Showing papers by "Richard J. Saykally published in 2004"

Nanoribbon waveguides for subwavelength photonics integration.

Abstract: Although the electrical integration of chemically synthesized nanowires has been achieved with lithography, optical integration, which promises high speeds and greater device versatility, remains unexplored We describe the properties and functions of individual crystalline oxide nanoribbons that act as subwavelength optical waveguides and assess their applicability as nanoscale photonic elements The length, flexibility, and strength of these structures enable their manipulation on surfaces, including the optical linking of nanoribbon waveguides and other nanowire elements to form networks and device components We demonstrate the assembly of ribbon waveguides with nanowire light sources and detectors as a first step toward building nanowire photonic circuitry

Energetics of hydrogen bond network rearrangements in liquid water.

Abstract: A strong temperature dependence of oxygen K-edge x-ray absorption fine structure features was observed for supercooled and normal liquid water droplets prepared from the breakup of a liquid microjet. Analysis of the data over the temperature range 251 to 288 kelvin (–22° to +15°C) yields a value of 1.5 ± 0.5 kilocalories per mole for the average thermal energy required to effect an observable rearrangement between the fully coordinated (“ice-like”) and distorted (“broken-donor”) local hydrogen-bonding configurations responsible for the pre-edge and post-edge features, respectively. This energy equals the latent heat of melting of ice with hexagonal symmetry (ice Ih) and is consistent with the distribution of hydrogen bond strengths obtained for the “overstructured” ST2 model of water.

Ultrafast Carrier Dynamics in Single ZnO Nanowire and Nanoribbon Lasers

Abstract: Time-resolved second-harmonic generation (TRSHG) and transient photoluminescence (PL) spectroscopy are utilized to probe the ultrafast creation and subsequent relaxation of excited carriers immediately following band-gap excitation in single ZnO nanowire and nanoribbon lasers. The TRSHG signal consists of a 1−5 ps recovery present only during strong lasing and a 10−80 ps intensity-dependent component. The transient PL response from single structures exhibits an 80 ps decay component independent of pump power (free exciton PL), and a < 10 ps power-dependent component (stimulated emission) that shifts to earlier delay by ca. 10 ps at high pump fluence.

Dissociative recombination of rotationally cold H 3

Abstract: This paper presents the first dissociative recombination (DR) measurement of electrons with rotationally and vibrationally cold $\mathrm{H}_{3}^{+}$ ions. A dc discharge pinhole supersonic jet source was developed and characterized using infrared cavity ringdown spectroscopy before installation on the CRYRING ion storage ring for the DR measurements. Rotational state distributions $({T}_{\mathrm{rot}}\ensuremath{\sim}30\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ produced using the source were comparable to those in the diffuse interstellar medium. Our measurement of the electron energy dependence of the DR cross section showed resonances not clearly seen in experiments using rotationally hot ions, and allowed calculation of the thermal DR rate coefficient for ions at interstellar temperatures, ${\ensuremath{\alpha}}_{\mathrm{DR}}(23\phantom{\rule{0.3em}{0ex}}\mathrm{K})=2.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. This value is in general agreement with recent theoretical predictions by Kokoouline and Greene [Phys. Rev. A 68, 012703 (2003)]. The branching fractions of the two breakup channels, $\mathrm{H}+\mathrm{H}+\mathrm{H}$ and $\mathrm{H}+{\mathrm{H}}_{2}$, have also been measured for rotationally and vibrationally cold $\mathrm{H}_{3}^{+}$.

Investigation of volatile liquid surfaces by synchrotron x-ray spectroscopy of liquid microjets

Abstract: Soft x-ray absorption spectroscopy is a powerful probe of surface electronic and geometric structure in metals, semiconductors, and thin films. Because these techniques generally require ultrahigh vacuum, corresponding studies of volatile liquid surfaces have hitherto been precluded. We describe the design and implementation of an x-ray experiment based on the use of liquid microjets, permitting the study of volatile liquid surfaces under quasi-equilibrium conditions by synchrotron-based spectroscopy. The liquid microjet temperatures are also characterized by Raman spectroscopy, which connects our structural studies with those conducted on liquid samples under equilibrium conditions. In recent experiments, we have observed and quantified the intermolecular surface relaxation of liquid water and methanol and have identified a large population of “acceptor-only” molecules at the liquid water interface.

