Coverage-dependent adsorption sites for K/Cu(001) and Cs/Cu(001) determined by surface X-ray diffraction

Surface X-ray diffraction has been used to analyze in situ the room-temperature adsorption behaviour and the structure of K and Cs on Cu(100) at submonolayer coverages. Adsorption of K takes place in fourfold hollow sites up to coverages of about 0.25 monolayers (ML), where 1 ML corresponds to 1.53 × 1015 atoms/cm2. At higher coverages the formation of a quasi-hexagonal incommensurate adlayer is observed. In contrast, for Cs adsorption we observe from the very beginning the formation of the quasi-hexagonal structure up to the completion of the adlayer at about 0.30 ML. For K adsorption in the hollow sites we determine an adsorption height, d = 2.25(15) Å, corresponding to an effective K radius of reff = 1.6(1) Å close to the ionic radius of 1.33 Å. We do not observe a change in the effective radius as a function of coverage. For the quasi-hexagonal Cs structure we find an (average) adsorption height d = 2.94 Å corresponding to an effective radius of reff = 2.18 and 1.93 Å, for the limiting ca hollow- and bridge-site adsorption, respectively. The analysis of the superlattice reflections corresponding to the quasi-hexagonal incommensurate structures indicated that the K adlayer is strongly modulated. The first Fourier component of the substrate-induced modulation was determined to u01 = 1.29(3) Å. In contrast, for Cs/Cu(001) static modulation is much less important (u01 0.2 Å). Variation of the Cs adlayer density by changing the substrate temperature allows continuous expansion and contraction of the adsorbate unit cell. No commensurate-incommensurate transition has been observed

Atomic Hydrogen and Star Formation in the Bridge/Ring Interacting Galaxy Pair NGC 7714/7715 (Arp 284)

We present high spatial resolution 21 cm HI maps of the interacting galaxy pair NGC 7714/7715. We detect a massive (2 x 10**9 M(sun)) HI bridge connecting the galaxies that is parallel to but offset from the stellar bridge. A chain of HII regions traces the gaseous bridge, with H-alpha peaks near but not on the HI maxima. An HI tidal tail is also detected to the east of the smaller galaxy NGC 7715, similarly offset from a stellar tail. The strong partial stellar ring on the eastern side of NGC 7714 has no HI counterpart, but on the opposite side of NGC 7714 there is a 10**9 M(sun) HI loop 11 kpc in radius. Within the NGC 7714 disk, clumpy HI gas is observed associated with star formation regions. Redshifted HI absorption is detected towards the starburst nucleus. We compare the observed morphology and gas kinematics with gas dynamical models in which a low-mass companion has an off-center prograde collision with the outer disk of a larger galaxy. These simulations suggest that the bridge in NGC 7714/7715 is a hybrid between bridges seen in systems like M51 and the purely gaseous `splash' bridges found in ring galaxies like the Cartwheel. The offset between the stars and gas in the bridge may be due to dissipative cloud-cloud collisions occuring during the impact of the two gaseous disks.Comment: 31 pages, Latex, 11 figures, to be published in the July 10, 1997 issue of the Astrophysical Journa

Niche complementarity and facilitation drive positive diversity effects on biomass production in experimental benthic diatom biofilms

Up to now, relatively few diversity-production experiments have been performed using microorganisms. Benthic diatom communities from estuarine intertidal mudflats are especially interesting for this purpose as they are relatively species poor and are thus more easy to simulate in laboratory conditions. We studied the effect of diversity on biomass production during microcosm experiments with diatoms assembled in combinations of up to eight species. Our results demonstrate a highly positive effect of biodiversity on production, with transgressive overyielding occurring in more than half of the combinations. These strong positive diversity effects could largely be attributed to positive complementarity effects (covering both niche complementarity and facilitation), although negative selection effects effects at higher diversities. We found a significant positive relation between functional diversity and the net biodiversity effects, indicating niche complementarity. In addition, we provide one of the first mechanistic evidences for facilitation by which biodiversity can enhance ecosystem functioning. This was demonstrated by the improved growth of Cylindrotheca closterium after addition of spent medium obtained from other diatom species. The stimulated growth of C. closterium was explained by a shift to mixotrophic growth with a down-regulation of the photosynthetic apparatus

Microphytobenthos production and biomass monitoring on intertidal mudflats using remote sensing

Coastal areas and estuaries are ecosystems with a high economic and ecological value. Microphytobenthos organisms form thin biofilms on intertidal mudflats and are important for stabilizing the sediment and transferring inorganic nutrients to the estuarine foodweb. Knowledge about the microphytobenthos is therefore essential for the management of these areas. As intertidal mudflats are not easily accessible for routine sampling and show high spatial variability, remote sensing techniques are increasingly used for monitoring these areas. Despite the wider use of remote sensing techniques in these systems, the potential of these techniques is still not fully explored. We obtained a variety of hyperspectral and satellite images from the Westerschelde and IJzer Estuary to obtain indices suitable for mapping microphytobenthos biomass. The most widely used marker pigment to quantify algal biomass is chlorophyll a. The microbial mats on intertidal mudflats are generally dominated by diatoms, but blooms of noxious algae such asUlva and cyanobacteria are also frequently observed. Using traditional HPLC-pigment analysis techniques, these different algal groups can readily be distinguished based on differences in their marker pigment composition (fucoxanthine/lutein/zeaxanthine). We will explore the potential for mapping different algal groups on hyperspectral images using these marker pigments. In addition, the data on the biomass and composition of the microphytobenthos will be used for modelling the primary production on the mudflats

Double-real contribution to the quark beam function at N3LO QCD

We compute the master integrals required for the calculation of the double-realemission contributions to the matching coefficients of 0-jettiness beam functions at next-to-next-to-next-to-leading order in perturbative QCD. As an application, we combine theseintegrals and derive the double-real gluon emission contribution to the matching coefficient Iqq(t;z) of the quark beam function

Colorectal Cancer in the Family: Psychosocial Distress and Social Issues in the Years Following Genetic Counselling

<p>Abstract</p> <p>Background</p> <p>This study examined: (1) levels of cancer-specific distress more than one year after genetic counselling for hereditary nonpolyposis colorectal cancer (HNPCC); (2) associations between sociodemographic, clinical and psychosocial factors and levels of distress; (3) the impact of genetic counselling on family relationships, and (4) social consequences of genetic counselling.</p> <p>Methods</p> <p>In this cross-sectional study, individuals who had received genetic counselling for HNPCC during 1986–1998 completed a self-report questionnaire by mail.</p> <p>Results</p> <p>116 individuals (81% response rate) completed the questionnaire, on average 4 years after the last counselling session. Of all respondents, 6% had clinically significant levels of cancer-specific distress (Impact of Event Scale, IES). Having had contact with a professional psychosocial worker for cancer risk in the past 10 years was significantly associated with higher levels of current cancer specific distress. Only a minority of the counselees reported any adverse effects of genetic counselling on: communication about genetic counselling with their children (9%), family relationships (5%), obtaining life insurance (8%), choice or change of jobs (2%), and obtaining a mortgage (2%).</p> <p>Conclusion</p> <p>On average, four years after genetic counselling for HNPCC, only a small minority of counselled individuals reports clinically significant levels of distress, or significant family or social problems.</p