This is in line with the experimental results showing that n=6 is a magic number in the mass distribution and An+W6 is relatively stable in metastable decay. [ABSTRACT FROM AUTHOR]

Time-resolved optical emission spectroscopy was employed to examine the temporal properties of a plume formed by laser ablation of a graphite target in He, N2, and Ar background gases. Time-dependent spectroscopic temperatures of electronically excited C2 and CN molecules generated in the plume at different background gases and pressures were derived by simulation of the emission spectra. The rotational temperature of CN molecules was significantly higher than the vibrational temperature. This indicates that they are formed in the gas phase by the chemical reaction C2+N2→CN. At pressure above 0.5 Torr, the temperature in each background gas was apparently in the order of Ar>N2>He while there was no such gas dependence at 0.1 Torr. The expansion dynamics and cooling rate of the plume turned out to be highly dependent on the background gas and its pressure. [ABSTRACT FROM AUTHOR]

Distinct hydrogen bonding patterns are predicted in the amide I and amide A vibrational bands of four dominant conformations of antamanide using anharmonic vibrational Hamiltonians constructed at the DFT and AM1 levels. We show how these conformations may be distinguished using coherent three pulse infrared measurements with several pulse polarization configurations in the amide I region. The amide A hydrogen bonded N-H stretching modes are highly localized and have conformation-dependent frequencies, but their anharmonicities are insensitive to local structure at the hydrogen bond distances in antamanide. [ABSTRACT FROM AUTHOR]

Vibrational exciton Hamiltonians for the amide I and amide A modes of both the α- and 3[sub 10]-helical conformations of a fifteen unit polyalanine oligomer CH[sub 3]-CO(Ala)[sub 15]-NHCH[sub 3] are constructed using density-functional calculations for smaller model peptides. Energy levels as well as the transition dipoles of all singly and doubly excited-state manifolds are calculated. A variety of C[sub 13]-substituted isotopic derivatives are examined with respect to their ability to reveal differences in local secondary structures in two-dimensional infrared spectra in the amide I region. Amide mode anharmonicities are predicted to be valid indicators of secondary helical structures. [ABSTRACT FROM AUTHOR]

Mass spectral distributions and time-of-flight spectra of C[sup +][sub n] (n = 1-24) ions escaping from a carbon plasma plume generated by laser (λ = 1064 nm) ablation of a graphite target are analyzed to shed light on their formation mechanisms, expansion dynamics, and laser-plume interaction. In particular, by shining the whole or a slice of the plume with a photodissociation laser (λ=266 nm) delayed with respect to the ablation laser, we examine the effects of the laser-plume interaction on the mass distribution as well as photodissociation channels of carbon-cluster cations. Also, we present experimental data that show a clear linear dependence of the velocities of C[sup +][sub n] ions on 1/&Mradic;. This result confirms that C[sup +][sub n] ions are mainly accelerated by a transient local electric field in the plume. [ABSTRACT FROM AUTHOR]

Presents information on a mass spectroscopic studies on a reactive laser ablation of YBa...Cu...O... in a pulsed oxygen jet. Ion mass spectra with oxygen jet on and off; Dependence of wavelength; Conclusion.

We have used the method of Shapiro, Hepburn, and Brumer to control the ionization rates of HCl and CO. This method is based on the principle of quantum mechanical interference between two competing paths. In this case the two paths are one- and three-photon excitation of an intermediate Rydberg state. A vacuum ultraviolet (VUV) (110–115 nm) laser beam was produced by third-harmonic generation from an UV beam in Kr gas. The relative phases of the two beams were controlled by passing them through a chamber containing either Ar or H2. The laser beams were focused into a molecular beam, and the ionization products were measured in a time-of-flight mass spectrometer. As the pressure of the phase-tuning gas was varied, the ionization signal was found to oscillate with a period that is proportional to the difference of the indices of refraction of the tuning gas at the UV and VUV wavelengths. We have applied this technique to transitions involving different intermediate electronic states [the j 3Σ-(0+), H 1Σ+(0+), and m 3Π(1) states of HCl and the B 1Σ+ state of CO], different rotational lines of the same electronic transition, and different competing processes (ionization vs dissociation) following excitation of the intermediate state. [ABSTRACT FROM AUTHOR]

