Sub-Doppler resonances in the back-scattered light from random porous media infused with Rb vapor

We report on the observation of sub-Doppler resonances on the back-scattered light from a random porous glass medium with rubidium vapor filling its interstices. The sub-Doppler spectral lines are the consequence of saturated absorption where the incident laser beam saturates the atomic medium and the back-scattered light probes it. Some specificities of the observed spectra reflect the transient atomic evolution under confinement inside the pores. Simplicity, robustness and potential miniaturization are appealing features of this system as a spectroscopic reference.Comment: 6 pages, 4 figure

Spectrum of the Relativistic Particles in Various Potentials

We extend the notion of Dirac oscillator in two dimensions, to construct a set of potentials. These potentials becomes exactly and quasi-exactly solvable potentials of non-relativistic quantum mechanics when they are transformed into a Schr\"{o}dinger-like equation. For the exactly solvable potentials, eigenvalues are calculated and eigenfunctions are given by confluent hypergeometric functions. It is shown that, our formulation also leads to the study of those potentials in the framework of the supersymmetric quantum mechanics

Calculation of the energy spectrum of a two-electron spherical quantum dot

We study the energy spectrum of the two-electron spherical parabolic quantum dot using the exact Schroedinger, the Hartree-Fock, and the Kohn-Sham equations. The results obtained by applying the shifted-1/N method are compared with those obtained by using an accurate numerical technique, showing that the relative error is reasonably small, although the first method consistently underestimates the correct values. The approximate ground-state Hartree-Fock and local-density Kohn-Sham energies, estimated using the shifted-1/N method, are compared with accurate numerical self-consistent solutions. We make some perturbative analyses of the exact energy in terms of the confinement strength, and we propose some interpolation formulae. Similar analysis is made for both mean-field approximations and interpolation formulae are also proposed for these exchange-only ground-state cases.Comment: 18 pages, LaTeX, 2 figures-ep

Hv1 Proton Channel Opening Is Preceded by a Voltage-independent Transition

AbstractThe voltage sensing domain (VSD) of the voltage-gated proton channel Hv1 mediates a H+-selective conductance that is coordinately controlled by the membrane potential (V) and the transmembrane pH gradient (ΔpH). Allosteric control of Hv1 channel opening by ΔpH (V-ΔpH coupling) is manifested by a characteristic shift of approximately 40 mV per ΔpH unit in the activation. To further understand the mechanism for V-ΔpH coupling in Hv1, H+ current kinetics of activation and deactivation in excised membrane patches were analyzed as a function of the membrane potential and the pH in the intracellular side of the membrane (pHI). In this study, it is shown for the first time to our knowledge that the opening of Hv1 is preceded by a voltage-independent transition. A similar process has been proposed to constitute the step involving coupling between the voltage-sensing and pore domains in tetrameric voltage-gated channels. However, for Hv1, the VSD functions as both the voltage sensor and the conduction pathway, suggesting that the voltage independent transition is intrinsic to the voltage-sensing domain. Therefore, this article proposes that the underlying mechanism for the activation of Hv1 involves a process similar to VSD relaxation, a process previously described for voltage-gated channels and voltage-controlled enzymes. Finally, deactivation seemingly occurs as a strictly voltage dependent process, implying that the kinetic event leading to opening of the proton conductance are different than those involved in the closing. Thus, from this work it is proposed that Hv1 activity displays hysteresis

On the effects of repeated tax amnesties

Abstract. We examine empirically the effect of tax amnesties on long term tax collection when such amnesties are used by a government as a regular source of revenue. We use data from the Tucuman province (Argentina) to test the main hypothesis of the model, namely, that amnesties lower the government’s revenue, as they reduce the penalties and make evasion more profitable. We find, however, that amnesties do not affect the long-term revenue. The other main result is in line with the theoretical predictions: the increase in short-run revenue is temporary and only accelerates the collection of the taxes but does not increase the amount collected. Thus, we conclude that amnesties were used only to obtain a short-run surge in revenue and to avoid more fundamental tax reforms.Keywords. Tax amnesties, Tax evasion.JEL. H27, H26, C32

The Energy Eigenvalues of the Two Dimensional Hydrogen Atom in a Magnetic Field

In this paper, the energy eigenvalues of the two dimensional hydrogen atom are presented for the arbitrary Larmor frequencies by using the asymptotic iteration method. We first show the energy eigenvalues for the no magnetic field case analytically, and then we obtain the energy eigenvalues for the strong and weak magnetic field cases within an iterative approach for $n=2-10$ and $m=0-1$ states for several different arbitrary Larmor frequencies. The effect of the magnetic field on the energy eigenvalues is determined precisely. The results are in excellent agreement with the findings of the other methods and our method works for the cases where the others fail.Comment: 13 pages and 5 table

Voltage-Controlled Enzymes: The New Janus Bifrons

The Ciona intestinalis voltage-sensitive phosphatase, Ci-VSP, was the first Voltage-controlled Enzyme (VEnz) proven to be under direct command of the membrane potential. The discovery of Ci-VSP conjugated voltage sensitivity and enzymatic activity in a single protein. These two facets of Ci-VSP activity have provided a unique model for studying how membrane potential is sensed by proteins and a novel mechanism for control of enzymatic activity. These facets make Ci-VSP a fascinating and versatile enzyme. Ci-VSP has a voltage sensing domain (VSD) that resembles those found in voltage-gated channels (VGC). The VSD resides in the N-terminus and is formed by four putative transmembrane segments. The fourth segment contains charged residues which are likely involved in voltage sensing. Ci-VSP produces sensing currents in response to changes in potential, within a defined range of voltages. Sensing currents are analogous to gating currents in VGC. As known, these latter proteins contain four VSDs which are entangled in a complex interaction with the pore domain - the effector domain in VGC. This complexity makes studying the basis of voltage sensing in VGC a difficult enterprise. In contrast, Ci-VSP is thought to be monomeric and its catalytic domain - the VSP\u27s effector domain - can be cleaved off without disrupting the basic electrical functioning of the VSD. For these reasons, VSPs are considered a great model for studying the activity of a VSD in isolation. Finally, VSPs are also phosphoinositide phosphatases. Phosphoinositides are signaling lipids found in eukaryotes and are involved in many processes, including modulation of VGC activity and regulation of cell proliferation. Understanding VSPs as enzymes has been the center of attention in recent years and several reviews has been dedicated to this area. Thus, this review will be focused instead on the other face of this true JanusBifrons and recapitulate what is known about VSPs as electrically active proteins

Solution of Massless Spin One Wave Equation in Robertson-Walker Space-time

We generalize the quantum spinor wave equation for photon into the curved space-time and discuss the solutions of this equation in Robertson-Walker space-time and compare them with the solution of the Maxwell equations in the same space-time.Comment: 16 Pages, Latex, no figures, An expanded version of paper published in International Journal of Modern Physics A, 17 (2002) 113

Energy Spectrum of a 2D Dirac Oscillator in the Presence of the Aharonov-Bohm Effect

We determine the energy spectrum and the corresponding eigenfunctions of a 2D Dirac oscillator in the presence of Aharonov-Bohm (AB) effect . It is shown that the energy spectrum depends on the spin of particle and the AB magnetic flux parameter. Finally, when the irregular solution occurs it is shown that the energy takes particular values. The nonrelativistic limit is also considered.Comment: Latex, 12 page