558 research outputs found

    Ab initio calculation of H + He+^+ charge transfer cross sections for plasma physics

    Full text link
    The charge transfer in low energy (0.25 to 150 eV/amu) H(nlnl) + He+(1s)^+(1s) collisions is investigated using a quasi-molecular approach for the n=2,3n=2,3 as well as the first two n=4n=4 singlet states. The diabatic potential energy curves of the HeH+^+ molecular ion are obtained from the adiabatic potential energy curves and the non-adiabatic radial coupling matrix elements using a two-by-two diabatization method, and a time-dependent wave-packet approach is used to calculate the state-to-state cross sections. We find a strong dependence of the charge transfer cross section in the principal and orbital quantum numbers nn and ll of the initial or final state. We estimate the effect of the non-adiabatic rotational couplings, which is found to be important even at energies below 1 eV/amu. However, the effect is small on the total cross sections at energies below 10 eV/amu. We observe that to calculate charge transfer cross sections in a nn manifold, it is only necessary to include states with nnn^{\prime}\leq n, and we discuss the limitations of our approach as the number of states increases.Comment: 14 pages, 10 figure

    Emergence and maintenance of biodiversity in an evolutionary food-web model

    Get PDF
    Ecological communities emerge as a consequence of gradual evolution, speciation, and immigration. In this study, we explore how these processes and the structure of the evolved food webs are affected by species-level properties. Using a model of biodiversity formation that is based on body size as the evolving trait and incorporates gradual evolution and adaptive radiation, we investigate how conditions for initial diversification relate to the eventual diversity of a food web. We also study how trophic interactions, interference competition, and energy availability affect a food web's maximum trophic level and contrast this with conditions for high diversity. We find that there is not always a positive relationship between conditions that promote initial diversification and eventual diversity, and that the most diverse food webs often do not have the highest trophic levels

    Cold collisions of C2_{2}^{-} anions with Li and Rb atoms in hybrid traps

    Full text link
    We present a theoretical investigation of reactive and non-reactive collisions of Li and Rb atoms with C2_{2}^{-} molecular anions at low temperatures in the context of sympathetic cooling in hybrid trap experiments. Based on recently reported accurate potential energy surfaces for the singlet and triplet states of the Li-C2_{2}^{-} and Rb-C2_{2}^{-} systems, we show that the associative electronic detachment reaction is slow if the colliding partners are in their ground state, but fast if they are excited. The results are expected to be representative of the alkali-metal series. We also investigate rotationally inelastic collisions in order to explore the cooling of the translational and rotational degrees of freedom of C2_2^- in hybrid ion-atom traps. The effect of micromotion is taken into account by considering Tsallis distributions of collision energies. We show that the translational cooling occurs much more rapidly than rotational cooling and that the presence of excited atoms leads to losses of anions on a timescale comparable to that of rotational cooling.Comment: ICPEAC 2019 conferenc

    Ab initio calculation of the 66 low lying electronic states of HeH+^+: adiabatic and diabatic representations

    Full text link
    We present an ab initio study of the HeH+^+ molecule. Using the quantum chemistry package MOLPRO and a large adapted basis set, we have calculated the adiabatic potential energy curves of the first 20 1Σ+^1 \Sigma^+, 19 3Σ+^3\Sigma^+, 12 1Π^1\Pi, 9 3Π^3\Pi, 4 1Δ^1\Delta and 2 3Δ^3\Delta electronic states of the ion in CASSCF and CI approaches. The results are compared with previous works. The radial and rotational non-adiabatic coupling matrix elements as well as the dipole moments are also calculated. The asymptotic behaviour of the potential energy curves and of the various couplings between the states is also studied. Using the radial couplings, the diabatic representation is defined and we present an example of our diabatization procedure on the 1Σ+^1\Sigma^+ states.Comment: v2. Minor text changes. 28 pages, 18 figures. accepted in J. Phys.