We have used the method of coherent phase modulation, first proposed by Brumer and Shapiro for controlling the rates of chemical reactions, to measure the refractive index of a gas in the vacuum ultraviolet. In these experiments we populated a Rydberg state of HCl or CO by simultaneous absorption of one VUV photon and three UV photons. Molecular ions were generated by absorption of an additional UV photon and detected with a time-of-flight mass spectrometer. The phase difference for the UV and VUV beams was altered by passing them through a cell containing either H2 or Ar. By varying the pressure in the cell we could control this phase difference and thereby modulate the ion signal. From the modulation frequency of the signal it was possible to determine the difference between the refractive indices of the gas at the UV and VUV wavelengths. Using reliable refractive indices in the UV, we extracted from our data values for the VUV indices. These values are in quantitative agreement with a calculation that uses dipole oscillator strengths constructed using constrained oscillator strength methods, taking into account the dispersion resulting from individual rotational transitions. The experimental method can be used very close to an optical transition of the refractive medium, provided that the refractive index does not vary appreciably across the bandwidth of the VUV laser. [ABSTRACT FROM AUTHOR]

Coherent control of the ionization of HCl was achieved by simulataneous 3+1 and 1+1 multiphoton excitation in a molecular beam machine, using either three 336 nm photons or one 112 nm photon to resonantly excite the intermediate j 3Σ-(Ω=0+) state. The phase difference between the two laser beams was continuously varied by passing the radiation through a cell containing either Ar or H2 gas. [ABSTRACT FROM AUTHOR]

Photocurrent of a single ZnO nanowire synthesized by a sol-gel route was investigated. In vacuum, the dark current was bigger but the photoresponse was slower than that in air, attributed to the release of the available charge carriers by the desorption of water molecules and the decrease of the exchange rates of molecular ions. Under the steady radiation of the ultraviolet light (λ=325 nm), a gradual decrease of the photocurrent was noticeable, which can be explained in terms of the annihilation of the carriers by the replacement of hydroxyl groups (OH-) by O2-, resulting in the decrease of charge carriers. [ABSTRACT FROM AUTHOR]

Ordered arrays of Er-doped Si nanodots were fabricated by pulsed laser deposition using an anodic aluminum oxide (AAO) nanopore membrane. Er-doped Si (Si:Er) nanodots with an average diameter of 60–80 nm and a periodicity of 100 nm were uniformly assembled into hexagonally ordered nanopores of AAO membrane. Photoluminescence (PL) spectra taken from Si:Er nanodots showed an emission peak at 1.54 μm, which is due to intra-4f shell transition (4I13/2→4I15/2) of Er3+ ions. Postannealing of nanodot arrays up to 500 °C resulted in the enhancement of PL intensities without enlargement or aggregation of the nanodots. [ABSTRACT FROM AUTHOR]

We have fabricated Er-doped amorphous SiOx films by laser ablation of a Si:Er[sub 2]O[sub 3] target in He atmosphere. The photoluminescence intensity at 1.54 µm was highly dependent on the oxygen content in the film, which turned out to be changed significantly by the ambient He pressure. Also, we have adopted time-of-flight quadrupole mass spectroscopy to obtain kinetic energies of ionic species in a plume produced by laser ablation. Si and Er ions do not overlap spatially as they expand toward the Si substrate and Er ions impinge on the preformed SiO[sub x] layer. [ABSTRACT FROM AUTHOR]

Photofragmentation of mass-selected aniline(+)(water)n (An(+)Wn, n=4-20) clusters is investigated over photon energies ranging from 1.65 to 4.66 eV by linear tandem time-of-flight mass spectrometry. The aniline ring turns out to survive irradiation of photons, and most of the absorbed photon energy flows to the hydrogen-bonding networks to be used up for liberation of water molecules. The average number of ejected water molecules measured as a function of photon energy reveals that the loss of water molecules is a photoevaporation process. The distributions of internal energies for parent ions and binding energies of water molecules are estimated from the plots of photofragment branching ratio versus photon energy, which give nice Gaussian fits. Also, density functional theory calculations are performed to obtain optimized structures of isomers for An(+)Wn clusters and binding energies. The authors find that the An(+)W6 cluster has a highly symmetric structure and its binding energy in An(+)W6-->An(+)W5+W stands out. This is in line with the experimental results showing that n=6 is a magic number in the mass distribution and An(+)W6 is relatively stable in metastable decay.