Site-dependent charge transfer at the Pt(111)-ZnPc interface and the effect of iodine

The electronic structure of ZnPc, from sub-monolayers to thick films, on bare and iodated Pt(111) is studied by means of X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and scanning tunneling microscopy (STM). Our results suggest that at low coverage ZnPc lies almost parallel to the Pt(111) substrate, in a non-planar configuration induced by Zn-Pt attraction, leading to an inhomogeneous charge distribution within the molecule and charge transfer to the molecule. ZnPc does not form a complete monolayer on the Pt surface, due to a surface-mediated intermolecular repulsion. At higher coverage ZnPc adopts a tilted geometry, due to a reduced molecule-substrate interaction. Our photoemission results illustrate that ZnPc is practically decoupled from Pt, already from the second layer. Pre-deposition of iodine on Pt hinders the Zn-Pt attraction, leading to a non-distorted first layer ZnPc in contact with Pt(111)-I $\left(\sqrt{3}\times\sqrt{3}\right)$ or Pt(111)-I $\left(\sqrt{7}\times\sqrt{7}\right)$, and a more homogeneous charge distribution and charge transfer at the interface. On increased ZnPc thickness iodine is dissolved in the organic film where it acts as an electron acceptor dopant.Comment: 12 pages, 9 figure

The proper motion of the Magellanic Clouds, I: first results and description of the program

We present the first results of a ground-based program to determine the proper motion of the Magellanic Clouds (MCs) relative to background quasars (QSO), being carried out using the Iréneé du Pont 2.5 m telescope at Las Campanas Observatory, Chile. Eleven QSO fields have been targeted in the Small Magellanic Cloud (SMC) over a time base of six years, and with seven epochs of observation. One quasar field was targeted in the Large Magellanic Cloud (LMC), over a time base of five years, and with six epochs of observation. The shorter time base in the case of the LMC is compensated by the much larger amount of high-quality astrometry frames that could be secured for the LMC quasar field (124 frames), compared to the SMC fields (an average of roughly 45 frames). In this paper, we present final results for field Q0557-6713 in the LMC and field Q0036-7227 in the SMC. From field Q0557-6713, we have obtained a measured proper motion of μαcos δ = +1.95 ± 0.13 mas yr-1, μδ = +0.43 ± 0.18 mas yr-1 for the LMC. From field Q0036-7227, we have obtained a measured proper motion of μα cosδ = +0.95 ± 0.29 mas yr-1, μδ = -1.14 ± 0.18 mas yr-1 for the SMC. Although we went through the full procedure for another SMC field (QJ0036-7225), on account of unsolvable astrometric difficulties caused by blending of the QSO image, it was impossible to derive a reliable proper motion. Current model rotation curves for the plane of the LMC indicate that the rotational velocity (V rot) at the position of LMC field Q0557-6713 can be as low as 50 km s-1, or as high as 120 km s-1. A correction for perspective and rotation effects leads to a center of mass proper motion for the LMC of μα cosδ = +1.82 ± 0.13 mas yr-1, μδ = +0.39 ± 0.15 mas yr-1 (V rot = 50 km s-1), and to μα cosδ = +1.61 ± 0.13 mas yr-1, μδ = +0.60 ± 0.15 mas yr-1 (V rot = 120 km s-1). Assuming that the SMC has a disk-like central structure, but that it does not rotate, we obtain a center of mass proper motion for the SMC of μα cosδ = +1.03 ± 0.29 mas yr-1, μδ = -1.09 ± 0.18 mas yr-1. Our results are in reasonable agreement with most previous determinations of the proper motion of the MCs, including recent Hubble Space Telescope measurements. Complemented with published values of the radial velocity of the centers of the LMC and SMC, we have used our proper motions to derive the galactocentric (gc) velocity components of the MCs. For the LMC, we obtain V gc,t = +315 ± 20 km s-1, V gc,r = +86 ± 17 km s-1 (V rot = 50 km s-1), and V gc,t = +280 ± 24 km s-1, V gc,r = +94 ± 17 km s-1 (V rot = 120 km s-1). For the SMC, we obtain V gc,t = +258 ± 50 km s-1, V gc,r = +20 ± 44 km s-1. These velocities imply a relative velocity between the LMC and SMC of 84 ± 50 km s-1, for V rot,LMC = 50 km s-1, and 62 ± 63 km s-1 for V rot,LMC = 120 km s-1. Albeit our large errors, these values are not inconsistent with the standard assumption that the MCs are gravitationally bound to each other.Fil: Costa, Edgardo. Universidad de Chile; ChileFil: Méndez, René A.. Universidad de Chile; ChileFil: Pedreros, Mario H.. Universidad de Tarapaca; ChileFil: Moyano, Maximiliano. Universidad de Chile; ChileFil: Gallart, Carme. Instituto de Astrofísica de Canarias; EspañaFil: Noël, Noelia. Instituto de Astrofísica de Canarias; EspañaFil: Baume, Gustavo Luis. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Carraro, Giovanni. European Southern Observatory; Chil

