Potential energy curves for the interaction of Ag(5s) and Ag(5p) with noble gas atoms

We investigate the interaction of ground and excited states of a silver atom with noble gases (NG), including helium. Born-Oppenheimer potential energy curves are calculated with quantum chemistry methods and spin-orbit effects in the excited states are included by assuming a spin-orbit splitting independent of the internuclear distance. We compare our results with experimentally available spectroscopic data, as well as with previous calculations. Because of strong spin-orbit interactions, excited Ag-NG potential energy curves cannot be fitted to Morse-like potentials. We find that the labeling of the observed vibrational levels has to be shifted by one unit

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

The charge transfer in low energy (0.25 to 150 eV/amu) H($nl$) + He$^+(1s)$ collisions is investigated using a quasi-molecular approach for the $n=2,3$ as well as the first two $n=4$ 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 $n$ and $l$ 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 $n$ manifold, it is only necessary to include states with $n^{\prime}\leq n$, and we discuss the limitations of our approach as the number of states increases.Comment: 14 pages, 10 figure

Cold collisions of C2−_{2}^{-} anions with Li and Rb atoms in hybrid traps

We present a theoretical investigation of reactive and non-reactive collisions of Li and Rb atoms with C$_{2}^{-}$ 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-C$_{2}^{-}$ and Rb-C$_{2}^{-}$ 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 C$_2^-$ 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

Interaction of CH3_3CN and CH3_3NC with He : potential energy surfaces and low-energy scattering

Several nitrogen-bearing molecules, such as methyl cyanide (or acetonitrile, CH$_3$CN) and methyl isocyanide (CH$_3$NC) of interest here, have been observed in various astrophysical environments. The accurate modeling of their abundance requires the calculation of rate coefficients for their collisional excitation with species such as He atoms or H$_2$ molecules at low temperatures. In this work we compute new three-dimensional potential energy surfaces for the CH$_3$NC-He and CH$_3$CN-He van der Waals complexes by means of the explicitly correlated coupled cluster approach with single, double and perturbative triple excitation CCSD(T)/F12a in conjunction with the aug-cc-pVTZ basis set. We find a global minimum with $D_e= 55.10$ and 58.61 cm$^{-1}$ for CH$_3$CN-He and CH$_3$NC-He, respectively, while the dissociation energy $D_0$ of the complexes are 18.64 and 18.65 cm$^{-1}$, respectively. Low energy scattering calculations of pure rotational (de-)excitation of CH$_3$CN and CH$_3$NC by collision with He atoms are carried out with the close-coupling method and the collisional cross sections of $ortho-$ and $para-$CH$_3$NC and CH$_3$CN are computed for kinetic energies up to 100 cm$^{-1}$. While the PESs for both complexes are qualitatively similar, that of CH$_3$NC-He is more anisotropic, leading to different propensity rules for rotational excitation. For CH$_3$NC-He, we find that |$\Delta j$| = 1 transitions are dominant at low kinetic energy and a propensity rule that favors odd $\Delta j$ transitions is observed, whereas for CH$_3$CN the dominant cross sections are associated to transitions with |$\Delta j$| = 2. We expect that the findings of this study will be beneficial for astrophysical investigations as well as laboratory experiments

‘Interests’ in medicine and the inadequacy of disclosure

Many papers published in the academic press sink without a trace, making no impression on clinical practice, health policy or public discourse. While the narrative review of melatonin-based therapies for depression published in the Lancet by Professor Ian Hickie and Associate Professor Naomi Rogers (2011) is unlikely to generate significant changes in clinical practice, it has generated enormous professional and media debate about the extent and impact of conflicts of interest in medicine arising from the relationships between health professionals and the pharmaceutical industry. In the disclosure accompanying the review, which highlighted the potential worth of the drug agomelatine, manufactured by Servier, both authors disclosed financial and professional links with Servier. The responses to this review, and to the disclosures made by the authors in the Lancet, on health media websites, in the lay press and in social media, have been largely critical – making either empirical claims, that the authors misreported the tolerability and/or efficacy of agomelatine, or moral claims, that the authors were conflicted or biased, had not adequately disclosed the extent of their interests, or that they or Elsevier (the publisher of the Lancet) may have gained some benefit from the publication (Barbui and Cipriani, 2012; Carroll, 2012; Dunlevy, 2012; Griffiths, 2012; Howland, 2012; Jureidini and Raven, 2012; Lloret-Linares et al., 2012; Serfaty and Raven, 2012). In a spirited response, the authors defended both their analysis and their professional ethics, arguing that they had appropriately attended to the processes of disclosure required by the Lancet for all contributors (Hickie and Rogers, 2012)

Hyperfine collisional excitation of ammonia by molecular hydrogen

Ammonia is one of the most widely observed molecules in space, and many observations are able to resolve the hyperfine structure due to the electric quadrupole moment of the nitrogen nucleus. The observed spectra often display anomalies in the satellite components of the lines, which indicate substantial deviations from the local thermodynamic equilibrium. The interpretation of the spectra thus requires the knowledge of the rate coefficients for the hyperfine excitation of NH$_3$ induced by collisions with H$_2$ molecules, the dominant collider in the cold interstellar medium. In this paper we present the first such calculations using a recoupling approach. The rate coefficients are obtained for all hyperfine levels within rotation-inversion levels up to $j=4$ and temperatures up to 100 K by means of quantum scattering close-coupling calculations on an accurate, five-dimensional, potential energy surface. We show that the rate coefficients depart significantly from those obtained with the statistical approach and that they do not conform to any simple propensity rules. Finally, we perform radiative transfer calculations to illustrate the impact of our new rate coefficients by modelling the hyperfine line intensities of the inversion transition in ground state para-NH$_3$ ($j_k=1_1$) and of the rotational transition $1_0\rightarrow 0_0$ in ortho-NH$_3$

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

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 $^1 \Sigma^+$, 19 $^3\Sigma^+$, 12 $^1\Pi$, 9 $^3\Pi$, 4 $^1\Delta$ and 2 $^3\Delta$ 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 $^1\Sigma^+$ states.Comment: v2. Minor text changes. 28 pages, 18 figures. accepted in J. Phys.

Global relationship between phytoplankton diversity and productivity in the ocean

The shape of the productivity–diversity relationship (PDR) for marine phytoplankton has been suggested to be unimodal, that is, diversity peaking at intermediate levels of productivity. However, there are few observations and there has been little attempt to understand the mechanisms that would lead to such a shape for planktonic organisms. Here we use a marine ecosystem model together with the community assembly theory to explain the shape of the unimodal PDR we obtain at the global scale. The positive slope from low to intermediate productivity is due to grazer control with selective feeding, which leads to the predator-mediated coexistence of prey. The negative slope at high productivity is due to seasonal blooms of opportunist species that occur before they are regulated by grazers. The negative side is only unveiled when the temporal scale of the observation captures the transient dynamics, which are especially relevant at highly seasonal latitudes. Thus selective predation explains the positive side while transient competitive exclusion explains the negative side of the unimodal PDR curve. The phytoplankton community composition of the positive and negative sides is mostly dominated by slow-growing nutrient specialists and fast-growing nutrient opportunist species, respectively.Marie Curie International Outgoing Fellowship (FP7)Gordon and Betty Moore FoundationSpain. Ministerio de Economía y Competitividad (Ramon y Cajal Contracts

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

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 $n=1-3$ 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 $n=4$ states, we then show that the experimental observation of the importance of the electronic states with $n>3$ cannot be well explained theoretically, especially for cold ($v=0$) ions. We also calculate the relative contributions to the cross section of the $\Sigma$ and $\Pi$ 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