Calculations of the Far-Wing Line Profiles of Sodium and Potassium in the Atmospheres of Substellar-Mass Objects

At the low temperatures achieved in cool brown dwarf and hot giant planet atmospheres, the less refractory neutral alkali metals assume an uncharacteristically prominent role in spectrum formation. In particular, the wings of the Na-D (5890 \AA) and K I (7700 \AA) resonance lines come to define the continuum and dominate the spectrum of T dwarfs from 0.4 to 1.0 \mic. Whereas in standard stellar atmospheres the strengths and shapes of the wings of atomic spectral lines are rarely needed beyond 25 \AA of a line center, in brown dwarfs the far wings of the Na and K resonance lines out to 1000's of \AA detunings are important. Using standard quantum chemical codes and the Unified Franck-Condon model for line profiles in the quasi-static limit, we calculate the interaction potentials and the wing line shapes for the dominant Na and K resonance lines in H$_2$- and helium-rich atmospheres. Our theory has natural absorption profile cutoffs, has no free parameters, and is readily adapted to spectral synthesis calculations for stars, brown dwarfs, and planets with effective temperatures below 2000 Kelvin.Comment: 14 pages, Latex, 7 figures in JPEG format, accepted for publication in the Astrophysical Journa

Electron transport across a quantum wire in the presence of electron leakage to a substrate

We investigate electron transport through a mono-atomic wire which is tunnel coupled to two electrodes and also to the underlying substrate. The setup is modeled by a tight-binding Hamiltonian and can be realized with a scanning tunnel microscope (STM). The transmission of the wire is obtained from the corresponding Green's function. If the wire is scanned by the contacting STM tip, the conductance as a function of the tip position exhibits oscillations which may change significantly upon increasing the number of wire atoms. Our numerical studies reveal that the conductance depends strongly on whether or not the substrate electrons are localized. As a further ubiquitous feature, we observe the formation of charge oscillations.Comment: 7 pages, 7 figure

A boundary condition determined wave function for the H-2 (X-1 Sigma(g)) molecule

Two relatively simple non-variational wave functions for two electron diatomic molecules are proposed. The electron-electron cusp condition is satisfied rigorously by a correlation function which has the correct behavior for r(12) --> 0 and r(12) --> infinity. The electron-nucleus cusp conditions are also rigorously satisfied by the proposed functional forms of the wave functions. The parameters are chosen to match the asymptotic conditions in two different approximations. Both wave functions yield very good energies for the chemical bond. This demonstrates that these local conditions have a big effect on the wave function

Simulating bacterial efflux: how molecular features affect functional rotation

The major tripartite efflux pump AcrAB-TolC is responsible for the intrinsic and acquired multidrug resistance in Escherichia coli. At heart of the extrusion machinery there is the homotrimeric transporter AcrB, which is in charge of the selective binding of structurally and chemically different substrates and energy transduction. The effects of conformational changes, which have been proposed as the key features of the extrusion of drugs, are investigated at molecular level using different computational methods like targeted molecular dynamics. Simulations, including almost half a million atoms, have been used to assess several hypotheses concerning the structure-dynamics-function relationship of the AcrB protein. The results indicate that, upon induction of conformational changes, the substrate detaches from the binding pocket and approaches the gate to the central funnel. In addition, we provide evidence for the proposed peristaltic transport involving a zipper-like closure of the binding pocket, responsible for the displacement of the drug. Using these atomistic simulations the role of specific amino acids during the transitions can be identified, providing an interpretation of sitedirected mutagenesis experiments. Additionally, we discuss a possible role of water molecules in the extrusion process