    Ro-vibrational analysis of the XUV photodissociation of HeH+^+ ions

    Full text link
    We investigate the dynamics of the photodissociation of the hydrohelium cation HeH+^+ by XUV radiation with the aim to establish a detailed comparison with a recent experimental work carried out at the FLASH free electron laser using both vibrationally hot and cold ions. As shown in previous theoretical works, the comparison is hindered by the fact that the experimental ro-vibrational distribution of the ions is unknown. We determine this distribution using a dissociative charge transfer set-up and the same source conditions as in the FLASH experiment. Using a non-adiabatic time-dependent wave packet method, we calculate the partial photodissociation cross sections for the n=13n=1-3 coupled electronic states of HeH+^+. We find a good agreement with the experiment for the total cross section into the He + H+^+ dissociative channel. By performing an adiabatic calculation involving the n=4n=4 states, we then show that the experimental observation of the importance of the electronic states with n>3n>3 cannot be well explained theoretically, especially for cold (v=0v=0) ions. We also calculate the relative contributions to the cross section of the Σ\Sigma and Π\Pi states. The agreement with the experiment is excellent for the He+^+ + H channel, but only qualitative for the He + H+^+ channel. We discuss the factors that could explain the remaining discrepancies between theory and experiment.Comment: 10 pages, 8 figure

    Robust estimation of microbial diversity in theory and in practice

    Get PDF
    Quantifying diversity is of central importance for the study of structure, function and evolution of microbial communities. The estimation of microbial diversity has received renewed attention with the advent of large-scale metagenomic studies. Here, we consider what the diversity observed in a sample tells us about the diversity of the community being sampled. First, we argue that one cannot reliably estimate the absolute and relative number of microbial species present in a community without making unsupported assumptions about species abundance distributions. The reason for this is that sample data do not contain information about the number of rare species in the tail of species abundance distributions. We illustrate the difficulty in comparing species richness estimates by applying Chao's estimator of species richness to a set of in silico communities: they are ranked incorrectly in the presence of large numbers of rare species. Next, we extend our analysis to a general family of diversity metrics ("Hill diversities"), and construct lower and upper estimates of diversity values consistent with the sample data. The theory generalizes Chao's estimator, which we retrieve as the lower estimate of species richness. We show that Shannon and Simpson diversity can be robustly estimated for the in silico communities. We analyze nine metagenomic data sets from a wide range of environments, and show that our findings are relevant for empirically-sampled communities. Hence, we recommend the use of Shannon and Simpson diversity rather than species richness in efforts to quantify and compare microbial diversity.Comment: To be published in The ISME Journal. Main text: 16 pages, 5 figures. Supplement: 16 pages, 4 figure

    Earthworm and belowground competition effects on plant productivity in a plant diversity gradient

    Get PDF
    Diversity is one major factor driving plant productivity in temperate grasslands. Although decomposers like earthworms are known to affect plant productivity, interacting effects of plant diversity and earthworms on plant productivity have been neglected in field studies. We investigated in the field the effects of earthworms on plant productivity, their interaction with plant species and functional group richness, and their effects on belowground plant competition. In the framework of the Jena Experiment we determined plant community productivity (in 2004 and 2007) and performance of two phytometer plant species [Centaurea jacea (herb) and Lolium perenne (grass); in 2007 and 2008] in a plant species (from one to 16) and functional group richness gradient (from one to four). We sampled earthworm subplots and subplots with decreased earthworm density and reduced aboveground competition of phytometer plants by removing the shoot biomass of the resident plant community. Earthworms increased total plant community productivity (+11%), legume shoot biomass (+35%) and shoot biomass of the phytometer C. jacea (+21%). Further, phytometer performance decreased, i.e. belowground competition increased, with increasing plant species and functional group richness. Although single plant functional groups benefited from higher earthworm numbers, the effects did not vary with plant species and functional group richness. The present study indicates that earthworms indeed affect the productivity of semi-natural grasslands irrespective of the diversity of the plant community. Belowground competition increased with increasing plant species diversity. However, belowground competition was modified by earthworms as reflected by increased productivity of the phytometer C. jacea. Moreover, particularly legumes benefited from earthworm presence. Considering also previous studies, we suggest that earthworms and legumes form a loose mutualistic relationship affecting essential ecosystem functions in temperate grasslands, in particular decomposition and plant productivity. Further, earthworms likely alter competitive interactions among plants and the structure of plant communities by beneficially affecting certain plant functional groups
    corecore