Propagation on networks: an exact alternative perspective

By generating the specifics of a network structure only when needed (on-the-fly), we derive a simple stochastic process that exactly models the time evolution of susceptible-infectious dynamics on finite-size networks. The small number of dynamical variables of this birth-death Markov process greatly simplifies analytical calculations. We show how a dual analytical description, treating large scale epidemics with a Gaussian approximations and small outbreaks with a branching process, provides an accurate approximation of the distribution even for rather small networks. The approach also offers important computational advantages and generalizes to a vast class of systems.Comment: 8 pages, 4 figure

Analysis of global water vapour trends from satellite measurements in the visible spectral range

International audienceGlobal water vapour total column amounts have been retrieved from spectral data provided by the Global Ozone Monitoring Experiment (GOME) flying on ERS-2, which was launched in April 1995, and the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) onboard ENVISAT launched in March 2002. For this purpose the Air Mass Corrected Differential Optical Absorption Spectroscopy (AMC-DOAS) approach has been used. The combination of the data from both instruments provides us with a long-term global data set spanning more than 11 years with the potential of extension up to 2020 by GOME-2 data, on Metop. Using linear and non-linear methods from time series analysis and standard statistics the trends of H2O contents and their errors have been calculated. In this study, factors affecting the trend such as the length of the time series, the magnitude of the variability of the noise, and the autocorrelation of the noise are investigated. Special emphasis has been placed on the calculation of the statistical significance of the observed trends, which reveal significant local changes of water vapour columns distributed over the whole globe. <br

Preliminary results of GOME-2 water vapour retrievals and first applications in polar regions

International audienceGlobal total water vapour columns have been derived from measurements of the Global Ozone Monitoring Experiment 2 (GOME-2) on MetOp. For this purpose, the Air Mass Corrected Differential Optical Absorption Spectroscopy (AMC-DOAS) method has been adapted, having previously been applied successfully to GOME (on ERS-2) and SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY, on ENVISAT) data. Comparisons between the derived GOME-2 and SCIAMACHY water vapour columns show a good overall agreement. This gives confidence that the temporal series of water vapour columns from GOME-type instruments (GOME/ERS-2, SCIAMACHY/ENVISAT), which began in 1995, is successfully continued by the MetOp instrumentation until at least 2020. The enhanced temporal and spatial resolution of GOME-2 enables the analysis of diurnal variations in the polar regions. This is especially important because atmospheric data sources in the polar regions are generally sparse. As an exemplary application, daily water vapour total columns over the polar research station Ny Ålesund (78°55'19" N/11°56'33" E) are investigated. At this latitude GOME-2 yields about six data points during daylight hours at varying local times. From these data diurnal variations of water vapour have been successfully retrieved

Retrieval of global water vapour columns from GOME-2 and first applications in polar regions

International audienceGlobal total water vapour columns have been derived from measurements of the Global Ozone Monitoring Experiment 2 (GOME-2) on MetOp. For this purpose, the Air Mass Corrected Differential Optical Absorption Spectroscopy (AMC-DOAS) method has been adapted, which has already been applied successfully to GOME (on ERS-2) and SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY, on ENVISAT) data. Comparisons between the derived GOME-2 and SCIAMACHY water vapour columns show a good overall agreement. This gives confidence that the time series of water vapour columns from GOME-type instruments which started in 1995 can be continued by the MetOp instrumentation until at least 2020. The enhanced temporal and spatial resolution of GOME-2 enables the analysis of short-term variations particularly in the polar regions. This is especially important since atmospheric data sources in the polar regions are generally sparse. As an exemplary application, daily water vapour concentrations over the polar research station Ny Ålesund (78°55'19" N/11°56'33" E) are investigated. At this latitude GOME-2 gives about six data points during daylight hours at varying local times. The results of this study show that it is possible to derive information about the diurnal variability of water vapour in polar regions from GOME-2 measurements

Validation of SCIAMACHY AMC-DOAS water vapour columns

International audienceA first validation of water vapour total column amounts derived from measurements of the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) in the visible spectral region has been performed. For this purpose, SCIAMACHY water vapour data have been determined for the year 2003 using an extended version of the Differential Optical Absorption Spectroscopy (DOAS) method, called Air Mass Corrected (AMC-DOAS). The SCIAMACHY results are compared with corresponding water vapour measurements by the Special Sensor Microwave Imager (SSM/I) and with model data from the European Centre for Medium-Range Weather Forecasts (ECMWF). In confirmation of previous results it could be shown that SCIAMACHY derived water vapour columns are typically slightly lower than both SSM/I and ECMWF data, especially over ocean areas. However, these deviations are much smaller than the observed scatter of the data which is caused by the different temporal and spatial sampling and resolution of the data sets. For example, the overall difference with ECMWF data is only -0.05 g/cm2 whereas the typical scatter is in the order of 0.5 g/cm2. Both values show almost no variation over the year. In addition, first monthly means of SCIAMACHY water vapour data have been computed. The quality of these monthly means is currently limited by the availability of calibrated SCIAMACHY spectra. Nevertheless, first comparisons with ECMWF data show that SCIAMACHY (and similar instruments) are able to provide a new independent global water vapour data set