The effect of charge separation on the phase behavior of dipolar colloidal rods

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Colloids with anisotropic shape and charge distribution can assemble into a variety of structures that could find use as novel materials for optical, photonic, electronic and structural applications. Because experimental characterization of the many possible types of multi-shape and multipolar colloidal particles that could form useful structures is difficult, the search for novel colloidal materials can be enhanced by simulations of colloidal particle assembly. We have simulated a system of dipolar colloidal rods at fixed aspect ratio using discontinuous molecular dynamics (DMD) to investigate how the charge separation of an embedded dipole affects the types of assemblies that occur. Each dipolar rod is modeled as several overlapping spheres fixed in an elongated shape to represent excluded volume and two smaller, embedded spheres to represent the charges that make up the extended dipole. Large charge separations predominately form structures where the rods link head-to-tail while small charge separations predominately form structures where the rods stack side-by-side. Rods with small charge separations tend to form dense aggregates while rods with large charge separations tend to form coarse gel-like structures. Structural phase boundaries between fluid, string-fluid, and "gel'' (networked) phases are mapped out and characterized as to whether they have global head-to-tail or global side-by-side order. A structural coarsening transition is observed for particles with large charge separations in which the head-tail networks thicken as temperature is lowered due to an increased tendency to form side-by-side structures. Triangularly connected networks form at small charge separations; these may be useful for encapsulating smaller particles.DFG, GRK 1524, Self-Assembled Soft-Matter Nanostructures at Interface

Stimulated Raman scattering metrology of molecular hydrogen

Frequency combs have revolutionized optical frequency metrology, allowing one to determine highly accurate transition frequencies of a wealth of molecular species. These progresses have only marginally benefited infrared-inactive transitions, due to their inherently weak cross-sections. Here we overcome this limitation by introducing stimulated-Raman-scattering metrology, where a frequency comb is exploited to calibrate the frequency detuning between the pump and Stokes excitation lasers. We apply this approach to the investigation of molecular hydrogen, which is a recognized benchmark for tests of quantum electrodynamics and of theories that describe physics beyond the standard model. Specifically, we measure the transition frequency of the Q(1) fundamental line of H2 around 4155 cm−1 with few parts-per-billion uncertainty, which is comparable to the theoretical benchmark of ab initio calculations and more than a decade better than the experimental state of the art. Our comb-calibrated stimulated Raman scattering spectrometer extends the toolkit of optical frequency metrology as it can be applied, with simple technical changes, to many other infrared-inactive transitions, over a 50-5000 cm−1 range that covers also purely rotational bands

Reactive scattering of H2 from Cu(100): comparison of dynamics calculations based on the specific reaction parameter approach to density functional theory with experiment

We present new experimental and theoretical results for reactive scattering of dihydrogen from Cu(100). In the new experiments, the associative desorption of H2 is studied in a velocity resolved and final rovibrational state selected manner, using time-of-flight techniques in combination with resonance-enhanced multi-photon ionization laser detection. Average desorption energies and rota- tional quadrupole alignment parameters were obtained in this way for a number of (v = 0, 1) ro- tational states, v being the vibrational quantum number. Results of quantum dynamics calculations based on a potential energy surface computed with a specific reaction parameter (SRP) density func- tional, which was derived earlier for dihydrogen interacting with Cu(111), are compared with the results of the new experiments and with the results of previous molecular beam experiments on sticking of H2 and on rovibrationally elastic and inelastic scattering of H2 and D2 from Cu(100). The calculations use the Born-Oppenheimer and static surface approximations. With the functional derived semi-empirically for dihydrogen + Cu(111), a chemically accurate description is obtained of the molecular beam experiments on sticking of H2 on Cu(100), and a highly accurate descrip- tion is obtained of rovibrationally elastic and inelastic scattering of D2 from Cu(100) and of the orientational dependence of the reaction of (v = 1, j = 2 − 4) H2 on Cu(100). This suggests that a SRP density functional derived for H2 interacting with a specific low index face of a metal will yield accurate results for H2 reactively scattering from another low index face of the same metal, and that it may also yield accurate results for H2 interacting with a defected (e.g., stepped) surface of that same metal, in a system of catalytic interest. However, the description that was obtained of the average desorption energies, of rovibrationally elastic and inelastic scattering of H2 from Cu(100), and of the orientational dependence of reaction of (v = 0, j = 3 − 5, 8) H2 on Cu(100) compares less well with the available experiments. More research is needed to establish whether more accurate SRP-density functional theory dynamics results can be obtained for these observables if surface atom motion is added to the dynamical model. The experimentally and theoretically found dependence of the rotational quadrupole alignment parameter on the rotational quantum number provides evidence for rotational enhancement of reaction at low translational energies.Fil: Sementa, L.. Leiden University; Países Bajos. Istituto per i Processi Chimico-Fisici of the Consiglio Nazionale delle Ricerche; ItaliaFil: Wijzenbroek, M.. Leiden University; Países BajosFil: Van Kolck, B. J.. Leiden University; Países BajosFil: Somers, M. F.. Leiden University; Países BajosFil: Al-Halabi, A.. Leiden University; Países BajosFil: Busnengo, Heriberto Fabio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); ArgentinaFil: Olsen, R. A.. Leiden University; Países Bajos. SINTEF Materials and Chemistry; NoruegaFil: Kroes, G. J.. Leiden University; Países BajosFil: Rutkowski, M.. Westfalische Wilhelms Universitat; AlemaniaFil: Thewes, C.. Westfalische Wilhelms Universitat; AlemaniaFil: Kleimeier, N. F.. Westfalische Wilhelms Universitat; AlemaniaFil: Zacharias, H.. Westfalische Wilhelms Universitat; Alemani