Water Dimers in the Atmosphere II: Results from the VRT(ASP-W)III Potential Surface

Abstract: We report refined results for the equilibrium constant for water dimerization (KP), computed as a function of temperature via fully coupled 6D calculation of the canonical (H2O)2 partition function on VRT(ASP-W)III, the most accurate water dimer potential energy surface currently available. Partial pressure isotherms calculated for a range of temperatures and relative humidities indicate that water dimers can exist in sufficient concentrations (e.g., 1018 m-3 at 30 °C and 100% relative humidity) to affect physical and chemical processes in the atmosphere. The determinations of additional thermodynamic properties (ΔG, ΔH, ΔS, CP, and CV) for (H2O)2 are presented, and the role of quasi-bound states in the calculation of KP is discussed at length.

Elucidating the role of many-body forces in liquid water. I. Simulations of water clusters on the VRT(ASP-W) potential surfaces

Abstract: We test two new potentials for water, fit to vibration-rotation tunneling (VRT) data by employing diffusion quantum Monte Carlo simulations to calculate the vibrational ground-state properties of water clusters. These potentials, VRT(ASP-W)II and VRT(ASP-W)III, are fits of the highly detailed ASP-W (anisotropic site potential with Woermer dispersion) ab initio potential to (D2O)2 microwave and far-infrared data, and along with the SAPT5s (five-site symmetry adapted perturbation theory) potentials, are the most accurate water dimer potential surfaces in the literature. The results from VRT(ASP-W)II and III are compared to those from the original ASP-W potential, the SAPT5s family of potentials, and several bulk water potentials. Only VRT(ASP-W)III and the spectroscopically “tuned” SAPT5st (with N-body induction included) accurately reproduce the vibrational ground-state structures of water clusters up to the hexamer. Finally, the importance of many-body induction and three-body dispersion are examined, and...

Infrared Cavity Ringdown Spectroscopy of Jet-Cooled Nucleotide Base Clusters and Water Complexes

Abstract: We present the first direct infrared absorption measurements of gas phase nucleotide base clusters and complexes with water. Spectra in the NH stretching region indicate the presence of several doubly H-bonded isomers of both thymine and uracil dimers, as well as both larger base clusters and uracil−water complexes, but no singly H-bonded species are observed. Three NH stretching bands are assigned to three different uracil−water dimer isomers. Bands due to larger complexes are also observed, but due to spectral congestion, assignment to specific clusters is tentative. The IR cavity ringdown laser absorption spectroscopy results show unusually large bonded OH stretch red shifts for uracil−water clusters, providing evidence of the extremely strong hydrogen bonds formed between these molecules predicted in ab initio calculations.

A "First Principles" Potential Energy Surface for Liquid Water from VRT Spectroscopy of Water Clusters

Abstract: We present results of gas phase cluster and liquid water simulations from the recently determined VRT(ASP-W)III water dimer potential energy surface. VRT(ASP-W)III is shown to not only be a model of high ''spectroscopic'' accuracy for the water dimer, but also makes accurate predictions of vibrational ground-state properties for clusters up through the hexamer. Results of ambient liquid water simulations from VRT(ASP-W)III are compared to those from ab initio Molecular Dynamics, other potentials of ''spectroscopic'' accuracy, and to experiment. The results herein represent the first time that a ''spectroscopic'' potential surface is able to correctly model condensed phase properties of water.

Infrared cavity ringdown spectroscopy of jet-cooled polycyclic aromatic hydrocarbons.