THE HUBBLE SPACE TELESCOPE WIDE FIELD CAMERA 3 EARLY RELEASE SCIENCE DATA: PANCHROMATIC FAINT OBJECT COUNTS FROM 0.2{2 MICRONS WAVELENGTH

We describe the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) Early Re- lease Science (ERS) observations in the Great Observatories Origins Deep Survey (GOODS) South eld. The new WFC3 ERS data provide calibrated, drizzled mosaics in the mid-UV lters F225W, F275W, and F336W, as well as in the near-IR lters F098W (Ys), F125W (J), and F160W (H) in 1{2 HST orbits per lter. Together with the existing HST Advanced Camera for Surveys (ACS) GOODS-South mosaics in the BVi\u27z\u27 lters, these panchromatic 10-band ERS data cover 40{50 square arcmin from from 0.2-1.7 m in wavelength at 00:007{ 000 :15 FWHM resolution and 000 :090 multidrizzled pixels to depths of AB\u3c 26.0{27.0 mag (5-sigma) for point sources, and AB\u3c 25.5{26.5 mag for compact galaxies. In this paper, we describe: a) the scientic rationale, and the data taking plus reduction procedures of the panchromatic 10-band ERS mosaics; b) the procedure of generating object catalogs across the 10 dierent ERS lters, and the specic star-galaxy separation techniques used; and c) the reliability and completeness of the object catalogs from the WFC3 ERS mosaics. The excellent 00:007{00:015 FWHM resolution of HST/WFC3 and ACS makes star- galaxy separation rather straightforward over a factor of 10 in wavelength to AB\u3c 25{26 mag from the UV to the near-IR, respectively. Our main scientic results are: 1) We present the resulting Galactic star counts and galaxy counts in 10 dierent lters. From the ERS data, these could be accurately determined from AB\u2719 mag to AB\u3c 26 from the mid-UV to the near-IR, respectively. 2) Both the Galactic stars counts and the galaxy counts show mild but signicant trends of decreasing count slopes from the mid{UV to the near-IR over a factor of 10 in wavelength. 3) We combine the 10-band ERS counts with the panchromatic GAMA counts at the bright end (10\u3c AB\u3c 20 mag), and with the ultradeep BVizYJH HUDF counts and other avail- able HST UV counts at the faint end (24\u3c AB\u3c 30 mag). The galaxy counts are now well measured over the entire magnitude range 10\u3c AB\u3c 30 mag from 0.2{2 m in wavelength. 4)We t simple galaxy evolution models to these panchromatic galaxy counts over this entire ux range, using the best available 10-band local galaxy luminosity functions (LFs) from the GAMA survey, as well as simple prescriptions of luminosity and/or density evolution. While these models can explain each of the 10-band counts from 10\u3c AB\u3c 30 mag well, no single one of these simple models can explain the counts over this entire ux range in all 10 lters simultaneously. 5) The 10-band panchromatic ERS data base is very rich in structural information at all rest-frame wavelengths where young or older stars shine during the peak epoch in the cosmic star-formation rate (z\u271{2), and constitutes a unique new HST data base for the community to explore in the future

Stellar Populations of Lyman Break Galaxies at z=1-3 in the HST/WFC3 Early Release Science Observations

We analyze the spectral energy distributions (SEDs) of Lyman break galaxies (LBGs) at z=1-3 selected using the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) UVIS channel filters. These HST/WFC3 observations cover about 50 sq. arcmin in the GOODS-South field as a part of the WFC3 Early Release Science program. These LBGs at z=1-3 are selected using dropout selection criteria similar to high redshift LBGs. The deep multi-band photometry in this field is used to identify best-fit SED models, from which we infer the following results: (1) the photometric redshift estimate of these dropout selected LBGs is accurate to within few percent; (2) the UV spectral slope (beta) is redder than at high redshift (z>3), where LBGs are less dusty; (3) on average, LBGs at z=1-3 are massive, dustier and more highly star-forming, compared to LBGs at higher redshifts with similar luminosities (0.1L*<~L<~2.5L*), though their median values are similar within 1-sigma uncertainties. This could imply that identical dropout selection technique, at all redshifts, find physically similar galaxies; and (4) stellar masses of these LBGs are directly proportional to their UV luminosities with a logarithmic slope of ~0.46, and star-formation rates are proportional to their stellar masses with a logarithmic slope of ~0.90. These relations hold true --- within luminosities probed in this study --- for LBGs from z~1.5 to 5. The star-forming galaxies selected using other color-based techniques show similar correlations at z~2, but to avoid any selection biases, and for direct comparison with LBGs at z>3, a true Lyman break selection at z~2 is essential. The future HST UV surveys, both wider and deeper, covering a large luminosity range are important to better understand LBG properties, and their evolution.Comment: Accepted for publication in ApJ (29 pages, 9 figures

HST-WFC3 Near-Infrared Spectroscopy of Quenched Galaxies at zeta approx 1.5 from the WISP Survey: Stellar Populations Properties

We combine Hubble Space Telescope (HST) G102 and G141 near-IR (NIR) grism spectroscopy with HST/WFC3- UVIS, HST/WFC3-IR, and Spitzer/IRAC [3.6 microns] photometry to assemble a sample of massive (log(Mstar/M solar mass) at approx 11.0) and quenched (specific star formation rate 2 and the zeta approx 1.5 RS. According to their estimated ages, the time required for quenched galaxies off the RS to join their counterparts on the z approx. 1.5 RS is of the order of approx. 1G/yr