Abstract: Infrared absorption spectra of the CH stretching region were observed for naphthalene, anthracene, phenanthrene, pyrene, and perylene using a heated, supersonic, slit-jet source and cavity ringdown spectroscopy. Band positions and intensities recorded with 0.2-cm 1 resolution were compared with previous gas-phase and argon matrix isolation experiments, as well as theoretical calculations. The largest matrix shift in the absorption maximum (-7.4 cm 1 ) was observed for anthracene, with all others shifted by 3.0 cm 1 or less. Spectral features in the supersonic jet spectrum were generally narrower than those observed in the Ar matrix, with the largest matrix broadening found for the perylene (80 % increase). Low number densities observed for the larger polycyclic aromatic hydrocarbons (PAHs) suggest that the lower vapor pressure of PAHs with catacondensed four-membered rings and with five-membered rings other than perylene will not be detectable using our current configuration.

Chromophore orientations in a nonlinear optical azopolymer diffraction grating: Even and odd order parameters from far-field Raman and near-field second harmonic generation microscopies

Abstract: A diffraction grating with a large diffraction efficiency (η = 25% on the first order) was inscribed on an azobenzene-containing polymer thin film, electrically poled, and characterized using spatially resolved Raman confocal microscopy and near-field scanning optical microscopy (NSOM) coupled with second-order nonlinear optical measurements. Linear (Raman) and nonlinear (second harmonic generation or SHG) polarized microscopic measurements were performed on a grating with ∼1.4 μm periodicity, revealing the molecular orientations in various regions of the sinusoidal relief surface. The most probable distribution functions f(θ) of the chromophore orientations were derived using the two first even parity Legendre polynomials 〈P2〉 and 〈P4〉 together with the two odd order parameters 〈P1〉 and 〈P3〉, as determined by Raman and SHG, respectively. These distribution functions show that the poling treatment is quite efficient and also emphasize the importance of determining both couples of the order parameters in s...

Shape control of near‐field probes using dynamic meniscus etching

Abstract: Dynamic etching methods for fabricating fibre optic tips are explored and modelled. By vertically translating the fibre during etching by an HF solution under an organic protective layer, a variety of tip shapes were created. The probe taper lengths, cone angles and geometrical probe shapes were measured in order to evaluate the dynamic meniscus etching process. Fibre motion, etching rate, meniscus distortion and etching time were all found to be important variables that can be used to control the final probe shape.

High-spectral-resolution multiplex CARS spectroscopy using chirped pulses

Abstract: A novel technique for achieving high spectral resolution with a femtosecond laser system is presented. Transform-limited 800 nm, 90 femtosecond (fs) pulses pass off two gratings, stretching the pulse in time to a pulse width of several picoseconds due to an induced linear temporal chirp directly proportional to the grating separation. This chirped pulse is the degenerate pump ( p ) and probe ( p' ) pulse for the CARS experiment. When overlapped in time with the 1050 nm, 90 fs transform-limited Stokes ( S ) pulse, only a fraction of the chirped p' pulse generates the CARS signal, thereby creating a temporal slit that defines the spectral resolution of the technique. Spectra for liquid methanol and liquid isooctane are presented, with ~6 cm -1 spectral resolution achieved for isooctane. Resonance enhancement and the mechanism of achieving high spectral resolution are shown by adjusting the S wavelength and p delay relative to the S pulse. Keywords: Coherent Anti-Stokes Raman Scattering (CARS), chirp, multiplex

Optical cavity resonances in water micro-droplets: Implications for shortwave cloud forcing

Abstract: [1] The influence of narrow optical resonances, which result from trapping of light rays via total internal reflection in water droplets, on the absorption of shortwave (SW) solar radiation has been estimated through high resolution Mie scattering calculations. Our results indicate that these resonances engender an increase in absorption of solar radiation by cloud droplets that is several W/m2 above the linear direct absorption process. Mie scattering calculations performed at the Δx = 0.1 (x = 2πr/λ) resolution typically implemented in cloudy sky radiative transfer models are shown to be insufficient for accurate determination of the attenuation of SW radiation when considered over relatively narrow wavelength ranges, consistent with the recent finding of Nussenzveig [2003]. However, for broadband calculations we find positive and negative errors in Mie calculations at Δx = 0.1 nearly cancel resulting in reasonable estimates of SW attenuation.